Swift/XRT observes the fifth outburst of the periodic Supergiant Fast
  X-ray Transient IGR J11215-5952
Astronomy & Astrophysics manuscript no. romanoigrj112 c© ESO 2018
October 24, 2018
Letter to the Editor
Swift/XRT observes the fifth outburst of the periodic Supergiant
Fast X–ray Transient IGR J11215–5952
P. Romano1,2, L. Sidoli3, V. Mangano4, S. Mereghetti3, and G. Cusumano4
1 INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
2 Università degli Studi di Milano, Bicocca, Piazza delle Scienze 3, I-20126 Milano, Italy
3 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via E. Bassini 15, I-20133 Milano, Italy
4 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, I-90146 Palermo, Italy
Received 1 March 2007/Accepted 4 April
ABSTRACT
Context. IGR J11215-5952 is a hard X–ray transient source discovered in April 2005 with INTEGRAL and a confirmed member of
the new class of High Mass X–ray Binaries, the Supergiant Fast X–ray Transients (SFXTs). Archival INTEGRAL data and RXTE
observations showed that the outbursts occur with a periodicity of ∼330 days. Thus, IGR J11215–5952 is the first SFXT displaying
periodic outbursts, possibly related to the orbital period.
Aims. We performed a Target of Opportunity observation with Swift with the main aim of monitoring the source behaviour around
the time of the fifth outburst, expected on 2007 Feb 9.
Methods. The source field was observed with Swift twice a day (2ks/day) starting from 4th February, 2007, until the fifth outburst,
and then for ∼ 5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of ∼ 73 ks. This
is the most complete monitoring campaign of an outburst from a SFXT.
Results. The spectrum during the brightest flares is well described by an absorbed power law with a photon index of 1 and NH ∼
1 × 1022 cm−2. A 1–10 keV peak luminosity of ∼1036 erg s−1 was derived (assuming 6.2 kpc, the distance of the optical counterpart).
Conclusions. These Swift observations are a unique data-set for an outburst of a SFXT, thanks to the combination of sensitivity and
time coverage, and they allowed a study of IGR J11215–5952 from outburst onset to almost quiescence. We find that the accretion
phase lasts longer than previously thought on the basis of lower sensitivity instruments observing only the brightest flares. The
observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit
with many flares superimposed, possibly due to episodic or inhomogeneous accretion.
Key words. X-rays: stars: individual: IGR J11215–5952
1. Introduction
The hard X–ray transient IGR J11215–5952 was discovered
with the INTEGRAL satellite during an outburst in April 2005
(Lubinski et al., 2005) and was associated with HD 306414
(Negueruela et al., 2005), a B1Ia supergiant located at a dis-
tance of 6.2 kpc (Masetti et al., 2006). The short duration of the
outburst together with the likely optical counterpart suggested
that IGR J11215–5952 could be a new member of the class
of Supergiant Fast X-ray Transients (SFXTs; Negueruela et al.
2006). Analysing archival INTEGRAL observations of the
source field, Sidoli, Paizis, & Mereghetti (2006, hereafter Paper
I) discovered two previously unnoticed outbursts (in July 2003
and in May 2004) which demonstrate the recurrent nature of this
transient and suggest a possible periodicity of ∼330 days. This
periodicity was confirmed by the detection of the fourth outburst
from IGR J11215–5952 with RossiXTE/PCA on 2006 March
16–17, 329 days after the third outburst (Smith et al., 2006b).
The RXTE/PCA observations showed strong flux variability and
a hard spectrum (power-law photon index of 1.7 ± 0.2 in the
range 2.5–15 keV) as well as a possible pulse period of ∼195 s
(Smith et al., 2006a). The periodicity was confirmed with RXTE
observations of the latest outburst, yielding P = 186.78 ± 0.3 s
(Swank et al., 2007). Follow-up observations with Swift/XRT
Send offprint requests to: P. Romano, patrizia.romano@brera.inaf.it
refined the source position and confirmed the association with
HD 306414 (Steeghs et al., 2006). A hard power-law with a high
energy cut-off around 15 keV is a good fit to the spectra ob-
served with INTEGRAL (Paper I). For the distance of 6.2 kpc,
the peak fluxes of the outbursts correspond to a luminosity of
∼ 3×1036 erg s−1 (5–100 keV). All these findings confirmed
IGR J11215–5952 as a member of the class of the SFXTs, and
the first object of this class of High Mass X–ray Binaries dis-
playing periodic outbursts.
