---
abstract: 'We present optical photometric and low-resolution spectroscopic observations of the Type II plateau supernova (SN) 2008in, which occurred in the outskirts of the nearly face-on spiral galaxy M 61. Photometric data in the X-rays, ultraviolet and near-infrared bands have been used to characterize this event. The SN field was imaged with the ROTSE-IIIb optical telescope about seven days before the explosion. This allowed us to constrain the epoch of the shock breakout to JD = 2454825.6. The duration of the plateau phase, as derived from the photometric monitoring, was $\sim$ 98 days. The spectra of  show a striking resemblance to those of the archetypal low-luminosity IIP SNe 1997D and 1999br. A comparison of ejecta kinematics of  with the hydrodynamical simulations of Type IIP SNe by @dessart10 indicates that it is a less energetic event ($\sim 5\times10^{50}$ erg). However, the light curve indicates that the production of radioactive $^{56}$Ni is significantly higher than that in the low-luminosity SNe. Adopting an interstellar absorption along the SN direction of $A_V \sim$ 0.3 mag and a distance of 13.2 Mpc, we estimated a synthesized  mass of $\sim0.015 M_{\sun}$. Employing semi-analytical formulae [@litvinova85], we derived a pre-SN radius of $\sim 126$, an explosion energy of $\sim 5.4\times10^{50}$ erg and a total ejected mass of $\sim 16.7$. The latter indicates that the zero age main-sequence mass of the progenitor did not exceed 20. Considering the above properties of and its occurrence in a region of sub-solar metallicity (\[O/H\] $\sim$ 8.44 dex), it is unlikely that fall-back of the ejecta onto a newly formed black hole occurred in SN 2008in. We therefore favor a low-energy explosion scenario of a relatively compact, moderate-mass progenitor star that generates a neutron star.'
author:
- 'Rupak Roy$^\star$, Brijesh Kumar, Stefano Benetti, Andrea Pastorello, Fang Yuan, Peter J. Brown, Stefan Immler, Timur A. Fatkhullin, Alexander S. Moskvitin, Justyn Maund, Carl W. Akerlof, J. Craig Wheeler, Vladimir V. Sokolov, Rorbert M. Quimby, Filomena Bufano, Brajesh Kumar, Kuntal Misra, S. B. Pandey, Nancy Elias-Rosa, Peter W. A. Roming and Ram Sagar'
title: ' $-$ Bridging the gap between normal and faint supernovae of type IIP'
---

introduction {#intro}
============

Core-collapse Type II supernovae (SNe) mark the violent death of stars with main-sequence masses greater than 8  and as is indicated by the presence of hydrogen lines in their optical spectra, they originate from a progenitor star with a significant amount of hydrogen still intact [@eldridge04]. Of special interest are the Type II plateau SNe which are characterized by a ‘plateau’ in their optical light curve and are more common, constituting about 75% of all Type II SNe [@smith10]. The IIP SNe show a wide range of plateau luminosities, plateau durations, expansion velocities and nickel masses [@hamuy03] and these observational properties are connected with the explosion mechanism as well as the physical properties of the progenitor star such as ejected mass, explosion energy and pre-SN radius [@nadyozhin03; @smartt09]. The IIP SNe are thought to result from progenitor masses in the range $8-25$ [@heger03] with an extended hydrogen envelope necessary to maintain the plateau phase. A detailed study of optical light curves and spectra of only a few nearby IIP SNe has been done so far and there exists a discrepancy in estimating the mass of their progenitors, e.g, for the three well studied events (namely 1999em, 2005cs and 2004et), the determination of progenitor mass from the hydrodynamical modeling of their light curve is found to be higher than that estimated from pre-SN imaging [@utrobin10; @bersten11].

Recently, a number of ‘low luminosity’ Type IIP events have been discovered viz. SNe 1999br, 1999eu, 1994N, 2001dc, 2005cs [@pastorello04; @pastorello09], 2008bk [@vandyk11] and 2009md [@fraser10]. These events have explosion energy ($\sim 10^{50}$ erg) and ejected $^{56}$Ni-mass ($2-7\times10^{-3}$ ), both lower by a factor of 10 than normal, and low expansion velocity $\sim$1000 (@pastorello09 and references therein). The low-luminosity IIP SNe are debated because of the unknown nature of their progenitors. The first reported faint SN was SN 1997D [@turatto98; @benetti01], and the observed properties of its light curves and spectra were explained in terms of significant fallback of ejected material on a newly formed black hole (BH), created through the core collapse of a massive progenitor ($M \ga 20M_\odot$, @zampieri98 [@zampieri03]). Alternatively, SN 1997D was interpreted as the explosion of a less massive progenitor ($8-12M_\odot$, @chugai00), close in mass to the lower limit for stars that can undergo core-collapse. @heger03 suggested that low-luminosity Type IIP events are electron capture SNe produced by low-mass progenitors giving rise to ONeMg cores. This is further supported through investigations of pre-explosion images [@maund05a; @maund05b], though @eldridge07 have ruled out the possibility of such a mechanism for the low-luminosity SN 2005cs. According to the formalism of @heger03 and @eldridge04, no star having initial mass less than 22 can form a BH, which can quench the ejected material and produce a low luminosity SN. Stars with masses above 25$M_\odot$, formed in metal-poor or slightly sub-solar metallicity regions can produce low-luminosity, BH-forming Type IIP SNe. Type IIL/b events can be produced through this process from stars having masses $\textgreater~ 25$ and generated in regions with solar (or super-solar) metallicity. So, the metallicity information at the SN location and the estimation of the initial mass are essential to constrain the triggering mechanisms of these explosions.

SN 2008in was discovered in the nearby galaxy M 61 (NGC 4303). The first unfiltered CCD images of  were taken by Koichi Itagaki on 2008 December 26.79 (all times in UT hereafter) and 27.69 at a magnitude of 14.9. Independent observations of this event by K. Kadota showed the transient at an unfiltered mag of 15.1, In addition W. Wells recorded the SN on 2008 December 28.46 at $V$ and $R$ band magnitudes of 14.3 and 13.2, respectively [@nakano08]. Low and mid-resolution spectroscopic observations indicate an early discovery for SN 2008in (within 1$-$2 weeks after core-collapse). The spectra showed highly blueshifted H$_\alpha$ and H$_\beta$ absorptions (by $\sim$ 9000 ) with weaker emission components [@chakraborti08; @foley08; @stritzinger08]. The presence of prominent P-Cygni profiles of Balmer lines leads to its classification as a Type II SN.

The broadband light curve and the initial spectral evolution of SN 2008in were similar to those of normal Type IIP SNe. However, from mid-plateau, the SN started to show a few spectral features (like ) which are similar to under-luminous events.  was also observed in the radio with the Very Large Array (VLA) on 2008 December 31.40 UT in two frequency bands at 8.4601 and 22.4601 GHz [@Stockdale08]. Observation for the second epoch was further reported by @Stockdale09 on 2009 January 27. Interacting Type II SNe, like Type IIn events and a few Type IIP events (e.g. SN 2004et) are supposed to be strong sources of radio emission (for review see @weiler02). However, both VLA observations produced null results for this proximate event.

