J. Rodriguez 1,2 - J. A. Tomsick3 - L. Foschini4,2 - R. Walter2 - A. Goldwurm1 - S. Corbel5,1 - P. Kaaret6
1 - CEA Saclay, DSM/DAPNIA/SAp (CNRS FRE 2591), 91191 Gif-sur-Yvette Cedex, France
2 - Integral Science Data Center, Chemin d'Ecogia, 16, 1290 Versoix,
Switzerland
3 - Center for Astrophysics and Space Sciences, Code 0424, University of California at San Diego, La Jolla, CA 92093, USA
4 - IASF/CNR, sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy
5 - Université Paris VII, Fédération APC, 2 place Jussieu, 75251 Paris Cedex 05, France
6 - Harvard-Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, USA
Received 7 April 2003 / Accepted 3 July 2003
Abstract
The hard X-ray sensitivity and arcminute position
accuracy of the
recently launched International Gamma-Ray Laboratory (INTEGRAL) has lead
to the (re-)discovery of a class of heavily absorbed hard X-ray sources
lying in the Galactic plane.
We report on the analysis of an XMM observation of such a source
IGR J16320-4751 = AX J1631.9-4752. Our analysis allowed us to obtain
the most accurate X-ray position to date (Rodriguez et al. 2003), and
to identify a likely infrared counterpart
(Tomsick et al. 2003). We present the detailed analysis of the IGR J16320-4751
XMM spectra. The PN spectrum
can be well represented by a single powerlaw or a comptonized spectrum
with a high equivalent
absorption column density of
cm-2.
The current analysis
and the comparison with the properties of other sources favor the
possibility that the source is a Galactic X-Ray Binary (XRB).
The identification of two candidate IR counterparts is in good
agreement with this identification. The hard
spectrum previously seen with ASCA, and the brightness of the
candidate counterparts indicate
that IGR J16320-4751 is most probably a highly absorbed High Mass X-ray Binary,
hosting a neutron star.
Key words: accretion, accretion disk - stars: individual: IGR J16320-4751 - X-rays: binaries - X-rays: general
The International Gamma-Ray Laboratory (INTEGRAL) has been launched on
October 17, 2002. Since then, the high sensitivity and position accuracy
of the IBIS imager has allowed for detections
and determinations of arcminute positions for faint hard X-ray sources
(e.g. IGR J16318-4848, Courvoisier et al. 2003; IGR J16358-4726,
Revnivtsev et al. 2003, in addition to IGR J16320-4751).
It is interesting to note that these sources belong
to a class of highly absorbed low luminosity X-ray sources, which renders
their detection in the soft X-rays (10 keV) difficult.
In fact, some of them
were missed with the All Sky Monitors of past X-ray missions, especially
those sensitive in the
1-10 keV spectral range. As a result, these sources have remained
poorly studied until the advent of INTEGRAL, and there may be many more than
previously realized. In that view the
IBIS/ISGRI detector on board INTEGRAL appears perfectly suited since it
works in a spectral
range (
15 keV) not affected by absorption. Once such a source is
detected,
given the good position accuracy of the IBIS/ISGRI detector, it is possible
to use
highly sensitive soft (1-10 keV) X-ray telescopes, such as XMM-Newton or
CHANDRA,
to 1) obtain a more precise position and allow for counterpart search,
and 2) obtain a
soft X-ray spectrum and try to identify the type of the source.
Such studies should then
allow for a better understanding of the nature of these highly
absorbed sources and the physics underlying the emission/absorption processes
(e.g. Revnivtsev et al. 2003b).
IGR J16320-4751 was detected on Feb. 1.4 UT (Tomsick et al. 2003a), as
a hard X-ray
source with the IBIS/ISGRI detector (Lebrun et al. 2001) on board INTEGRAL
at RA
32
0,
51
(
2
), during an
observation of the Galactic
Black Hole Candidate (BHC) 4U 1630-47 (PI Tomsick). The source was observed
to vary
significantly in the 15-40 keV energy range on time-scale of
1000 s,
and was detected in some occasions above 60 keV (Tomsick et al. 2003a).
