SUBARU HDS Observations of a Balmer-Dominated Shock in Tycho's Supernova
  Remnant
Draft version October 28, 2018
Preprint typeset using LATEX style emulateapj v. 04/21/05
SUBARU HDS OBSERVATIONS OF A BALMER-DOMINATED SHOCK IN TYCHO’S SUPERNOVA
REMNANT ∗
Jae-Joon Lee,
Bon-Chul Koo,
John Raymond,
Parviz Ghavamian,
Tae-Soo Pyo,
Akito Tajitsu,
Masahiko Hayashi
Draft version October 28, 2018
ABSTRACT
We present an Hα spectral observation of a Balmer-dominated shock on the eastern side of Tycho’s
supernova remnant using SUBARU Telescope. Utilizing the High Dispersion Spectrograph (HDS), we
measure the spatial variation of the line profile between preshock and postshock gas. Our observation
clearly shows a broadening and centroid shift of the narrow-component postshock Hα line relative to
the Hα emission from the preshock gas. The observation supports the existence of a thin precursor
where gas is heated and accelerated ahead of the shock. Furthermore, the spatial profile of the emission
ahead of the Balmer filament shows a gradual gradient in the Hα intensity and line width ahead of
the shock. We propose that this region (∼ 1016 cm) is likely to be the spatially resolved precursor.
The line width increases from ∼ 30 km s−1 up to ∼ 45 km s−1 and its central velocity shows a redshift
of ∼ 5 km s−1 across the shock front. The characteristics of the precursor are consistent with a cosmic
ray precursor, although a possibility of a fast neutral precursor is not ruled out.
Subject headings: ISM:supernova remnants – ISM: individual (Tycho, G120.1+1.4) – Shock Waves –
line: profiles
1. INTRODUCTION
Balmer-dominated filaments are the signature of
non-radiative shocks propagating into partially neu-
tral medium (Chevalier & Raymond 1978). The Hα
line profile is composed of two distinctive components
(narrow and broad) representing the velocity distri-
bution of preshock and postshock gases, respectively
(Chevalier et al. 1980). High resolution spectroscopic ob-
servations of several of these shocks have revealed that
the width of the narrow component is unusually large
(30 ∼ 50 km s−1) for ambient neutral hydrogen, and it
was proposed that the gas was heated in a precursor
thin enough (. 1017 cm) to avoid complete ionization
of hydrogen (see Ghavamian et al. 2001; Sollerman et al.
2003, and references therein).6 Two likely candidates are
cosmic-ray (CR) and fast neutral precursors (Smith et al.
1994; Hester et al. 1994; Ghavamian et al. 2001). Both
scenarios predict significant Doppler shifts of preshock
gas. No clear indication of such a shift of the Hα nar-
row component is reported (but see Lee et al. 2004). A
careful comparison of postshock and preshock line pro-
files with high spectral resolution is crucial for confirming
the existence of such a precursor.
The blast wave of Tycho’s SNR, the historical
∗BASED ON DATA COLLECTED AT SUBARU TELESCOPE,
WHICH IS OPERATED BY THE NATIONAL ASTRONOMI-
CAL OBSERVATORY OF JAPAN
1 Astronomy Program, Department of Physics and Astronomy,
Seoul National University, Seoul 151-742, Korea
2 jjlee@astro.snu.ac.kr
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138
4 Department of Physics and Astronomy, Johns Hopkins Univer-
sity, 3400 North Charles Street, Baltimore, MD 21218
5 Subaru Telescope, National Astronomical Observatory of
Japan, 650 North A’ohōkū Place, Hilo, HI 96720
6 We refer to this precursor explicitly as a “thin precursor”, to
avoid potential confusion with a photoionization precursor which
will be shortly introduced.
remnant of the 1572 supernova, has been known
to exhibit Balmer-dominated emission (van den Bergh
1971; Kamper & van den Bergh 1978). The re-
gion of the brightest Hα emission (Knot g from
Kamper & van den Bergh 1978) is located along the
north-eastern edge of the remnant. Faint, diffuse optical
emission extends ∼1 pc ahead of the Balmer-dominated
filaments (Ghavamian et al. 2000, G00 hereafter). This
feature has been identified as a photoionization precur-
sor (PIP) produced by photoionization of the preshock
gas by He II λ304Å emission from behind the blast
wave (G00). The estimated temperature of the PIP is
∼ 1.2 × 104 K, which is not high enough to explain the
observed width of the Balmer narrow component.