In this paper we present optical and near-infrared photometric and optical spectroscopic observations of . The photometric data cover a time span of about 410 days since the discovery. The [*Swift*]{}/XRT [@burrows05] and [*Swift*]{}/UVOT [@roming05] data covering a time span of 60 days are also presented. In Sections \[res:broadband\] and \[res:spec\] we study the the photometric and spectroscopic evolutions respectively. In Sections \[res:DistExt\] and \[res:ColBol\] we cover the estimates of distance, reddening, intrinsic color and bolometric fluxes. The main physical parameters of the explosion and the mass of the progenitor are derived in Section \[res:parameter\] while a comparison of its properties with other SNe is given in Section \[res:comp\]. The conclusion of the paper is given in the last section. The epoch of explosion JD = 2454825.6 (§\[res:broadband\]) is considered throughout the paper and the times of pre-/post-explosion are referred with $-/+$ signs respectively.

observation and data reduction {#obs}
==============================

Photometric Observation {#obs:phot}
-----------------------

The prompt follow-up of the event was carried out by the ground-based ROTSE-IIIb telescope[^1] having sensitivity in the wavelength region from 0.35 to 1.0  with a peak around 0.6 [@quimby07]. The SN was first detected in the ROTSE-IIIb images on 2008 December 24.45 and it was monitored at 58 phases until +115d. The initial detections of the SN evaded the automated pipeline identification due to poor image quality and low signal-to-noise ratio (S/N). The data reduction was performed afresh on all the available ROTSE data. In order to remove the contamination of the true SN-flux from the underlying galaxy, a pre-SN galaxy template was constructed from images taken in early 2008 and each SN frame was reduced using the galaxy-template subtraction scheme developed by @alard00. The point spread function (PSF) photometry was performed at the SN location in the galaxy-template subtracted images. The unfiltered instrumental magnitudes were calibrated using the USNO B1.0 $R$-band magnitudes of about 15 isolated stars. The light curve thus produced was found to be 0.15 mag off from the Cousins $R$ band light curve produced by multi-band observation carried out at ARIES (described below) and the ROTSE magnitudes were scaled accordingly.

The SN 2008in was also monitored with the Ultraviolet Optical Telescope (UVOT) on board [*Swift*]{} from +5d to +60d. The UVOT filters $uvw2, uvm2, uvw1, u, b$, and $v$ have their effective wavelength at 2030, 2231, 2634, 3501, 4329, and 5402 Å, respectively (Poole et al. 2008). The UVOT data reduction was performed following the prescriptions of @brown09a. A 5 aperture is used to measure the counts for the coincidence loss correction whereas a 3 aperture was used for the photometry. For the filters $uvw2, uvm2$, and $uvw1$, the last epoch (obtained about 60 days after explosion when the UV flux was very weak) data were used to subtract the galaxy light, while in the optical the SN remains much brighter than the underlying light so contamination was negligible. An aperture correction (based on an average PSF in [*Swift*]{} CALDB) as well as zeropoints from @poole08 was applied to put the magnitudes on the UVOT photometric system. The UVOT magnitudes are listed in Table \[tab:uvotsn\].

From +5d to +416d, the follow-up of SN 2008in in optical broadband Johnson $UBV$ and Cousins $RI$ was performed with the 1-m Sampurnanand Telescope (ST) at ARIES, Nainital[^2]. An identification chart showing the field of the galaxy M61 along with the locations of  as well as the local standards is presented in Figure \[fig:snid\]. The photometry is performed using standard tasks of IRAF [^3] and [*DAOPHOT*]{} [^4] as described in the paper by @roy11. Bias subtraction and flat fielding were performed on the raw frames and the cosmic removal was done using LACOSMIC routine [@dokkum01]. As the SN lies in the outskirts of the galaxy on a relatively faint and smooth background, the photometry at the initial phases (mostly during the plateau phase when the SN is bright) is estimated using the profile fitting method. During nebular phases, when the SN becomes faint, the true SN flux is estimated using the galaxy template subtraction method following the procedures of @roy11. As a galaxy template, we used post-explosion (+600d) images observed on 2011 January 04 under good seeing conditions. Figure \[fig:subimg\] shows an example of this procedure applied to a late-time $V$-band image of . In the template image of 2011 January 04, we can see a clear flux enhancement located approximately 5away (a linear distance of $\sim$ 343pc) from the SN position. This knot is also present in a deep image from the Sloan Digital Sky Survey (SDSS) and it is identified as a   region in the NED catalog. The progenitor of  may have a possible association with this star-forming region. The field of  was calibrated using @landolt09 standard stars of the field SA98 observed on the same night as the SN. A sample of 10 bright and isolated non-variable stars in the field of  was used as local standards to derive the zero points for the SN at each epoch. The location and magnitudes of these local standards are listed in Table \[tab:photstar\]. The entire time span of photometric observation is about 410 days and in Table \[tab:photsn\], we report the $UBVRI$ photometry of the event.

From +4d to +116d, the  was also observed in $JHK$ near-infrared (NIR) bands with the 0.6-m REM/REMIR at La Silla [@zerbi04]. The object was clearly visible in the $J$ and $H$ passbands, while it remained undetected in $K$. The instrumental magnitudes were standardized using the 2MASS standards available in the field of . The calibrated $JH$ magnitudes are listed in Table \[tab:remsn\].

X-ray Observations {#obs:xrt}
------------------

The [*Swift*]{} XRT observations were obtained simultaneously with the UVOT observations. To search for X-ray emission from , we extracted X-ray counts from a circular region with a 10 pixel radius ($23\farcs7$, corresponding to the XRT on-axis 90% encircled energy radius) centered on the optical position of the SN. The background was extracted locally from a source-free region of $40\arcsec$ radius to account for the detector, sky background, and the diffuse emission from the host galaxy.

No X-ray source is detected in the merged 27.1 ks XRT data obtained in photon-counting mode. The $3\sigma$ upper limit to the XRT net count rate is $7.2 \times 10^{-4}~{\rm cts~s}^{-1}$, corresponding to an unabsorbed (0.2–10 keV band) X-ray flux of $f_{0.2-10} < 3.4 \times 10^{-14}~{\rm erg~cm}^{-2}~{\rm s}^{-1}$ and a luminosity of $L_{0.2-10} < 7.0 \times 10^{38}~{\rm erg~s}^{-1}$ for an adopted thermal plasma spectrum with a temperature of $kT = 10~{\rm keV}$ (see @fransson96 and references therein), a Galactic foreground column density of $N_{\rm H} = 1.67 \times 10^{20}~{\rm cm}^{-2}$ [@dickey90] and a distance of 13.19 Mpc (§\[res:DistExt\]).

![image](sn2008in.epsi){width="18cm"}

![image](100120v_SN.ps){width="5cm"}![image](110104v_bg.ps){width="5cm"}![image](100120v_sub.ps){width="5cm"}

Optical Spectroscopic Observation {#obs:spec}
---------------------------------

Long-slit low-resolution spectra ($\sim$ 6 to 14 Å) in the optical range (0.33 $-$ 1.0 ) were collected at eleven phases from +7d to +143d, including five phases from the 2-m IGO, three phases from the 9.2-m HET, two phases from the 6-m BTA and one epoch from the 3.6-m NTT [^5]. A journal of spectroscopic observations is given in Table \[tab:speclog\]. The spectroscopic data acquired from IGO, NTT and HET were reduced under the IRAF environment. Bias and flat-fielding were performed on all the frames. Cosmic-ray rejection was done using the Laplacian kernel detection method [@dokkum01]. All the data obtained from the BTA were reduced using programs in the IDL software environment.