Predicting a fifth outburst for 2007 Feb 9, we obtained a
Target of Opportunity (ToO) observing campaign with Swift,
which commenced on Feb 4. The source started showing re-
newed activity on Feb 8 (Romano et al., 2007) and under-
went a powerful outburst on Feb 9 (Mangano et al., 2007a,b;
Sidoli et al., 2007; Swank et al., 2007). This paper presents our
observations of IGR J11215–5952 and it is organized as follows.
In Sect. 2 we describe our observations and data reduction; in
Sect. 3 we describe our spatial, timing and spectral data anal-
ysis. Finally, in Sect. 4 we discuss our findings and draw our
conclusions.
2. Observations and Data Reduction
Table 1 reports the log of the Swift/XRT observations used for
this work. Thanks to Swift’s fast-slewing and flexible observing
http://arxiv.org/abs/0704.0543v1
2 P. Romano et al.: The fifth outburst of IGR J11215–5952 observed by Swift
140 145 150 155
MJD (−54,000)
(a) 1−10 keV
Feb  9 14 19 24
0 2×104 4×104 6×104 8×104
Time (s since 2007−02−09 00:03:05 UT)
1 2 3
(b) 1−10 keV
1−4 keV(c) Flare 1
te 4−10 keV
5200 5400 5600 5800 6000
Time (s)
4−10/1−4
1−4 keV (d) Flare 2
3 4−10 keV
1.05×104 1.1×104 1.15×104
Time (s)
4−10/1−4
Fig. 1. XRT light curves, cor-
rected for pile-up, PSF losses,
vignetting and background-
subtracted. a) 1–10 keV light
curve for the whole campaign.
Different colours denote dif-
ferent observations (Table 1),
and points before Feb 6 (MJD
54,137) and after Feb 15 (MJD
54,146) are drawn from the
sum of several observations.
Filled circles are full detections
(S/N>3), triangles marginal
detections (2 <S/N< 3), while
downward-pointing arrows are
3-σ upper limits. The vertical
lines mark our time selections
for spectroscopic analysis
(observation 6, observations
7–8, observations 9–12, end
of observation 18). The hor-
izontal line marks the region
shown enlarged in panel b).
b) Detail of observation 6,
with a binning that allowed to
achieve a S/N in excess of 6.
The numbers mark the 5 flares
on which we performed spec-
troscopic analysis. c) Detail of
flare 1, showing the 1–4 keV,
4–10 keV count rates (top and
middle panel) and the hardness
ratio 4–10/1–4 (bottom). The
data were rebinned in order
to have at least 20 counts per
bin in both the 1–4 keV and
4–10 keV band. d) Same as c),
for flare 2.
scheduling, the ToO observations started on 2007 Feb 4 with
2 ks per day evenly spread throughout the day to maximize the
chances of detection of the outburst onset, and were increased
to 5 ks afterwards for a total of 23 days and a total on-source
exposure of ∼ 73 ks. We also retrieved from the Swift Archive
the data from a 643 s ToO performed on 2006 Mar 20 during the
fourth outburst of this source (Steeghs et al., 2006).
The XRT data were processed with standard procedures
(xrtpipeline v0.10.6), filtering and screening criteria by us-
ing FTOOLS in the Heasoft package (v.6.1.2). Given the low
rate of the source during the whole campaign, we only consid-
ered photon counting data (PC) and further selected XRT grades
0–12 (Burrows et al. 2005). With the exception of observation
6 (2006 Feb 9) the data show an average count rate of < 0.5
counts s−1 and no pile-up correction was necessary. We there-
fore extracted the source events from a circular region with a
radius of 11 pixels (1 pixel ∼ 2.37′′). During observation 6, pile-
up correction was required and we adopted an annular source
extraction region with radii 4 and 30 pixels. To account for the
background, we also extracted events within an annular region
centered on the source, and with radii 40 and 100 pixels, free
from background sources. Ancillary response files were gen-
erated with xrtmkarf, and account for different extraction re-
gions, vignetting and PSF corrections. We used the latest spec-
tral redistribution matrices (v008) in the Calibration Database
maintained by HEASARC. For timing analysis, the arrival times
of XRT events were converted to the Solar System barycentre
with the task barycorr and source events were extracted from
the circular region with 30 pixels radius to maximize statistics.