The existence of Hα emission from the PIP makes
Tycho an unique target for the study of nature of the
thin precursor, as we can investigate the change of the
line profile across this precursor by comparing the line
emission from the PIP (preshock) and that of Knot g
(postshock). In this Letter, we present high resolution
(Echelle) long slit Hα spectra of Tycho Knot g and its
PIP using the SUBARU High Dispersion Spectrograph
(Noguchi et al. 2002). Our observations reveal the line
broadening and the Doppler shift of the the narrow com-
ponent, providing strong evidence for the existence of a
cosmic ray or fast neutral precursor.
2. OBSERVATIONS AND RESULTS
The spectrosopic observation was carried out on 2004
October 1. Longslit Echelle spectroscopy of the Hα line
was performed using order-blocking filters (HDS stan-
dard setup “stdHa”). This gives a spectral coverage
of 6540 Å∼ 6690 Å over the 60′′ of slit length. The
slit was centered at α(2000), δ(2000) = (00h 25m 56.s5,
64◦ 09′ 28′′) with position-angle of 76◦ (measured E of
N), covering both Knot g and its PIP simultaneously
(Fig. 1(a)). A total of 3×30 minutes of source expo-
sure was obtained, and the same amount of exposure for
http://arxiv.org/abs/0704.1094v1
2 Lee et al.
nearby sky. The spectrum was binned by 2 along the
slit direction and 4 along the dispersion direction before
the readout. The pixel scale after the binning is 0.27′′
pixel−1 (9.3 × 1015 cm pixel−1 at a distance of 2.3 kpc
(Kamper & van den Bergh 1978)) and 0.08 Å respec-
tively. The slit width was 2′′, which gives velocity resolu-
tion of 17 km s−1. The seeing was 0.′′5±0.′′1. The process-
ing of the SUBARU data included a typical CCD prepro-
cessing (including overscan correction and flat fielding)
and two-dimensional spectral extraction. A wavelength
calibration solution is obtained from the spectrum of a
Th-Ar lamp. The source spectrum was sky-subtracted,
and normalized using the spectra of standard stars. The
uncertainty in the wavelength calibration is estimated to
be around 0.2 km s−1 at the wavelength of Hα.
In Fig. 1(b), we present the fully processed two dimen-
sional spectrum of Hα line. The bright patch at the bot-
tom with three distinct local emission peaks corresponds
to Knot g, and the faint emission extending to the top
of the image corresponds to the PIP. The average Hα
spectrum of Knot g and that of PIP are shown together
in Fig. 2. The velocity width of the Knot g narrow com-
ponent line is clearly larger than that of the PIP Hα line.
In addition, the velocity centroid of the Knot g narrow
component is slightly redshifted (5.5± 0.6 km s−1) rela-
tive to that of the PIP Hα line. And as clearly seen in
Fig. 2(b), the Knot g spectrum shows a very broad (∼
1,000 km s−1), faint Hα line. The small peak near 900
km s−1 is the [N II] λ6583.4 Å line from the PIP.
The spectrum of the PIP is well fitted by a single Gaus-
sian, and yields a FWHM of 33.8± 0.8 km s−1 and cen-
troid velocity of −35.8 ± 0.6 km s−1 (in LSR frame).7
The measured FWHM of [N II] λ6583.4 Å in the PIP
is ∼ 23 km s−1. If the broadening were purely thermal,
the widths of the lines whould be inversely proportional
to the square root of their atomic masses. The expected
width of the [N II] line in this case is 1/
14 = 0.267
of Hα. As this is significantly narrower than what is
observed, a significant amount of nonthermal broaden-
ing is suggested. If we simply assume that the observed
line widths are a convolution of thermal and nonther-
mal broadenings, the estimated thermal temperature is
∼ 13, 000 K, consistent with 12, 000 K of G00. It is also
possible that the large line width is due to a residual of
Galactic Hα emission.