The instrumental FWHM resolution of 2-m IGO spectra as measured from the [\[\]]{}$\lambda$5577  emission skyline was found to lie between 6 and 10Å ($\sim$ 322 - 510 ). Flux calibration was done using standard spectrophotometric fluxes from @hamuy94, assuming a mean extinction for the site. For HET, BTA and NTT, the resolution near 6000Å is about 10Å, 14Å, and 12Å respectively.

Light Curve Evolution {#res:broadband}
=====================

According to theoretical interpretation, the entire broadband evolution of Type II SNe can be segmented into three phases: the rising phase, the plateau phase and the nebular phase.

![image](ROTSE_obs.ps){width="13cm"}

The Rising Phase {#res:rise}
----------------

The rising phase of the light curve is associated with the shock breakout phenomenon having been theoretically predicted for an SN explosion, in which, the radiation-dominated shock wave, generated through the reversal of iron core-collapse, starts to propagate outward through the onion shell-like layers of the progenitor; when the shock reaches regions with an optical depth of a few tens to unity, the radiation behind the shock escapes the outer surface giving rise to a hot (T $>10^5$ K) fireball which emits quasi-black body radiation in UV and soft-Xrays. This phenomenon is called shock breakout and depending on envelope mass, density structure and wind properties of the progenitor, the breakout light curve may last from a few hours to few a days (@grassberg71 [@chevalier76; @falkarnett77]). Due to the short timescale, the detection of shock breakout is rare, and only recently were, the UV light curves of the entire shock breakout phase lasting several hours was reported for two Type IIP SNe $-$ SNLS-04D2dc and SNLS-06D1jd (@gezari08 [@schawinski08]). Observation of the earliest UV and optical light curves of IIP SNe is crucial to model the shock breakout light curves and constrain the properties of SN progenitors [@tominaga09]. The earliest SNe IIP optical light curves have been studied in the past for SN 2005cs by @pastorello09 and for SN 2006bp by @quimby07 and the optical rising phase is rarely observed.

![image](lightcurve_2008in.ps){width="15cm"}

We model the early optical (ROTSE-IIIb $R$-band) light curves of using a simple model and put an observational constraint on the epoch and duration of shock breakout. Following the formulation of @waxman07, it can be shown that just after the shock breakout, the intensity of the SN light at a fixed wavelength is proportional to the intensity of blackbody radiation at that wavelength (see Equation 1 of @cowen10), while for the plateau phase @arnett80 derived an analytical expression (Equation 39 of that paper) and hence by combining these two equations one can approximate the overall intensity profile of a Type II SN during the rising phase through the plateau with the following expression :

I\_[SN]{}(t) = (t-t\_0)\^[1.6]{}

$$+ C\times exp[-\{(t/D) + (t/E)^2\}]$$

Here, the first term represents the phase associated with the shock breakout while the second term represents the plateau phase. Here ‘$t$’ is the time measured in days since 2008 December 23.95 (JD = 2454823.5), the epoch when ROTSE had a non-detection of the SN with limiting magnitude 16.16 in the $R$ band. The time of shock breakout, ‘$t_0$’ along with the constants $A, B, C, D$ and $E$ are free parameters of the fit. The values of these constants depend on the nature of the progenitor and the kind of explosion. The above model is fitted to the ROTSE data using the $\chi$ minimization technique [@press92] and the fit is shown in Figure \[fig:rotse\]. We obtained a value of the shock breakout time as $t_0 = 2.13\pm0.83$ day. This value of $t_0$ is consistent with the first data point observed from ROTSE. We shall adopt $t_0$, corresponding to JD = 2454825.6 as the explosion epoch for all the phases of . It is noted that at $V$ and $R$ bands, the SN was detected by @nakano08, just 2.4 d after the shock breakout. We note, however, that depending on the extent of the envelope the true core reversal marking the SN explosion would have occurred a few hours earlier (e.g., SN 1987A) or a few days earlier for a red supergiant envelope $\sim 50000{\mbox{$R_{\odot}$}}$ [@quimby07].

In Figure \[fig:rotse\], ‘$t_{p}$’ (+5.3 d) corresponds to the peak of the ROTSE $R$-band light curve. A similar peak in the UV light curves of the post-shock breakout phase of the Type IIP supernova SNLS-04D2dc lasting several days was identified as a secondary peak and this peak was explained as the shift of the spectral energy distribution toward longer wavelengths due to the rapid fall in temperature during free adiabatic expansion of optically thick plasma lead by a collisionless shock (@schawinski08 [@tominaga09]). For we identify the peak in the ROTSE $R$-band curve as a secondary peak and also speculate that for a few other type IIP SNe such as 1999em, 1999gi and 2005cs, where very tiny peaks before the plateau light curve were observed, such features are basically the footprints of the secondary peaks which could be a consequence of shock breakout.

The Plateau and Nebular Phase {#res:PlatNeb}
-----------------------------

The entire UV, optical and NIR light curves of  are shown in Figure \[fig:lightcur\] and for comparison, the light curves of the archetypal low-luminosity SN 2005cs [@brown09b; @pastorello09] are also plotted by scaling them in magnitude to match it with the observed plateau of . In the early plateau phase, a rapid drop in the UV flux and a slowly declining or constant optical and NIR flux are clearly apparent. The $uvm2$ magnitude declined from about 14.7 to 18.7 mag within 12 days. The decline rates of the flux in UVOT, $U$, $B$, and $V$ bands are almost identical with that of SN 2005cs. Starting from the $uvw2$ until the $R$ band, the measured decline rates from shock breakout to the plateau phase are approximately 0.32, 0.34, 0.25, 0.13, 0.04, 0.01 and 0.01 mag d$^{-1}$. The $IJH$ light curves are almost flat even at early phases of the plateau. For this SN, the plateau is well sampled in the $BVRI$ bands, so we can accurately determine the plateau duration. As discussed in §\[res:rise\], the secondary peak ($t_{p} \sim +5.3$ d) in the prompt light curve is generated by the gradually cooling shock heated SN atmosphere and after that the plateau mechanism starts to dominate. On the other hand, between the plateau and the nebular phase there is another transitional state, when an inflection in the light curve can be seen (§\[res:nick\]). Hence the plateau duration is precisely the time span between the secondary peak and the inflection. The inflection ($t_{i}$) in the $V$ band light curve is observed to occur nearly 103.2 days after the shock breakout. Hence the time interval between the $t_{i}$ and $t_{p}$ is assumed as the duration of the plateau i.e. $(103.2-5.3) \sim 98$ days.

Once the hydrogen envelope is fully recombined and the ejecta becomes optically thin, the light curve enters into the nebular phase and it is sustained mainly by the energy output from the radioactive decays of the iron-group elements. During the plateau to nebular transition phase, the $V$-band magnitude drops from $\sim$ 16.0 mag at around +90d (still in the plateau) to 18.1 mag at +122d (in the exponential light curve tail), i.e. $\sim$ 2 mag in about one month. This drop is remarkably smaller than that of SN 2005cs, but consistent with the 2-3 mag drop observed in normal Type IIP SNe [@olivares10]. A linear fit to the tail from +120d to +400d gives the following decline rates \[in mag (100d)$^{-1}$\]: $\gamma_B~\sim~0.44$, $\gamma_V~\sim~0.84$, $\gamma_R~\sim~0.91$, $\gamma_I~\sim~1.03$ at $B, V, R,$ and $I$ bands which are similar to the values found in normal IIP SNe and comparable with the decay slope of $^{56}$Co to $^{56}$Fe, i.e. 0.98 mag (100d)$^{-1}$.