This source has a position consistent with that of AX J1631.9-4752, which
was observed with ASCA in 1994, and 1997. The ASCA spectrum was fitted with
a powerlaw with a hard photon index (
,
Sugizaki et al. 2001),
which
may suggest that the source belongs to the High Mass X-ray Binary (HMXB)
class. Analysis
of archival BeppoSAX-WFC data revealed that this source was
persistent for at
least 8 years (in't Zand et al. 2003). Their 2-28 keV spectral
analysis shows a quite
different result, since they obtain a soft photon index (
)
for
the powerlaw.
This evolution and the persistence of the source may indicate the presence
of an absorbed XRB. Two possible infrared counterparts have been identified
(Tomsick et al. 2003b).
We report here the detailed spectral analysis of an XMM public Target of Opportunity (ToO) observation of IGR J16320-4751, and compare it to the former observations of the source. In Sect. 2 we provide details about the XMM observation and data reduction methods that were employed for the analysis. We describe the spectral analysis in Sect. 3, and present results on the time variability of the source in Sect. 4. The infrared counterparts will be discussed in Sect. 5, and the results of our analysis will be discussed in the last section of this paper.
IGR J16320-4751 was observed with XMM-Newton on March 4, during a public ToO
pointing on the INTEGRAL position that started around 21 h UTC.
The data were processed using the Science Analysis Software v.5.4.1.
Images were then obtained both from the EPIC
MOS (Turner et al. 2001) and EPIC PN (Strüder et al. 2001) cameras.
The EPIC-PN was operating in imaging mode with large window and
a medium filter, the
EPIC MOS2 in imaging mode with a full window and a medium filter. The
EPIC-MOS1 was operating in timing mode (medium filter).
During the processing, the data were screened by rejecting periods of
high background, and by filtering the bad pixels.
Correction for vignetting (Lumb 2002) has not been applied, because
the source is close to
the center of the field of view (<
).
One source was detected by the MOS2 within the 1 arcmin INTEGRAL error circle.
The source position (obtained following the procedures given in the
Introduction to XMM-Newton Data Analysis
)
is
32
01.9
and
52
29
(
4
at the
90% confidence level, Rodriguez et al. 2003).
It should be noted that
this position is also consistent with the ASCA (at 0.4
from the
XMM position with an uncertainty of 1
), and BeppoSAX
(at 0.7
from the XMM position, uncertainty of 1.7
)
positions of AX J1631.9-4752.
Due to soft proton flares during this observation, the MOS2 data
are not usable for spectral and timing analysis,
and only 4.9 ks out of a total of 25 ks are exploitable for
scientific (spectral and timing) studies with the PN camera.
The spectrum and light curve for the source were extracted from a
circular region centered on
the source with a radius of 45
(which
gives an encircled energy
fraction of about 85%). The background spectrum and light
curve were extracted
from a source free region, with a radius of 2 arcmin.
The response matrices were generated with the SAS package
(arfgen, rmfgen). The spectrum was grouped with a minimum of 25 counts
per channels and was fitted with the XSPEC v11.2 package.
![]() |
Figure 1: EPIC-PN spectrum of IGR J16320-4751 and residuals to the absorbed powerlaw model. |
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Table 1:
Spectral fit parameters. The given fluxes are corrected for
an encircled energy fraction of 85%. The errors are the 1
confidence level.
The PN spectrum was fitted with different models.
In a first run, only single component models were tested.
An absorbed powerlaw gives a good representation of the spectrum,
with a reduced
of 0.81 (Rodriguez et al. 2003 and Fig. 1).