An adequate fit to the Knot g spectrum requires three
Gaussian components. They have velocity widths of
45.3 ± 0.9 km s−1 (narrow), 108 ± 4 km s−1 (intermedi-
ate) and 931± 55 km s−1 (broad), with central velocities
of −30.3± 0.2, −25.8± 0.8 and 29± 18 km s−1, respec-
tively. This result confirms that the Hα narrow com-
ponent line of Knot g is redshifted and broadened rela-
tive to that of the PIP. The broad component (FWHM∼
1, 000 km s−1) should correspond to the previously re-
ported ∼ 2, 000 km s−1 component (Ghavamian et al.
2001). The relatively narrower width of our observation
is probably due to the insensitivity of our spectroscopic
configuration to the very broad line, although the pos-
sibility of temporal variation (e.g., by crossing a density
7 throughout this paper, we give line widths corrected for instru-
mental broadening and velocity in LSR frame.
jump) does exist. The abrupt density increase might be
possible if the shock is propagating into the edge out-
skirts of the dense cloud (Lee et al. 2004). The existence
of the intermediate width component has already been
reported by G00. It may be produced when slow pro-
tons picked up by the postshock magnetic field undergo
a secondary charge exchange. Alternatively, it might be
an artifact of the assumption of Gaussian distributions,
which would not necessarily be appropriate if the mo-
tions are non-thermal.
The characteristics of the narrow component Hα line
of Knot g are consistent with G00, except that the ve-
locity centroid of the line in our data is significantly dif-
ferent from theirs (vLSR = −30.3± 0.2 from our data vs.
−53.9 ± 1.3 km s−1 from G00). We have carefully as-
sessed our wavelength calibration, but couldn’t find any
significant source of error. The wavelengths of night sky
lines observed near Hα matched well with those of VLT
UVES night-sky emission line catalog (Hanuschik 2003)
within error of 0.02 Å (∼ 1 km s−1). The Hα spectrum of
blank sky is also consistent with nearby WHAM spectra
(Haffner et al. 2003). Furthermore, the central velocity
of [N II] λ6583.4 Å emission line from the PIP is con-
sistent with that of Hα. Although we believe that our
wavelength calibration solution is quite secure, the dis-
crepancy with the previous observation should be con-
firmed by an independent observation. For the rest of
this paper, we mainly concentrate on the line width and
the relative variation of central velocity.
3. LOCATION OF THE SHOCK FRONT AND DISCOVERY
OF A THIN (1016 CM) PRECURSOR
Fig. 3 shows the spatial variation of the Hα line profile
along the slit. In the top panel, we separately plot the
integrated intensities of the representative Hα of narrow
and broad components, together with their sum (refer
to the figure caption for the velocity range of each com-
ponent). In the middle and bottom panels, we plot the
central velocities and FWHMs at each slit position from
the single Gaussian fit to the full profile. In all three
panels, the x-axis represents the pixel offset from the
southwest end of the slit. Although three Gaussian com-
ponents are actually required to adequately fit the Hα
profile of Knot g, the non-uniqueness of the multiple-
component fitting can complicate the interpretation of
the different emission line components. Therefore, we
plot the fit result from a single Gaussian and describe
our interpretation of this fit in the region of Knot g as
following: First, the central velocities closely match those
of the narrow component. Fitting with multiple compo-
nents leaves the fitted line centroids unchanged to within
the errors. Second, the FWHMs basically trace the spa-
tial variation of the narrow component width, but the
presence of the intermediate and the broad components
contribute significantly to the width. When fit with three
Gaussian components, the large widths of the Hα nar-
row component in Knot g from the single profile models
(60 ∼ 80 km s−1) are reduced to 40 ∼ 50 km s−1.
Inspection of Fig. 3 reveals that there exists only one
location (pixel offset 27, marked as a dotted vertical line)
where the intensity and width of the Hα line exhibits an
abrupt jump. The jump is most noticeable for the broad
component, where the flux is virtually zero toward the di-
rection of ambient medium (i.e., toward positive pixel off-
SUBARU HDS Observation of Tycho 3
6559 6560 6561 6562 6563 6564
A(htgnelevaW
(a) (b)
Fig. 1.— (a) Hα image of Tycho Knot g with position of the
2′′ ×60′′ slit overlaid. (b) The fully-processed Hα 2-d spectrum
(only the western part of the slit is shown). The bright emission
knot at the bottom is Knot g.
sets). The central velocity of the narrow component also
shows rapid change around this location. The FWHM
also steeply increases behind this point, but this might
be largely due to the sudden appearance of the broad
component. The fact that the intensity of the broad
component, which is associated with the postshock gas,
shows a significant jump at this location suggests that it
corresponds to the location of the shock front.