Spectroscopic Evolution {#res:spec}
=======================

The spectra of  at 10 phases from +7d to +143d are shown in Figure \[fig:spec\_all\]. The spectra are corrected for the recessional velocity of the host galaxy ($\sim$ $1567\pm3$ )[^6]. Spectral features are mainly identified as per previously published line identifications for IIP SNe [@leonard02; @pastorello04]. The two earliest spectra (+7d and +14d) show the blue continuum of blackbody emission and have broad P-Cygni profiles of ,  and  $\lambda$5876. The next two spectra (+54d and +60d) represent the mid-plateau phase and are marked by a decrease in the continuum and the appearance of more number of P-Cygni profiles for the permitted metallic (, ,, , , ),  $\lambda$7773,  D, and  IR triplet lines similar to other normal Type IIP SNe (see @roy11 and references therein). The spectra at phases +87d, +90d and +99d represent the end stages of the plateau and these are marked by a redder continuum and decreasing line widths of hydrogen lines. The +118d, +119d and +143d spectra represent the nebular phase having negligible continuum and are marked by pronounced emissions of forbidden lines [\[\]]{} $\lambda\lambda$7291, 7323, and fading of the absorption features of hydrogen and metallic lines.

In Figure \[fig:spec\_ccsne\], we compare the +99d spectrum of  with the spectra of other Type IIP SNe observed at similar phases that is at roughly the time when the light curve changes from the photospheric plateau to the nebular phase. The phases quoted for each event are with reference to the moment of inflection ($ti$; see Figure \[fig:rotse\]). The narrow P-Cygni  profile and the presence of strong lines of  at $\lambda$6142 and $\lambda$6497 of  show striking resemblance to the low-luminosity SNe 1997D, 1999br, 1999eu, 2001dc and 2005cs. On the other hand, the normal Type IIP SNe 2004et, 2008gz show broader profiles of  and weaker lines of . It is noted that in the blue wing of  more metallic lines get resolved than that in the normal IIP SNe and this arises due to smaller line widths of hydrogen lines seen in low-luminosity IIP SNe (@fraser10 and references therein). In addition, the  line in the low-luminosity SNe show stronger absorption components due to lower ejecta temperature than that in the normal luminosity Type IIP SNe 2004et and 2008gz [@turatto98]. In Figure \[fig:line\_id\], we have identified the spectral features in a late-plateau phase (+99d) spectrum which covers the full wavelength range from 0.4 to 0.95 . For line identification we have followed @pastorello04, where different spectral lines were identified for the +102d spectrum of a low-luminosity SN 1999br. We are able to identify all the features and the spectral profiles of all the elements are similar to the archetypal low-luminosity IIP SNe 1997D, 2005cs and 1999br (@pastorello04 and references therein).

![image](line_id.ps){width="15cm"}

![image](velocity_all.ps){width="15cm"}

Figure \[fig:vel\_line\] shows the spectral evolution of , , D, , , and  lines. In Figure \[fig:vel\_bulk\] (left panel), we show the expansion velocities of the ejecta derived from Balmer and lines. The later is a good indicator of photospheric velocity. The expansion velocities of the H-envelope are estimated using absorption minima of the P-Cygni profiles and at the two earliest phases (+7d and +14d)  and  show a broad P-Cygni profile which, with time, becomes narrower at later phases keeping the position of the emission peak centered near zero velocity. The  line velocity starts at about 7000  at +10d, reaches 4000  at +50d and flattens at a level of 1200  in the nebular phases. It can be seen from the right panel of Figure \[fig:vel\_bulk\] that in the comparable phases, the  line velocities of low-luminosity SNe are less than half those of normal IIP SNe 1999em and 2004et, whereas the  velocities are more like low-luminosity SN 2005cs. To estimate the photospheric velocity of the transient we have computed the velocities of different  lines $\lambda$4924, $\lambda$5018, and $\lambda$5169 at different phases. The first marginal detection of these lines is in the +14d spectrum and they became prominent in later stages of evolution. For , the average velocity of these lines (and hence roughly the photospheric velocity) at +14d is about 4450 , which is comparable with that of low luminosity SNe and much less than ordinary Type IIP events (see Figure 12 of @pastorello09).

![image](velocity_bulk.ps){width="9cm"}![image](velocity_bulkcomp.ps){width="9cm"}

![image](spectra_OI.ps){width="15cm"}

The emergence of absorption dips in the blue wings of  is clearly seen in the post +60d spectra [^7] and the absorption dips have been marked with A, B and C in Figure 8. We distinguish ‘A’ as an absorption dip due to  and  and its equivalent width (EW) increases from 12.85 Å at +60d to 19.45 Å at +143d. This progressively stronger absorption dip requires a steep deceleration of the  emitting zone along with its depletion by inner metal shells (like ). The broader absorption dip ‘B’ is identified as a blend of  $\lambda$6497, ,  and . An exceptionally strong  lines is quite typical for cooler ejecta of less energetic fain SNe. The segment ‘C’ is speculated to be a footprint of  multiplets as the evolution of its linewidths seems to be correlated with the other  lines. It is noted, however, that in a few normal Type IIP SNe 1999em, 2004et and 2008gz, the spectral feature near ’C’ at early plateau phases was also identified as a signature of high velocity  component, and in the present spectra its presence cannot be completely ruled out.

The evolution of  is similar to the evolution of . The P-Cygni profile of  is clearly visible in the +7d spectrum, and later on, the emission component is blanketed by various metal lines, mainly due to  $\lambda$4924, $\lambda$5018 and $\lambda$5169. Traces of  and   absorption dips indicate that from the beginning of the nebular phase H shells of ejecta and the regions containing  ions move with a comparable velocity, reaching an asymptotic value $\sim$ 1000   nearly 140 days after the explosion. In the +14d spectrum,  lines are marginally detected while in the spectra between +60d and +99d they are prominent. We investigate the temporal change of EW of a relatively less blended  $\lambda$4924 and from the spectra normalized to the  peak value, we found that at an EW $\sim$ 0.67 Å at +14d, increased to 17.45 Å at +54d and to 26.53 Å in the +99d spectrum. Later on, the EW decreased to 19.07 Å in the +118d spectrum and finally reaches to 18.27 Å in the +143d spectrum. This rapid fall in the EW of inner metal-rich shells plausibly indicates a decrease in the opacity of the SN inner ejecta.

Traces of the  $\lambda$5876 line are also seen in the +7d spectrum (Figure \[fig:vel\_line\]). The ratio of EWs between  and  for this spectrum is EW()/EW() $\sim 1.99$. In the +14d spectrum, this ratio has increased to 5.36. The steep decrement in EW of  seems to be due to a quick recombination of  ions as a result of the rapid fall in temperature of the constantly rarefying ejecta. From +54d, the emerging P-Cygni feature of  D becomes prominent. This feature seems to be a perfect P-Cygni throughout our entire spectral sequence. It simply indicates an uniform spherical distribution of  in the SN ejecta. In the three high S/N spectra labeled with +54d, +60d and +87d, a tiny absorption dip overlaid on the emission component of the  D P-Cygni is seen and since these spectra are redshift corrected, these dips are impressions of intervening interstellar matter present in the host galaxy. The absence of any similar absorption dip in the blue wing of the D profile, expected from the Galactic interstellar matter, confirms that there is little absorption from the Milky Way in the line of sight of .