BAT always observed IGR J11215–5952 simultaneously
with XRT, but only survey data products, in the form of Detector
Plane Histograms (DPH) with typical integration time of ∼
300 s, are available. The BAT data were analysed using the stan-
dard BAT analysis software distributed within FTOOLS. DPH
data were calibrated with the task baterebin using the proper
BAT gain/offset files from the housekeeping data directory, and
sky images of each observation were extracted in the 15–25,
25–50, 50–100, 100–150 and 15–150 keV energy bands. The
batcelldetect task never detected the source above a signal-
to-noise ratio (S/N) threshold of 4. This is consistent with the ex-
trapolation at the high energies of the XRT data fit (Sect. 3) with
an absorbed power-law with exponential cutoff, with e-folding
energy of 15 ± 2 keV drawn from the RXTE fit (Swank et al.,
2007).
Throughout this paper the uncertainties are given at 90%
confidence level for one interesting parameter (i.e., ∆χ2 = 2.71)
unless otherwise stated. The spectral indices are parameterized
as Fν ∝ ν
−α, where Fν (erg cm
−2 s−1 Hz−1) is the flux density as
a function of frequency ν; we also use Γ = α + 1 as the photon
index, N(E) ∝ E−Γ (ph cm−2 s−1 keV−1).
P. Romano et al.: The fifth outburst of IGR J11215–5952 observed by Swift 3
0 0.5 1 1.5
Phase
Fig. 2. Folded 0.2–10 keV light curve of the combined obser-
vations 6 though 11, using a period of 186.78 s (Swank et al.,
2007).
3. Analysis and Results
A refined position was obtained by summing all data taken in
2007 with the exclusion of observation 6 (affected by pile-up) at
RA(J2000) = 11h21m46.s90, Dec(J2000) = −59◦ 51′46.′′9, with
an error, drawn from the cross-correlation with the USNO-B1.0
catalogue, of 1.′′1 (90% confidence). This position is 1.′′2 from
the optical counterpart HD 306414.
We extracted light curves in the 1–10 keV (total), 1–4 keV
(soft) and 4–10 keV (hard) bands. The 0.2–1 keV band was not
used in our analysis because, given the high absorbing column
density, its signal was significantly lower than the one of the
other bands. The light curves were corrected for Point-Spread
Function (PSF) losses, due to the extraction region geometry,
bad/hot pixels and columns falling within this region, and for
vignetting, by using the task xrtlccorr (v0.1.9), which gen-
erates an orbit-by-orbit correction based on the instrument map.
We then subtracted the scaled background rate in each band from
their respective source light curves and calculated the 4–10/1–4
hardness ratio. The IGR J11215–5952 light curve (Fig. 1) shows
an increase in count rate by a factor of ∼ 10 in less than 1.5
hours, and of a factor of ∼ 65 in 17 hours on 2007 Feb 9.
However, no significant variation in the hardness ratio can be ev-
idenced (panels c,d of Fig. 1). Indeed, fitting the hardness ratio
as a function of time (or as a function of count rate) to a constant
model yields a value of 0.49±0.03 and χ2 = 1.13 for 80 degrees
of freedom, d.o.f.
We folded the data at the period of 186.78 s reported
by Swank et al. (2007) based on Feb 9 01:20–03:20 UT
RXTE/PCA observations and obtained the 0.2–10 keV light
curve shown in Fig. 2.