The Hα intensity in the PIP region is nearly constant
along the slit, but shows an rapid rise just before the
shock front, i.e., from pixel offset 32 to 27 in Fig. 3. The
intensity of the Hα line in the PIP region is about 10%
of the observed peak value in Knot g, and it reaches
about half the peak very near the shock. The line width
also increases from 30 to 45 km s
within this region.
We propose that the steep increase of the Hα intensity
accompanied with line broadening corresponds to a thin
precursor where gas is heated and accelerated. Unlike
the line broadening, the observed velocity centroids shift
slightly behind the shock instead of the precursor region.
This does not necessarily indicate that the broadening
and the Doppler shift takes at different region as this
is likely due to geometrical projection effects. The Hα
intensity profile gives an e-folding thickness of 1.4 ± 0.4
pixel (after accounting for seeing), which corresponds to
(1.4± 0.4)× 1016 cm at a distance of 2.3 kpc.
The observed emission of Knot g shows a few local
peaks indicating possible substructure, e.g., a collection
of shock tangencies seen in projection along the line of
sight. This leads to a possibility that the ‘precursor’
is simply the results of geometric projection of fainter
Balmer-dominated filament lying just ahead of Knot g.
If we assume that this filament has a line profile simi-
lar to that of Knot g, i.e., have a same broad-to-narrow
ratio, then a detectable amount of broad component is
expected. The flux profile of broad component plotted
in Fig. 3 would show similar gradual increase in the pre-
cursor region, which is not seen. Also, no hint of a broad
component is found in a summed spectra of precursor
region, which is expected to show up. Examining the
archival Chandra observation of Tycho’s SNR (the anal-
ysis is presented by Warren et al. 2005), we did not
find evidence of X-ray emission extending ahead of Knot
g. Therefore, there is no strong supporting evidence in
favor of a projection, although we cannot rule out the
possibility.
4. NATURE OF THE THIN PRECURSOR
As the shock is nearly tangential to our line of sight,
the actual amount of bulk gas acceleration could be much
-200 -100 0 100
Knot g : observed
Knot g : fit (sum)
Knot g : fit (individual)
PIP : observed
-2000 -1500 -1000 -500 0 500 1000
Velocity (km/s)
[NII]
Knot g : observed
Knot g : fit (broad)
PIP : observed
Fig. 2.— (a) Hα spectrum of Knot g and the PIP. The spectrum
of Knot g is fitted with three Gaussian components, and two nar-
rowest components are displayed (thin dashed) together with their
sum (thin solid). (b) Same as (a) except the full observed velocity
range is presented and the spectra are binned. The Knot g broad
component from the above fit is displayed as a thin solid line.
larger than the observed Doppler shift. The shock an-
gle can be inferred from the radial velocity shift of the
broad component of Knot g relative to the narrow com-
ponent (Chevalier et al. 1980), which is measured to be
∼ 60 km s−1. As our observation could be insensitive to
this broad component, using this value is rather conser-
vative. On the other hand, Ghavamian et al. (2001) re-
ported a redshift of ∼ 130 km s−1, but their field of view
is different from ours. We take these two values as limits
and give shock angle of 2◦−5◦ assuming a shock velocity
of 2,000 km s
. We consider 5 km s
to be the repre-
sentative redshift of the narrow component compared to
unperturbed medium, which gives actual acceleration of
60 ∼ 130 km s−1.
The line width of the narrow component at the shock
front (∼ 45 km s−1) corresponds to a neutral hydrogen
temperature of 40, 000 K, if the line broadening is purely
thermal, or lower, if there is a significant non-thermal
broadening such as a wave motion. Neutral hydrogen
atoms and protons may have similar velocity distribu-
tions due to their charge exchange interactions. But that
of electrons can be different, which would depend on the
heating mechanism in the precursor. In the following,
we estimate the electron temperature in the thin precur-
sor (TP) from the observed intensity increase of factor
5. Since the observed spectrum is an integrated emis-
sion along the line of sight where a significant contribu-
tion from PIP region is expected, the actual emissivity
increase within the precursor should be much greater.