The spectral evolution of  $\lambda$6248 and the s-process element  $\lambda$6142 is clearly seen in . In the +7d and +14d spectra, there is no clear evidence for the presence of these two elements; however in the +54d spectrum they are prominent. The absorption components become stronger with time and persist until the +143d spectrum. In contrast, in the normal Type IIP SNe 2004et [@sahu06], 1999em [@elmhamdi03a] and 2008gz [@roy11] these features disappeared at $\sim$ 170 day after the explosion. The ejecta of underluminous Type IIP SNe expand at a lower velocity than those of normal ones. So, the Ba lines in low luminosity events persist for a longer time just because the ejecta takes more time to cool-down. As a result it takes longer time to become optically thin.

In the nebular phase, the forbidden lines: \[\] $\lambda\lambda$7291, 7323, \[\] $\lambda\lambda$6300, 6364 and \[\] $\lambda$7155 are among the strongest features visible in the spectra. Also, the line ratios of emission lines of  and the  doublet in late nebular phases is a good indicator of the progenitor mass [@fransson87]. In Figure \[fig:spec\_oi\], we compare the +139d and +283d spectra of SN 19997D (Benetti et al. 2001) with the nebular phase spectra of . The \[\] doublet is seen in the +99d, +119d and +143d spectra of , whereas a small footprints of the  doublet can be seen in the +143d spectrum. Presence of \[\] line is not seen in our spectra. Considering that the explosion epoch of SN 1997D was quite uncertain, it is likely that the spectral evolution of  and forbidden lines of  is quite similar to that of SN 1997D.

extinction and distance toward  {#res:DistExt}
===============================

In order to determine the bolometric light curve and other physical parameters, a correct estimate of the extinction and distance toward the SN is essential. We adopt the Galactic reddening along the line-of-sight of as derived from the 100 all sky dust extinction map [@schlegel98], i.e. = $0.0224\pm0.0003$ mag. In order to estimate reddening due to the host galaxy M61, we used the spectrum of  taken on 2009 February 17 from the 2-m IGO telescope having good S/N ($\sim$ 40) and corrected for mean heliocentric radial velocity of the host ($cz \approx 1567\pm3$ ). Near the zero velocity, the spectrum showed a tiny absorption feature overlaid on the emission component of the P-Cygni profile of D (§ \[res:spec\]). The EW of this absorption feature was computed as 0.535$\pm$0.713 Å. The error in EW is estimated using Equation 6 of @vollmann06. It is known that the EW of the interstellar absorption bands is well correlated with the reddening  estimated from the tail of SN Ia color curve [@barbon90; @richmond94; @turatto03] and hence using the empirical relation $=$ $-0.01 + 0.16$EW (EW in Å), given by @turatto03, we obtain the host contribution as $\approx 0.076\pm0.104$, which is considerably higher than the Galactic contribution. This is consistent with the absence of any absorption feature due to Galactic interstellar matter in the blue wing of the emission component of D profile. Consequently, we adopt (estimated as a sum of Galactic and host galaxy reddening) of 0.0984$\pm$0.104 mag for . This corresponds to a visual extinction ($A_V$) of $0.305\pm0.322$. Considering the uncertainty in the  estimated using D lines, a lower value of  of 0.0448$\pm$0.0006 mag (twice the Galactic reddening) and the corresponding $A_V$ of $0.139\pm0.002$ cannot be ruled out. We will discuss its implication for the derived properties of the SN.

The Hubble flow distance of the host galaxy M 61, after correction for the Virgo infall, is estimated as 13.7$\pm$1.1 Mpc [^8]. The distance estimated through the Tully-Fisher method is $12.1\pm2.7$ Mpc[^9]. Additionally, we also calculated the distance following the Standard Candle Method for Type II SNe [@hamuy02; @hamuy05; @hendry05]. It is found that there is a strong correlation between the distance of Type IIP SNe along with their mid-plateau apparent $V$ and $I$ band magnitudes and the mid-plateau photospheric velocity, for a given cosmological model. For we estimate the mid-plateau ($\sim +50$d) apparent magnitudes $\sim 15.56\pm0.05$ mag for $V$ and $\sim 14.77\pm0.06$ mag for the $I$ band. Considering the $V$ and $I$ band extinctions toward the SN, $0.305\pm0.322$ mag and $0.183\pm0.193$ mag respectively, we derive a distance of $\sim 12.23\pm1.87$ Mpc. The adopted value of mid-plateau photospheric velocity is $2694.67\pm70$ (see §\[res:proge\]). Combining the above three measurements, we adopt the weighted mean distance of $13.19\pm1.09$ Mpc, which corresponds to a distance modulus of $30.6\pm0.2$.

![image](colorcurve_2008in.ps){width="15cm"}

Color Evolution and Bolometric Flux {#res:ColBol}
===================================

Figure \[fig:colorcur\] shows the temporal evolution of the reddening-corrected broadband colors of . For comparison, we overplot the color curves for well studied SNe 1987A [@suntzeff90], 1999em [@elmhamdi03a], 2004et [@sahu06; @maguire10], 2005cs [@pastorello09] and SN 2008gz [@roy11]. After small differences during the initial phases, the plateau phase color evolution of all Type II SNe is more or less similar. The $(U-B)_{0}$ and $(B-V)_{0}$ colors are blue during early photospheric phases and they become redder by about 1-2 mag in the plateau phase while the $(V-R)_{0}$ and $(V-I)_{0}$ colors evolves slowly and become red only by about 0.5 mag. The $(J-H)_{0}$ color remains constant at $\sim$ 0.25 mag. During the transition phase from plateau to nebular, the low luminosity SN 2005cs showed a striking red peak in the $(B-V)_0$, $(V-R)_0$ and $(V-I)_0$ colors. For , this red peak is not present and its color evolution is found to be consistent with the normal Type IIP SNe.

The quasi-bolometric light curve estimated from the UV, optical and IR broadband ($UVOIR$) magnitudes of  is shown in Figure \[fig:bolcur\] along with those of other Type II SNe. The extinction-corrected magnitudes are first converted into fluxes using zero-points given by @bessell98 and then the total flux in $UVOIR$ bands is obtained after a linear interpolation and integration between 0.203 and 1.67 ${\mbox{$\mu{\rm m}$}}$. The fluxes were calculated on those nights for which we had complete observations in $UBVRI$ bands. For the initial two weeks the SN was detected by [*Swift*]{}/UVOT in near ultraviolet bands with a significant flux density. This contribution has been accounted for while measuring the net $UVOIR$ flux. It has been assumed that contribution in the $U$ band is mainly important during the plateau and decreases rapidly to about 5% in the nebular phase (see @misra07 [@roy11] and references therein). From the light curve it is noticeable that the near ultraviolet flux is almost negligible beyond 20 days after explosion. The $JH$ contribution during the plateau is calculated from our data, whereas for the nebular phase, due to lack of data, we are not able to make any direct measurement. For most low luminosity SNe IIP, the NIR flux contribution in the nebular phase is about 50% of the total flux [@pastorello09]. We have therefore increased the net flux by 55% to account for the maximal contributions from the $U$ and NIR bands at phases later than +140d. Figure \[fig:spec\_ccsne\] and \[fig:bolcur\] clearly demonstrate that spectroscopically  appears to be like low-luminosity SNe IIP, but photometrically it appears quite normal. In the following section, the $UVOIR$ curve is used to estimate the amount of radioactive $^{56}$Ni and other physical parameters that characterize the explosion and the progenitor star.

