139 research outputs found
Near-Infrared Counterparts to Chandra X-ray Sources toward the Galactic Center. I. Statistics and a Catalog of Candidates
We present a catalog of 5184 candidate infrared counterparts to X-ray sources
detected towards the Galactic center. The X-ray sample contains 9017 point
sources detected in this region by the Chandra X-ray Observatory, including
data from a recent deep survey of the central 2 x 0.8 deg of the Galactic
plane. A total of 6760 of these sources have hard X-ray colors, and the
majority of them lie near the Galactic center, while most of the remaining 2257
soft X-ray sources lie in the foreground. We cross-correlated the X-ray source
positions with the 2MASS and SIRIUS near-infrared catalogs, which collectively
contain stars with a 10-sigma limiting flux of K_s<=15.6 mag. In order to
distinguish absorbed infrared sources near the Galactic center from those in
the foreground, we defined red and blue sources as those which have H-K_s>=0.9
and <=0.9 mag, respectively. We find that 5.8(1.5)% of the hard X-ray sources
have real infrared counterparts, of which 228(99) are red and 166(27) are blue.
The red counterparts are probably comprised of WR/O stars, HMXBs, and
symbiotics near the Galactic center. We also find that 39.4(1.0)% of the soft
X-ray sources have blue infrared counterparts; most of these are probably
coronally active dwarfs in the foreground. There is a noteworthy collection of
~20 red counterparts to hard X-ray sources near the Sagittarius-B H II region,
which are probably massive binaries that have formed within the last several
Myr. For each of the infrared matches to X-ray sources in our catalog we
derived the probability that the association is real, based on the results of
the cross-correlation analysis. The catalog will serve spectroscopic surveys to
identify infrared counterparts to X-ray sources near the Galactic center.Comment: Submitted to ApJ January 16, 2009; accepted July 21, 2009; 30 pages,
6 figure
Interstellar Extinction Law toward the Galactic Center II: V, J, H, and Ks Bands
We have determined the ratios of total to selective extinction directly from
observations in the optical V band and near-infrared J band toward the Galactic
center. The OGLE (Optical Gravitational Lensing Experiment) Galactic bulge
fields have been observed with the SIRIUS camera on the IRSF telescope, and we
obtain A(V)/E(V-J)=1.251+-0.014 and A(J)/E(V-J)=0.225+-0.007. From these
ratios, we have derived A(J)/A(V) = 0.188+-0.005; if we combine A(J)/A(V) with
the near-infrared extinction ratios obtained by Nishiyama et al. for more
reddened fields near the Galactic center, we get A(V) : A(J) : A(H) : A(Ks) = 1
: 0.188 : 0.108 : 0.062, which implies steeply declining extinction toward the
longer wavelengths. In particular, it is striking that the Ks band extinction
is \approx 1/16 of the visual extinction A(V) much smaller than one tenth of
A(V) so far employed.Comment: 8 pages, 7 figures, Accepted for publication in Ap
A lack of classical Cepheids in the inner part of the Galactic disk
Recent large-scale infrared surveys have been revealing stellar populations
in the inner Galaxy seen through strong interstellar extinction in the disk. In
particular, classical Cepheids with their period-luminosity and period-age
relations are useful tracers of Galactic structure and evolution. Interesting
groups of Cepheids reported recently include four Cepheids in the Nuclear
Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga
et al. and those spread across the inner part of the disk reported by Dekany
and collaborators. We here report our discovery of nearly thirty classical
Cepheids towards the bulge region, some of which are common with Dekany et al.,
and discuss the large impact of the reddening correction on distance estimates
for these objects. Assuming that the four Cepheids in the NSD are located at
the distance of the Galactic Centre and that the near-infrared extinction law,
i.e. wavelength dependency of the interstellar extinction, is not
systematically different between the NSD and other bulge lines-of-sight, most
of the other Cepheids presented here are located significantly further than the
Galactic Centre. This suggests a lack of Cepheids in the inner 2.5 kpc region
of the Galactic disk except the NSD. Recent radio observations show a similar
distribution of star-forming regions.Comment: 8 pages, 4 figures, accepted for publication in MNRA
Cepheids and other short-period variables near the Galactic Centre
We report the result of our near-infrared survey of short-period variable
stars (P<60d) in a field-of-view of 20'x30' towards the Galactic Centre.
Forty-five variables are discovered and we classify the variables based on
their light curve shapes and other evidence. In addition to 3 classical
Cepheids reported previously, we find 16 type II Cepheids, 24 eclipsing
binaries, one pulsating star with P=0.265d (RR Lyr or delta Sct) and one
Cepheid-like variable whose nature is uncertain. Eclipsing binaries are
separated into the foreground objects and those significantly obscured by
interstellar extinction. One of the reddened binaries contains an O-type
supergiant and its light curve indicates an eccentric orbit. We discuss the
nature and distribution of type II Cepheids as well as the distance to the
Galactic Centre based on these Cepheids and other distance indicators. The
estimates of R0(GC) we obtained based on photometric data agree with previous
results obtained with kinematics of objects around the GC. Furthermore, our
result gives a support to the reddening law obtained by Nishiyama and
collaborators, A(Ks)/E(H-Ks)=1.44, because a different reddening law would
result in a rather different distance estimate.Comment: 13 pages, 10 figures, 7tables, accepted for publication in MNRA
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