We assume that regions of the PIP and the TP are
represented by two concentric shells with thicknesses of
δPIP ≃ 1016 cm and δTP ≃ 1 pc (G00), respectively, and
that both have an inner diameter of the Tycho (5.4 pc).
Then the ratio of path length through each shell along
the tangential direction of the shock front is ≃ 18. This
implies that the emissivity increase in the TP could be
as large as a factor of 90. This value should be regarded
4 Lee et al.
total
narrow
broad
0 10 20 30 40 50 60
pixel offset
Fig. 3.— (top) Integrated fluxes along the slit position of
Hα spectrum for a given velocity range. The thin solid line
shows emission in the range −60 < vLSR < 10 km s
−1, rep-
resentative of Hα narrow component, while the thick solid line
shows emission in the range −400 < vLSR < −120 km s
−1 and
+100 < vLSR < +350 km s
−1, for the broad component. The
thick dashed line shows the summed intensity for both components.
(−400 < vLSR < +350 km s
−1). Middle and bottom panels: cen-
tral velocities and FWHMs from fits with a single Gaussian. The
proposed location of shock front is marked as a vertical dotted line.
as an upper limit as it is likely that the filament is nearly
flat and tangent to the line of sight. The collisional ex-
citation rate of Hα at Te ∼ 40, 000K is 2, 000 times
greater than that of 12, 000K which is the temperature
of the PIP region (G00). This value greatly exceeds the
emissivity increase of 90, and implies a few possibilities.
The electron temperature can be less than 40, 000K, ei-
ther if observed Hα line width has significant nonthermal
broadening, or if Te is intrinsically less than Tp. On the
other hand, the estimated emissivity increase might be
explained if the emissivity increase due to high Te is sup-
pressed by ionization of neutrals. Since the existence of
Blamer-dominated filaments requires a significant frac-
tion of neutral hydrogen atoms to survive within precur-
sor, this possibility is less favored.
The two likely candidates for this precursor are fast
neutral and CR precursors. The momentum and en-
ergy carried by upstreaming fast neutrals can be large
enough to explain the observed heating and accelera-
tion in the precursor, but the available model calcula-
tions do not predict significant heating by these neutrals
(Lim & Raga 1996; Korreck 2005). Furthermore, It is
difficult to reproduce the small range of FWHMs ob-
served for narrow component Hα lines from shocks of dif-
ferent velocities (Smith et al. 1994; Hester et al. 1994).
The observed characteristics of the precursor are consis-
tent with a CR precursor. CR acceleration in the shock
does require a precursor (e.g., Blandford & Eichler 1987)
and there has been growing evidence of CR acceleration
in young SNRs including Tycho itself (e.g., Warren et al.
2005). Significant heating and acceleration are expected
to happen in the CR precursor (Blandford & Eichler
1987). Interaction of CR particles in the upstream gen-
erates Alfv́en waves and significant amplification of mag-
netic field has been suggested (Bell & Lucek 2001). Al-
though a measurement of the preshock magnetic field
is hardly available, a high value (40µG) of preshock
magnetic field is suggested for Tycho (Völk et al. 2002),
which may explain the line width of the narrow compo-
nent.
To conclude, our SUBARU observation clearly has
shown the line broadening and the Doppler shift between
the preshock gas and postshock gas. This strongly sug-
gests the existence of the thin precursor. Furthermore,
the precursor itself is likely to be resolved. Given the
lack of observational constraints on CR ion acceleration
in SNR shocks, the tentatively measured width (∼ 1016
cm) of the thin precursor whose primary candidate is the
CR precursor is promising. Follow-up observation, such
as a high resolution imaging with HST, would clearly
resolve the structures of the precursor.
We thank to the anonymous referee for valuable com-
ments. This work was supported by the Korea Research
Foundation (grant No. R14-2002-058-01003-0). JJL has
been supported in part by the BK 21 program.
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