![image](bolometric_2008in.ps){width="15cm"}

Physical Parameters {#res:parameter}
===================

The physical entities that seem to govern the entire scenario are mainly associated with the nature of the progenitor and the radioactive elements (mainly $^{56}$Ni), generated inside the inner portion of the ejecta during the explosion. $^{56}$Ni is synthesized by the explosive burning of Si and O during the shock breakout [@arnett80; @arnett96]. Over time this material is eventually converted to $^{56}$Co and then to $^{56}$Fe by means of radioactive transitions having decay times of 8.77 and 111.3 days respectively. The $\gamma$-rays and neutrinos emitted during this process sustain the nebular phase light curve and consequently, the observed tail luminosity becomes a good tracer for the ejected synthesized $^{56}$Ni.

Produced Radioactive Nickel {#res:nick}
---------------------------

We use different methods to estimate mass of $^{56}$Ni. @hamuy03 proposed a relation between bolometric tail luminosity and the synthesized $^{56}$Ni during core collapse SNe, by considering the underlying assumption that all $\gamma$-rays emitted during the radioactive decay make the ejecta thermalised. For , the time interval spanned by the observations is about 416 days, where the first $\sim$ 100 days are reserved for photospheric evolutionary processes. The average $V$ band magnitude during the nebular phase calculated using the data between +114d and +416d is $\sim$ 18.91, which corresponds to the $V$ magnitude at $\sim$ +222d. Taking the extinction correction as ($A_V = 0.305\pm0.322$ mag; §\[res:DistExt\]), a bolometric correction of $0.26 \pm 0.06$ mag [@hamuy01] and a distance modulus $30.6\pm0.2$, the derived tail luminosity at this fiducial time is $(3.02\pm1.95)\times10^{40}\,{\rm erg\,s^{-1}}$. Within the errors, this value is consistent with the bolometric flux at a comparable epoch, determined in §\[res:ColBol\]. This implies that the amount of $^{56}$Ni produced is this process is $M_{\rm Ni} =0.0157\pm0.0102$.

All Type IIP SNe show an inflection in the light curve during the transition from the plateau to the nebular phase. Statistically it has been shown that the steepness of the $V$-band light curve slope (defined as S=d$m_{\rm V}$/dt) at the inflection time ($t_{i}$) is anti-correlated with mass [@elmhamdi03b]. For , the $V$ band light curve with its well-sampled transition phase, shows a value of steepness ${\rm S}=0.151\pm0.044$ mag d$^{-1}$ (Figure \[fig:steep\]) and the epoch of inflection is $t_{i} \approx$ +101.5d with respect to date of discovery. This corresponds to $M_{\rm Ni}=0.0175\pm0.002$. This result is consistent with the value measured using the @hamuy03 scheme. According to @elmhamdi03b, the amount of $^{56}$Ni can also be derived from the plateau absolute $V$-band magnitude using the relation ${\rm log} M_{\rm Ni} = -0.438 M_{V}(t_{i}-35)-8.46$. Here $M_{V}(t_{i}-35)$ is the absolute $V$ magnitude 35 days prior to the day of inflection. For , $M_{V}(t_{i}-35) \approx -15.23$, which again corresponds to a $^{56}$Ni mass around 0.016.

Comparison of the tail luminosity with that of SN 1987A $-$ a well studied proximate event, is also used for estimation of the $^{56}$Ni mass. A linear least square fit on the nebular light curve tail shows that at +222d the luminosity of  is about 1.54$\times10^{40}\,{\rm erg\,s^{-1}}$, while SN 1987A had a luminosity $\sim 1.10\times10^{41}\,{\rm erg\,s^{-1}}$ (Figure \[fig:bolcur\]). Since the $^{56}$Ni mass produced by SN 1987A is about 0.075, the amount of $^{56}$Ni in the case of  is $[(1.54/1.10)\times10^{-1}]\times0.075 \approx 0.0105$.

The above estimates are consistent with each other and hence we adopt a mean value for $^{56}$Ni mass as $0.015\pm0.003$

Explosion Energy and Mass of Progenitor Star {#res:proge}
--------------------------------------------

We use the radiation-hydrodynamical simulations of core-collapse IIP SNe by @dessart10 to infer the explosion energy ($E_{0}$ - kinetic plus thermal, expressed hereafter in units of $10^{51}$ erg or foe). These simulations suggest that in a given progenitor larger explosion energies yield larger ejecta velocities and this implies that the ejecta kinematics can be used to put constraint on $E_{0}$. For , the expansion rate of the H-rich progenitor envelope as derived from absorption minima in  at 15d after shock breakout is about 6300  (see Figure \[fig:vel\_line\]), which is higher than 4700 (for SN 2005cs; @pastorello09) and lower than 8800 (for SN 1999em; @elmhamdi03a). For the SNe 2005cs and 1999em, the simulation results (for explosion of a non-rotating solar metallicity pre-SN stars) predicted $E_{0} \sim$ 0.3 foe and $\ga$ 1 foe respectively and these values are found to be consistent with that determined from the actual hydrodynamical modeling of the SN light curves, e.g. 0.4 foe [@utrobin08] and 0.3 foe [@pastorello09] for SN 2005cs; 1.3 foe [@utrobin07a] and 1.25 foe [@bersten11] for SN 1999em. For , the simulations suggest $E_{0}$ $\sim$ 0.5 foe.

Accurate determinations of explosion parameters such as $E_{0}$, the ejected mass ($M_{\rm ej}$) and the pre-SN radius ($R_{0}$) of the progenitor require detailed hydrodynamical modeling of the light curves and spectra and this being a non-trivial task (beyond the scope of this paper) has only been attempted for a few SNe. In order to have an estimate of explosion parameters of here, we employ the analytical relations derived by @litvinova85 correlating the observed parameters ($M_{Vmp}$ - the mid-plateau absolute magnitude at $V$, $v_{mp}$ - the mid-plateau photospheric velocity and $\Delta t_{p}$ -the plateau duration) with the physical parameters ($E_{0}$, $M_{\rm ej}$ and $R_{0}$) based on a grid of hydrodynamical models for different values of physical parameter for Type IIP SNe. We note, however, that these approximate formulae have limitations owing to the poorly-measured observables and the simplified physical conditions such as non-inclusion of the effect of nickel heating, use of old opacity tables, neglecting the effect of line opacity and using outdated pre-SN models (@smartt09 [@bersten11] and references therein). As a result, only approximate values of the physical parameters can be derived using these relations. Fortunately, the observed parameters are derived very accurately for . The $\Delta t_{p}$ is $\sim 98$ days (§\[res:PlatNeb\]), the $v_{\rm mp}$ is $2694\pm70$[^10] and the $M_{Vmp}$ is estimated as $-15.32\pm0.38$ mag[^11] Now, employing analytical relations, we estimate $E_{0}$ $\sim$ 0.54 foe, $M_{\rm ej} \sim 16.7$  and $R_{0}$ $\sim 127 R_{\odot}$. The explosion energy derived in this way is consistent with that predicted from the hydrodynamical simulations of the ejecta kinematics.

The explosion energy of  indicates that the event was less energetic than the standard IIP SNe 1999em, 2004et and more energetic than SN 2005cs. Assuming a net mass loss $\sim 0.5$M$_\odot$ due to stellar wind [^12] and accounting for a compact remnant with mass $\sim 1.5-2$M$_\odot$ [@sahu06], we find that the initial mass of the progenitor to be $\leq 20$M$_\odot$.