Upon examination of the light curve presented in Fig. 1 and
the available counting statistics, we selected different time bins
over which we accumulated spectra. These include i) the qui-
escent phase before the 2007 Feb 9 outburst, ii) the Feb 9 out-
burst (observation 6) and iii) the tail phase of the outburst (ob-
servations 7–8, 9–12, 7–12, 7–18). We further selected 5 flaring
episodes from observation 6 (see Fig. 1b). A comparison with
the data collected during the tail of the 2006 outburst was also
performed. The data were rebinned with a minimum of 20 counts
per energy bin to allow χ2 fitting. However, for the 2006 observa-
tion performed (28 counts), before the onset of the outburst (41
0.5 1 1.5 2 2.5 3
NH (10
22 cm−2)
0.5 1 1.5 2 2.5 3
NH (10
22 cm−2)
Fig. 3. XRT time-selected spectroscopy. The ∆χ2 =
2.3, 4.61, 9.21 contour levels for the column density in units of
1022 cm−2 vs. the photon index, with best-fit values indicated by
crosses. Orange contours are from observation 6, while the blue
ones are from the combined observations 7 though 18.
counts), for the late flares, and the late observations, the Cash
statistic (Cash, 1979) and spectrally unbinned data were used.
The spectra were all fit with XSPEC (v11.3.2) in the 0.5–9 keV
energy range, adopting the typical pulsar spectral model, an ab-
sorbed power law model.
The best fit parameters are reported in Table 2 along with
the mean luminosity of each time selection. The spectrum of
the brightest part of the outburst (observation 6) could be fit
with a single power law, with a photon index Γ = 1.00+0.16
−0.14 and
an absorbing column density of NH = (1.04
+0.25
−0.20) × 10
22 cm−2
(χ2red = 1.04/83 d.o.f.), while the combined observations 7–18
yielded a photon index of 2.08+0.41
−0.37 and NH = (2.04
+0.62
−0.50) × 10
cm−2 (χ2red = 1.19/19 d.o.f). We note that the 1–10 keV count
rate to unabsorbed 1–10 keV flux conversion factor, obtained
from the best fit model for observation 6, is 2.9×10−10 erg cm−2
count−1. We also performed fits of observation 6 with an ab-
sorbed black-body. Since the column density assumed a value
significantly below that resulting from the interstellar medium,
the power law fit was favoured. In all cases the best-fit absorb-
ing column density is consistent (within 2-σ) with the Galactic
absorption along the line of sight of IGR J11215–5952. This
value is significantly lower than the column density measured
with RXTE/PCA during the 2006 outburst (Smith et al., 2006b),
(11±3)×1022 cm−2, which is likely overestimated because it was
derived with RXTE/PCA in the energy range 2.5–15 keV.
To investigate spectral variations, we created the contour lev-
els for the column density vs. the photon index. The most inter-
esting example is shown in Fig. 3. They indicate that the photon
index showed significant variations, with the spectrum soften-
ing as the outburst progresses, confirming the 2006 observation
of the outburst tail; there is also evidence of an increasing ab-
sorbing column density. These findings are independent on our
choice of an absorbed power-law model.
4. Discussion and Conclusions
We have carried out the most complete monitoring campaign of
an outburst from a SFXT, thanks to the known periodicity of the
4 P. Romano et al.: The fifth outburst of IGR J11215–5952 observed by Swift
outburst activity from IGR J11215–5952 (Paper I). This is re-
markable, since the transient and unpredictable nature of the out-
bursts from all other SFXTs hampers a similar extensive study,
from the almost “quiescent” level up to the “flaring” activity. The
entire “outburst event” was monitored for 23 days. The source
was under the threshold of detectability in the early days of the
campaign, with a luminosity below 3.7×1033 erg s−1. On Feb 9
the source underwent a bright outburst up to ∼ 1.1×1036 erg s−1.