An absorbed blackbody or disc blackbody give acceptable fits,
with a reduced
of 0.81 (bbody) and 0.83 (diskbb), but the temperature
returned from the fits (kT=2.1 keV for the bbody and 3.8 keV for the diskbb)
is a bit higher (especially for the diskbb component) than what is observed in general
for accreting compact objects, even during the
high luminosity states where a thermal component contributes strongly
to the spectrum (see Tanaka & Shibazaki 1996 for a review).
Furthermore, the detection of the source at higher energies (
15 keV,
Tomsick et al. 2003a) indicates the need for an additional component to account
for this hard part. A comptonized spectrum
(comptt, Titarchuk 1994) gives a good representation of the spectrum,
with a reduced
of 0.83. The spectral parameters are not well
constrained, however, if they are all left free. Fixing the electron
temperature to 10 keV (which is an reasonable value, Barret 2001),
leads to a reduced
of 0.84, with spectral parameters
compatible with what is commonly observed in accreting neutron stars
(Table 1 and Barret 2001). Since both models are strongly correlated to
,
the 68% and 90% confidence intervals are shown in Fig. 2,
for the photon index and the absorption column density.
We have re-analysed the publicly available ASCA data, and fitted the SIS and GIS energy
spectra simultaneously in XSPEC, with a simple model of an absorbed powerlaw.
Our best result gives an equivalent absorption column density of
cm-2,
and a photon index of
(errors at 1
),
somewhat softer than that obtained by Sugizaki et al (2001). The error contour
plot is shown in Fig. 2, allowing for a direct comparison with
that of the XMM observation. The BeppoSAX spectrum was fitted with an
absorbed powerlaw of photon
index 2.5, and
cm-2 (in't Zand et al. 2003).
![]() |
Figure 2:
Left: error contours for the column density (![]() ![]() ![]() ![]() |
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The PN light curve is displayed in Fig. 3. The source
shows variability on a time scale of 50-100 s. A slow decrease
and a period of relatively quiet emission is first observed until
2000 s.
Then, the source undergoes two flares: the first starts at 2000 s after time 0
(begining of the good time intervals),
reaches its maximum about 300 s later. The flux increases by
a factor of 2.3 in the mean time. This flare lasts for
900 s. The second flare starts around 3050-3100 s,
and reaches the maximum around 3450 s, the flux increases by a
factor of
3.3, during this time.
A third flare may start around 4500 s, but our observation stops soon after,
and does not allow us to follow it.
We produced light curves in two different energy bands (2-5 keV
and 5-12 keV, Fig. 3), and the hardness ratio between
these two. The flare episodes occur in the two ranges in a similar
manner (Fig. 3). With the time binning of 200 s, the
flux increases by a factor of 4
between 2 and 5 keV, and
3 between 5 and 12 keV during a time
interval of 400 s
(first flare), and by a factor of
3 between 2 and 5 keV, and
2 between 5 and 12 keV
during 400 s (second flare)
This similarity indicates that the flares
are related to broadband flux increase rather than
variations of the absorption (since the hard band would be much less
affected in this case). The hardness ratio does not show significant
changes between the low flux periods and the flares. The same behavior is
observed if
the light curves are produced in other energy bands (e.g. 2-3 keV and
2-4 keV).
We searched for pulsations and quasi-periodic oscillations in the
power spectra of the source. However, the low counting statistics and
short exposure time does not enable us to obtain strong constraints.
Indeed taking into account a net average counting rate of
0.22 cts/s for the source, and 0.16 cts/s for the background, during a
4.8 ks observation, leads to a 3
upper
limit on the amplitude of a periodic signal between
5 mHz and 10 Hz
(where the power spectrum is dominated by Poissonian noise)
of 12.25%. The 3
upper limit for QPOs is higher than this
value in the given frequency range (due to the non-zero width of the QPO).
![]() |
Figure 3: Upper panel: 0.5-12 keV PN light curve of IGR J16320-4751 measured after background subtraction. The time sampling is 50 s, which shows the fast rise during the flare episodes. The lower panels represent a zoom on the region of the flares (starting around relative time 1500 s). The 2-5 keV, 5-12 keV and hardness ratio between 2-5 and 5-12 keV are shown. The time sampling is 200 s. |
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From the improved position obtained with XMM-Newton (Rodriguez et al.