![Determination of steepness parameter from the apparent $V$-band light curve of . See text for details.[]{data-label="fig:steep"}](elmhamdi_fit.ps){height="13cm"}

![image](abs_lightcurve.ps){width="15cm"}

Discussion in context of other Type IIP SNe {#res:comp}
===========================================

In Figure \[fig:abs\_lc\], we compare the light curve of  in absolute $V$-band magnitude with a sample of 13 other well studied Type II SNe taken from the literature (see @misra07 and @roy11 for the references). The sample includes two low-luminosity Type IIP SNe 1997D, 2005cs; nine normal Type IIP SNe 1990E, 1992H, 1999gi, 1999em, 2003gd, 2004A, 2004dj, 2004et, 2008gz and two peculiar Type II SNe 1987A, 1998A. It is seen that the Type II SNe show a wide range of mid-plateau luminosity, i.e. from $-14$ to $-17$ mag. The data on low luminosity SNe (see also Table 4 in @pastorello06) indicate that their $M_{Vmp}$ lie between $-14$ and $-15$ mag, with the only exception of SN 2002gd, whereas the other SNe are seen to lie between $-16$ and $-17$ mag. With the $M_{Vmp}$ of $-15.32$ mag[^13], the  present another case which occupies the gap between a low and normal luminosity Type IIP events.

The tail luminosity is an important indication of ejected nickel mass. The average tail luminosity of  (Mv $\sim -12.16$) is nearly 2.3 mag brighter than those of the low luminosity SNe 1997D and 2005cs ($\sim -9.6$ mag), and about 1.3 mag fainter than the normal Type IIP SNe 1992H, 2004et and 2008gz ($\sim -13.5$ mag). A close inspection shows that the average tail luminosity of  is roughly similar to Type IIP SNe 2004dj in NGC 2403 and 2003gd in M74, whereas their plateau is brighter ($\sim 0.5-0.8$ mag at mid-plateau) than . These comparison of nebular phase luminosities with other SNe are consistent with a quite modest radioactive $^{56}$Ni production for  (§\[res:nick\]).

The luminosity and shape of the tail depend on the $^{56}$Ni mass and the radiant energy per unit ejected mass ($E/M_{\rm ej}$). The first parameter determines the absolute magnitude, while the second determines its decay rate (see @turatto98 and references therein). The measured value of ejected $^{56}$Ni mass for SNe 2003gd, 2004dj and 2008in are respectively $\sim 0.016$ [@hendry05], 0.017 [@venko06] and 0.015. These are more than twice the $^{56}$Ni amount synthesized by the low-luminosity SNe 1997D ($\sim 0.002$, @turatto98) and 2005cs ($0.003-0.004$, @pastorello09), although at least three times less than the $^{56}$Ni produced by normal Type IIP SNe such as 1992H ($\sim 0.075$, @clocchiatti96), 2004et ($\sim 0.06$, @sahu06 [@maguire10]) and 2008gz ($\sim 0.05$, @roy11). Similarly, the measured value of $E/M_{\rm ej}$ ratio for  is larger than that for the low-luminosity SN 1997D and lower than other Type IIP events [^14].

Though we have an approximate value of the pre-SN radius (128 ), it can be seen that the progenitor radius of  is smaller than or comparable to that of low-luminosity SNe 1997D ($\sim 300$, @turatto98), 2005cs ($100-600$, @pastorello09, @utrobin08), 1999br ($114$, @zampieri03) and much smaller than that of normal Type IIP SNe like 1992H ($\textgreater 600$, @clocchiatti96), 2004et ($\sim 530$, @misra07). We also note that  progenitor is only slightly larger than the blue supergiant progenitors of SNe 1987A and 1998A [@pastorello05].

Information about the metallicity of the SNe location is essential to constrain the triggering mechanism of the SN explosion [@heger03]. The oxygen abundance \[O/H\] of  is about 8.44 dex [^15] which is marginally lower than the solar value of 8.65 dex [@asplund09]. A comparison [^16] of \[O/H\] of all the events in our sample indicate that there is no clear trend in the type of event and the metallicity.

The photometric and spectroscopic comparisons of  with some Type II SNe covering a wide range of physical parameters puts observational constraints on the nature of the progenitor of this event, pointing toward a star that was more compact than a typical M-Type red supergiants, and closer to a blue supergiant, may be an yellow supergiant. However, the direct detection of the progenitors of a few faint SNe IIP in pre-explosion archive [*Hubble Space Telescope*]{} images seems to contradict this conclusion (see @smartt09 for a review). Our spectroscopic study suggests that in  the hydrogen envelope ejected by the explosion is smaller than in most Type II events but larger than that ejected in low luminosity events (such as SN 1997D, SN 1999br and SN 1999eu). This may be due to a significant mass loss of the parent star in the latest stages of its life. This conclusion is also partially supported by the upper limit mass loss rate of the progenitor revealed from the X-ray study (§\[res:proge\]). In view of the upper mass limit of 20 M$_\odot$ progenitor, occurrence of the event in a sub-solar metallicity region and following the evolution of a single massive star as a function of metallicity [@heger03], we rule out the possibility of “fall back of ejecta to BH” scenario in case of , supporting the scenario of a weak explosion of a relatively compact progenitor.

conclusion {#concl}
==========

Low-luminosity Type IIP SNe belong to a poorly known class of events due to the unknown nature of their progenitors as well as the explosion mechanism. Spectroscopic as well as photometric characteristics of these events are significantly different from the normal and peculiar Type IIP SNe. The number of such underluminous events discovered so far is relatively small. This could plausibly be a selection effect. If we confine the search of core-collapse SNe to a small volume, the majority of SNe discovered in the local universe are of Type IIP ($\sim 48.2\%$, @smith10) and probably a large fraction of them would turn out to be underluminous. Incidentally SN 1999br, SN 2005cs and SN 2009md are the only events of this group to have been discovered soon after the core-collapse and whose data are publicly available.

In this study we have reported the results of an extensive photometric and spectroscopic follow-up campaign for in the ultraviolet, optical and near-infrared domains. The SN was also observed (although not detected) at X-ray and radio wavelengths. The SN was observed soon after its explosion, during the fast rise of the light curve to the optical maximum. Evidence for a shock breakout in  was primarily derived through the analysis of the ROTSE-IIIb $R$-band light curve. An upper limit of 16.16 mag in the $R$-band was estimated about two days before the discovery.

Modeling the $R$-band light curve allowed us to estimate a reliable epoch of the shock breakout, with an uncertainty of about two days. The plateau phase in  lasts about 98 days which is marginally shorter than in normal Type IIP SNe. The evolution of the bolometric light curve indicate that the event is somewhat in between the normal and faint Type IIP event. The luminosity of the nebular phase light curve indicates an ejected $^{56}$Ni mass of $\sim0.015{\mbox{$M_{\odot}$}}\,$, a factor two higher than that derived for low-luminosity IIP SNe. The spectroscopic evolution of  is similar to those of low-luminosity IIP SNe (1997D, 1999br and 1999eu), indicated particularly by the strong presence of  lines, the narrow line widths of  lines, and the expansion velocities of SN during the photospheric phases ($\sim 3000$ ) and the nebular phases ($\sim$ 1200 ). The ejecta kinematics of  are consistent with less-energetic ($\sim 5\times10^{50}$ erg) Type IIP SNe.