The bright part of the outburst (Feb 9; see Fig. 1b) is composed
of at least five flares, with variable peak flux, each with a du-
ration of ∼ 15 min–2 hours. This bright flaring activity lasted
about 1 day, then the source underwent a decline phase, not flat,
but composed of other equally short flares, one order of magni-
tude fainter. This decline phase lasted about 5 days, and then the
source faded to a much fainter level, almost below the thresh-
old of detectability. The whole outburst lasted about 15 d, after
which the source became fainter than 1.2 ×1033 erg s−1. Thus,
IGR J11215–5952 reached the typical luminosity of SFXTs dur-
ing outburst (around 1036 erg s−1), showing a dynamic range
larger than 103 and a hard X–ray spectrum, proper of this kind of
sources. The brightest part of the outburst lasted less than a day,
on 9th February, and would have been the only flaring activity
seen with less sensitive instruments. Indeed up to now, observa-
tions of outbursts from SFXTs have been mostly performed with
instruments on-board RXTE and/or INTEGRAL, which could
only catch the brightest flares, and missed the complete evo-
lution of the phenomenon, from the onset of the outburst, and
down again to the level of almost quiescence, which is expected
at a level around ∼ 1032 erg s−1 (possible magnetospheric emis-
sion plus the contribution from the soft X–ray emission from the
OB supergiant). Only during XMM-Newton and Chandra ob-
servations of IGR J17544-2619 (González-Riestra et al., 2004;
in’t Zand, 2005) outbursts were observed starting from the qui-
escent emission, but the “post-flare” phases could not be com-
pletely followed and thus the duration of the entire outburst
phase could not be measured.
For IGR J11215–5952 we could exceptionally observe the
whole phenomenon, which for the first time reveals that the
“short outbursts” (the “flares” lasting minutes or few hours), are
actually part of a much longer “outburst event” (lasting several
days), which we believe is triggered at the periastron passage in a
wide, highly eccentric orbit. Indeed, the IGR J11215–5952 out-
burst recurrence time (329 days) is remarkably stable and reveals
an underlying clock, which can be naturally associated with
the orbital motion in a non-circular orbit. The short flares most
prominent on Feb 9 are probably produced by the episodic accre-
tion of clumps from the massive wind (Owocki & Cohen, 2006),
or by an inhomogeneous accretion stream near periastron (sim-
ilar to what proposed to explain the periodic outbursts from the
eccentric X-ray pulsar GX 302-1, e.g. Leahy 1991). Thus, both
mechanisms originally proposed to explain the SFXTs outbursts,
seem to be at work in IGR J11215–5952, i.e., accretion at perias-
tron passage in a wide eccentric orbit (Negueruela et al., 2006),
and accretion from clumpy winds, (in’t Zand, 2005). Applying
a spherically symmetric homogeneous wind model to a B1 Ia
spectral type companion, with a mass of 39 M⊙, 42 solar radii,
and a wind mass loss of 3.67×10−6 M⊙ yrs
−1 (Vink et al., 2000),
the short outburst duration implies an eccentricity larger than
0.9. From the spectroscopy of the single flares there is evidence
for only minor variations in the local absorbing column density
(which would suggest the clear presence of clumps). This may
be partly due to the high column density along the line of sight
that absorbs most of the radiation below 1 keV, thus preventing
us from detecting comparatively small variations of the intrinsic
Table 2. Spectral fit results.
Spectruma NH Γ χ
2 (d.o.f.)/ L1−10 keV
(1022 cm−2) C-stat(%)c (erg s−1)
001 (2006) 0.88+0.96
−0.62 1.89
+1.07
−0.92 167.7 (65.8%) 2.35 × 10
001–005 2.28+2.21
−1.50 1.34
+1.11
−0.96 225.9 (65.8%) 4.32 × 10
006 1.04+0.25
−0.20 1.00
+0.16
−0.14 1.04 (83) 4.78
006 Flare 1 0.85+0.46
−0.32 0.94
+0.31
−0.28 0.66 (17) 8.68
006 Flare 2 1.11+0.79
−0.49 0.91
+0.42
−0.32 1.16 (25) 8.32
006 Flare 3 0.83+0.62
−0.42 0.82
+0.44
−0.40 0.93 (10) 11.1
006 Flare 4 0.88+0.38
−0.31 1.03
+0.32
−0.31 623.6 (39.0%) 4.86
006 Flare 5 2.02+1.01
−0.79 1.51
+0.58
−0.53 436.0 (54.6%) 2.75
007–008 1.75+1.03
−0.83 1.94
+0.64
−0.60 518.3 (66.0%) 2.22 × 10
009–012 1.04+0.47
−0.36 1.48
+0.39
−0.35 576.9 (53.6%) 8.82 × 10
007–012 1.86+0.68
−0.44 1.92
+0.43
−0.38 1.09 (18) 1.40 × 10
007–018 2.04+0.62
−0.50 2.08
+0.41
−0.37 1.19 (19) 5.96 × 10
a Last three digits of observation numbers, see Table 1, column 1.