2003), two candidate
infrared counterparts have been identified (Tomsick et al. 2003b,
Fig. 4).
The first one is located at
32
01
75,
= -47
52
28
9 (1
uncertainty) with magnitudes
,
,
J > 14.08 (97% confidence).
The second source (
,
,
)
is on the southeast
edge of the X-ray error circle. From the Palomar Observatory Sky Survey (POSS)
epoch I and II, we can derive R>21 for the first source,
and R=14.6 (
0.3, 1
error POSS epoch I, period 1949-1965) and R=15.4
(
0.3,
error POSS epoch II, period 1985-2000), for the second one (note
that the evidence for variability is only marginal).
To estimate the equivalent absorption column density along the line of
sight we used a web-based tool that uses data from Dickey & Lockman (1990).
For the position of IGR J16320-4751, the average value is
cm-2. Then,
assuming
(Predehl & Schmitt 1995), we derived
AV=11.7. With this value, we can calculate the
dereddened fluxes in the three bands, and compare them to tabulated objects.
The second source appears to be a star with a peak in the J band. The colors (J-H)=0.9 and
(H-K)=0.4 are typical for a M-type star with a temperature around 3000 K.
![]() |
Figure 4:
2-MASS K-band
![]() ![]() |
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With this visual extinction, the first source, the faintest and most probable counterpart,
presents some very interesting features. The dereddened flux increases toward radio wavelenghts
(from J to K band)
and the colors (J-H)>3.1 and (H-K)=2.0 suggest an infrared excess that is likely to be
due to circumstellar matter (probably hot plasma or warm dust). However, the absorption
column density along the line of sight may be different from this value. The
returned from
XMM spectral fits leads to an extinction of AV=106, giving a dereddened
magnitude close
to 0 for both objects. In addition, some molecular clouds and inhomogeneities in the ISM can
make the AV increase. If, for example, we take a value of
AV=30 (i.e.
cm-2), the dereddened magnitudes
would suggest a K-giant or
supergiant, rendering the object more usual. Furthermore, the wavelength dependence of the
interstellar extinction is poorly constrained when studying sources close to the Galactic ridge. We
thus cannot exclude other types for the companion star. We remark for example that the color, and
magnitude of the second candidate counterpart could indicate a high mass star (O or B spectral type).
From their analysis of ASCA data, Sugizaki et al. (2001) suggested that
AX J1631.9-4752 was an HMXB with a pulsar, given the flat spectrum they
obtained. Our best model, a powerlaw or a comptt spectrum, is also
consistent with the source being a Galactic XRB.
A comptonized spectrum would not be surprising for such an object, but alone
it is not a sufficient
argument since extragalactic sources have similar X-ray spectra. For
a distance range of 5-15 kpc (the latter giving an upper limit on the
distance for the object to be Galactic), the powerlaw model leads to
unabsorbed 2-10 keV isotropic luminosities of
0.5-
erg/s, which may be compatible with the observe
luminosity of either a
neutron star or a black hole in a low hard state. The spectral parameters we
obtain are also compatible with those commonly observed for this kind of
objects
(e.g. Tanaka & Shibazaki 1996). The persistence of the source over 8 years
(in't Zand et al. 2003), and the evolution of the spectral parameters
resembles the spectral transitions seen in XRBs.