Spectroscopically  appears to be like low-luminosity SNe IIP, but photometrically it appears close to a normal type IIP event.

From the light curve and spectra of , we could determine quite accurate values of the observed properties such as plateau duration, mid-plateau luminosity as well as the photospheric velocity and this has helped us to comment on the properties of the explosion and the progenitor star. Using semi-analytical formulae by @litvinova85, we could estimate approximate values of the explosion energy $\sim5.4\times10^{50}$ erg, the ejected mass $\sim17{\mbox{$M_{\odot}$}}$ and the pre-SN radius $\sim127{\mbox{$R_{\odot}$}}$. The explosion energy of  is smaller than the normal ($\ge 10^{51}$ erg) Type IIP events, although higher than that estimated in underenergetic ($\sim 10^{50}$ erg) SNe IIP. We could provide an upper limit to the mass loss rate of the progenitor as $5\times10^{-6}~M_{\odot}~{\rm yr}^{-1}~(v_{\rm w}/10~{\rm km~s}^{-1})$ whereas the upper limit for the main-sequence mass of the progenitor star is estimated as 20 .

We thank all the observers at Aryabhatta Research Institute of Observational Sciences (ARIES) who provided their valuable time and support for the observations of this event. We are thankful to the observing staffs of ROTSE, REM, 2-m IGO, 3.6-m NTT, 6-m BTA and 9.2-m HET for their kind cooperation in observation of . This work was supported by the grant RNP 2.1.1.3483 of the Federal Agency of Education of Russia. Timur A. Fatkhullin and Alexander S. Moskvitin were supported by the grant of the President of the Russian Federation (MK-405.2010.2). This is also partially based on observations collected at the European Southern Observatory, Chile under the program 083.D-0970(A). Stefano Benetti and Filomena Bufano are partially supported by the PRIN-INAF 2009 with the project “Supernovae Variety and Nucleosynthesis Yields”. The research of J. Craig Wheeler is supported in part by NSF Grant AST-0707669 and by the Texas Advanced Research Program grant ASTRO-ARP-0094. This research is supported by NASA grant NNX08AV63G and NSF grant PHY-0801007. This work is partially based on observations made with the REM Telescope, INAF Chile. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. We are indebted to the Indo-Russian (DST-RFBR) project No. RUSP-836 (RFBR-08-02:91314) for the completion of this research work.

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[^1]: The Robotic Optical Transient Search Experiment (ROTSE-III) is a set of four 45 cm fully robotic optical telescopes, installed at Siding Spring Observatory, Australia (ROTSE-IIIa), McDonald Observatory, Texas (ROTSE-IIIb), H.E.S.S. site, Namibia (ROTSE-IIIc) and TUBITAK National Observatory, Turkey (ROTSE-IIId; @akmrs03 [@yuan_thesis]).

[^2]: A 2048 $\times$ 2048 CCD camera mounted at the f/13 Cassegrain focus of the telescope, has a square pixel of 24 micron on a side, and with a plate scale of 0.38 arcsec per pixel, it covers a square area of 13$\arcmin$ on a side in the sky. The gain and readout noise of the CCD camera are 10 electrons per analog-to-digital unit and 5.3 electrons, respectively. To improve the S/N ratio and optimize the sampling, the observations were carried out in a binned mode of 2$\times$2 pixel.

[^3]: IRAF stands for Image Reduction and Analysis Facility distributed by the National Optical Astronomy Observatory which is operated by the Association of Universities for research in Astronomy, Inc. under co-operative agreement with the National Science Foundation

[^4]: [*DAOPHOT*]{} stands for Dominion Astrophysical Observatory Photometry [@stetson87]

[^5]: The full names of the telescope are provided in Table \[tab:speclog\]

[^6]: http://leda.univ-lyon1.fr/

[^7]: This is the first time that the rapidly changing evolution of  profiles during the late plateau to nebular phase has been so densely covered in a low-luminosity SN

[^8]: The cosmological model with $H_0$ = 70 Mpc$^{-1}$,$\Omega_{m}$ = 0.3 and $\Omega_{\Lambda}$ = 0.7 is assumed throughout the paper and the uncertainty corresponds to a local cosmic thermal velocity of 208 [@terry02].

[^9]: http://nedwww.ipac.caltech.edu/

[^10]: This is the mean value of the velocities computed from the lines of $\lambda$4924, $\lambda$5018 and $\lambda$5169 in the +54d spectrum.

[^11]: These observed values of $M_{Vmp}$ and $v_{mp}$ can be compared with the estimates derived empirically between the mass of $^{56}$Ni, and the $v_{mp}$ and $M_{Vmp}$ for a larger sample of IIP SNe (see Section 8.3 of @roy11). Using the $^{56}$Ni mass of $0.015\pm0.003$ we find $v_{mp} =2916\pm 220{\mbox{$\rm{\,km\,s^{-1}}$}}\,$ and $M_{Vmp} = -15.37\pm0.23$ mag which are consistent with those measured observationally.

[^12]:  was not detected in X-ray. The lack of X-ray emission can be used to constrain the mass-loss rate of the progenitor system that could be heated by the outgoing shock to X-ray emitting temperatures. Following the discussion in @immler07 and references therein, an upper limit to the pre-SN mass-loss rate of $5 \times 10^{-6}~M_{\odot}~{\rm yr}^{-1}~(v_{\rm w}/10~{\rm km~s}^{-1})$ with an uncertainty of a factor of two to three is obtained.

[^13]: Considering a lower value of $A_V = 0.139\pm0.002$, we get $M_{Vmp} = -15.1\pm0.2$

[^14]: The measured value of the $E/M$ ratio for  is $3.5\times\,10^{49}$erg $^{-1}$, which is twice the E/M ratio measured for SNe 1997D ($1.7\times\,10^{49}$erg $^{-1}$, @turatto98), but it is half those of SN 2008gz ($7.3\times\,10^{49}$erg $^{-1}$, @roy11), SN 2004dj ($7.3\times\,10^{49}$erg $^{-1}$, @venko06) and SN 2004et ($7.5\times\,10^{49}$erg $^{-1}$, @sahu06). Finally, it is barely a third the E/M ratios of SNe 1987A ($11.3\times\,10^{49}$erg $^{-1}$, @hamuy03) and 2003gd ($13.0\times\,10^{49}$erg $^{-1}$, @hendry05), and about seven times smaller than the luminous event like SN 1998A ($25.4\times\,10^{49}$erg $^{-1}$, @pastorello05).

[^15]: Derived from the \[O/H\]($= 12 + log(N_O/N_H)$)-$M_{B}$ relation given by @pilyugin04 for a given deprojected radius and galaxy type. The host of  is a spiral galaxy of SBbc Type and the SN location is $\sim 1.8\arcmin$ away from the center of the host. This corresponds to a deprojected distance $\sim 7.03$ kpc.

[^16]: We have taken the \[O/H\] value from @smartt09, though for a few cases the values are estimated using the \[O/H\]$-M_B$ relation mentioned in @pilyugin04. The \[O/H\] for normal Type IIP SNe 2008gz, 1999em and low-luminosity SNe 1999eu, 2005cs is about 8.6. For the low-luminosity SNe 1994N, 1999br, 2001dc, 2003gd and 2004dj, 1997D and the normal SN 2004et, it is about 8.4. In many cases, we have adopted the \[O/H\] value from @smartt09. For some cases we have estimated the value by adopting the \[O/H\]$-M_B$ relation mentioned in @pilyugin04.