b Luminosity in the 1–10 keV band in units of 1035 erg s−1 obtained
from the spectral fits.
c Cash statistics (C-stat) and percentage of Monte Carlo realizations
that had statistic < C-stat. We performed 104 simulations.
column density. However, XRT data show evidence of softening
of the spectrum in the long decay to quiescent state (thus con-
firming the 2006 observation of the outburst tail) and a possible
evidence of an NH growth connected with the same transition.
Acknowledgements. We thank the Swift team for making these observations
possible, in particular the duty scientists and science planners M. Chester, S.
Hunsberger, J. Kennea, C. Pagani and J. Racusin; we thank N. Gehrels for ap-
proving this ToO and D. Burrows for a winning observing strategy. We thank
S. Campana, P. D’Avanzo, A. Paizis, P. Persi, V.F. Polcaro, and S. Vercellone
for insightful discussions. This research has made use of NASA’s Astrophysics
Data System Bibliographic Services as well as the NASA/IPAC Extragalactic
Database (NED) which is operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics and Space
Administration. This work was supported by MIUR grant 2005-025417, and
contract ASI/INAF I/023/05/0. PR thanks INAF-IASFMi, where most of the
work was carried out, for their kind hospitality.
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Table 1. Observation log.
Sequence Start time (MJD) Start time (UT) End time (UT) Net Exposurea
(yyyy-mm-dd hh:mm:ss) (yyyy-mm-dd hh:mm:ss) (s)
00030384001 53814.7853 2006-03-20 18:50:47 2006-03-20 19:01:53 643
00030881001 54135.5060 2007-02-04 12:08:34 2007-02-04 23:38:58 2048
00030881002 54136.5092 2007-02-05 12:13:12 2007-02-05 23:44:57 1865
00030881003 54137.1798 2007-02-06 04:18:55 2007-02-06 17:28:17 1941
00030881004 54138.1244 2007-02-07 02:59:06 2007-02-07 17:13:56 1213
00030881005 54139.0604 2007-02-08 01:27:02 2007-02-08 16:03:57 1403
00030881006 54140.0021 2007-02-09 00:03:05 2007-02-09 23:59:57 4668
00030881007 54141.6747 2007-02-10 16:11:33 2007-02-11 00:16:56 3141
00030881008 54142.0696 2007-02-11 01:40:15 2007-02-12 00:10:11 4232
00030881009 54143.6078 2007-02-12 14:35:17 2007-02-12 19:38:57 3337
00030881010 54144.0085 2007-02-13 00:12:17 2007-02-13 16:30:58 3091
00030881011 54145.0182 2007-02-14 00:26:09 2007-02-14 21:25:56 4521
00030881012 54146.0142 2007-02-15 00:20:26 2007-02-15 09:58:57 4590
00030881013 54147.6084 2007-02-16 14:36:05 2007-02-16 19:44:56 5230
00030881014 54148.6848 2007-02-17 16:26:09 2007-02-17 21:23:57 4295
00030881015 54149.2812 2007-02-18 06:44:59 2007-02-18 12:01:58 4804
00030881016 54150.2187 2007-02-19 05:14:52 2007-02-19 12:02:56 4636
00030881017 54151.6337 2007-02-20 15:12:28 2007-02-20 20:19:56 4847
00030881018 54152.0412 2007-02-21 00:59:16 2007-02-21 17:11:57 5194
00030881019 54153.4431 2007-02-22 10:38:04 2007-02-22 15:39:58 3814
00030881021 54155.1683 2007-02-24 04:02:20 2007-02-24 13:46:57 2963
00030881023 54157.5108 2007-02-26 12:15:37 2007-02-26 18:48:58 786
a The exposure time is spread over several snapshots (single continuous pointings at the target) during each observation.
	Introduction
	Observations and Data Reduction
	Analysis and Results
	Discussion and Conclusions