It is interesting to note the
similarities of IGR J16320-4751 with the
persistent X-ray source 1E 1743.1-2843 whose primary type is still a
matter of debate. Both sources are persistent, heavily absorbed, and
undergo XRB-like spectral transitions between hard states
(Porquet et al. 2003), and softer ones
(Cremonesi et al. 1999). Although for 1E 1743.1-2843 the neutron star
hypothesis could
not be ruled out, Porquet et al. (2003) have suggested that this object
might be a black hole in
a hard state. In the case of IGR J16320-4751, the BeppoSAX and XMM observations alone
would rather indicate a persistent black hole undergoing spectral
transitions between
hard states and softer ones. However, the low value of the photon
index during the ASCA observation,
and its large variations between ASCA (Sugizaki et al. 2001),
BeppoSAX (in't Zand et al. 2003), and XMM (current study) observations
are rather
unusual for a black hole. On the other hand, such very hard photon indices
and large
variations (from 0 to 2.4) have already been observed in the neutron star
system
Sco X-1 (D'Amico et al. 2001), but also in some other sources (Sugizaki et al.
2001).
In addition, the powerlaw model gives extrapolated 1-20 keV
luminosities (between 5-15 kpc), of
erg/s, and
extrapolated 20-200 keV
luminosities of
erg/s (the comptt model leads to
slightly lower values). With these values, the
source would lie in the neutron star box in Fig. 1 of Barret et al. (1996).
The relatively good fit obtained with the
comptt model is compatible with a Comptonization of soft photons on hot
electrons surrounding
the compact object. However, the lack of data above 12 eV,
does not allow us to obtain a good constraint on the cut-off energy.
The detection of hard emission with INTEGRAL (Tomsick et al. 2003a)
appears in good agreement with a comptonized spectrum. The hard photon index obtained
during the ASCA observation, however, is not easy to understand in the framework of thermal
Comptonization.
The identification of the infrared counterpart remains
difficult given the proximity of the two candidates. The faintest one
(number 1 in Fig. 4), however, is more likely related to
the X-ray source.
This candidate could either favor a reprocessing of high energy photons
by a cloud or some dust, or the emission
from a supergiant or K-giant star depending on the visual extinction on the
line of sight.
Both hypothesis are in good agreement with the high absorption obtained
from the X-ray
spectrum, and the spectral parameters obtained from our fits, which are
similar to those
observed in the case of HMXB (e.g. GX 301-2, Saraswat et al. 1996, 4U1700-37,
Boroson et al. 2003),
although we do not detect lines in IGR J16320-4751 with a 3
upper
limit on the equivalent width of a narrow 6.4 keV Iron (emission) line
of 112 eV.
The position of IGR J16320-4751 in the Galactic plane, and along a spiral
arm (the Norma arm), a region of star formation, where a number of young (massive)
stars can be found, appear in good agreement with the system being an HMXB.
It is interesting to note the change of equivalent absorption density between
the different observations, (from
cm-2 to
cm-2). This strongly favors an absorption
intrinsic to the source,
similar to what observed in heavily absorbed sources (e.g. Revnivtsev et al. 2003
and reference therein).
In the case of a Galactic XRB, the absorption could be due to the
accretion flow, and the changes could be associated with variations of the accretion rate.
Although the source appears more likely to be a Galactic XRB (given its similarities with known neutron stars or black holes XRB) we can not exclude totally an extragalactic source, e.g. an ULX seen through the Galactic plane. Assuming a typical luminosity of 1040 erg/s (Foschini et al. 2002), the simple power law model would lead to a distance to the source of about 2.2 Mpc. Although compatible with an extragalactic origin, this hypothesis appears not to be favored given such a short distance. Longer X-ray observations and more multiwavelength studies (e.g. infrared spectra), should allow for more precise answers to all the above mentioned points. Note that a better X-ray position would be useful to select a unique IR counterpart, allowing for a better understanding of the nature of the source.
Acknowledgements
J.R. thanks D. Barret, S. Chaty, Y. Fuchs, A. Paizis, and M. Revnivtsev, for useful discussions and careful reading of the manuscript. The authors thanks the anonymous referee for useful comments which allowed to improve the quality of the paper. J.R. acknowledges financial support from the French spatial agency (CNES). J.A.T. acknowledges partial support from NASA grant NAG5-12703. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the NASA and the National Science Foundation. This reasearch has made use of The Digitized Sky Surveys that were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166.