60 research outputs found
Determining X-Ray Source Intensity and Confidence Bounds in Crowded Fields
We present a rigorous description of the general problem of aperture
photometry in high energy astrophysics photon-count images, in which the
statistical noise model is Poisson, not Gaussian. We compute the full posterior
probability density function for the expected source intensity for various
cases of interest, including the important cases in which both source and
background apertures contain contributions from the source, and when multiple
source apertures partially overlap. A Bayesian approach offers the advantages
that it allows one to (a) include explicit prior information on source
intensities, (b) propagate posterior distributions as priors for future
observations, and (c) use Poisson likelihoods, making the treatment valid in
the low counts regime. Elements of this approach have been implemented in the
Chandra Source Catalog.Comment: 32 pages, 12 figures. Fixed typos and incorrect statement at end of
4.2.3. Results unchange
Dwarf Galaxies of the Local Group
XMM-Newton and Chandra have ushered in a new era for the study of dwarf
galaxies in the Local Group. We provide an overview of the opportunities,
challenges, and some early results. The large number of background sources
relative to galaxy sources is a major theme. Despite this challenge, the
identification of counterparts has been possible, providing hints that the same
mechanisms producing X-ray sources in larger galaxies are active in dwarf
galaxies. A supersoft X-ray source within 2" of the supermassive black hole in
M32 may be a remnant of the tidal disruption of a giant, although other
explanations cannot be ruled out.Comment: 6 pages, 4 figures, proceedings for 2004 IAU symposiu
Chandra and Hubble Study of a New Transient X-ray Source in M31
We present X-ray and optical observations of a new transient X-ray source in
M31 first detected 23-May-2004 at R.A.=00:43:09.940 +/- 0.65'',
Dec.=41:23:32.49 +/- 0.66''. The X-ray lightcurve shows two peaks separated by
several months, reminiscent of many Galactic X-ray novae. The location and
X-ray spectrum of the source suggest it is a low mass X-ray binary (LMXB).
Follow-up HST ACS observations of the location both during and after the
outburst provide a high-confidence detection of variability for one star within
the X-ray position error ellipse. This star has B ~ 1 mag, and there is
only a ~1% chance of finding such a variable in the error ellipse. We consider
this star a good candidate for the optical counterpart of the X-ray source. The
luminosity of this candidate provides a prediction for the orbital period of
the system of 2.3 days.Comment: 17 pages, 3 figures, 4 tables, accepted for publication in Ap
Supersoft X-ray Sources in M31: I. A Chandra Survey and an Extension to Quasisoft Sources
We report on very soft X-ray sources (VSSs) in M31. In a survey which was
most sensitive to soft sources in four 8'x8' regions covered by Chandra's
ACIS-S S3 CCD, we find 33 VSSs that appear to belong to M31. Fifteen VSSs have
spectral characteristics mirroring the supersoft X-ray sources studied in the
Magellanic Cloud and Milky Way ( eV); we therefore call
these ``classical'' supersoft sources, or simply supersoft sources (SSSs).
Eighteen VSSs may either have small (< 10%) hard components, or slightly higher
effective temperatures (but still < 350 eV). We refer to these VSSs as
quasisoft sources (QSSs). While hot white dwarf models may apply to SSSs, the
effective temperatures of QSSs are too high, unless, e.g., the radiation
emanates from only a small portion of surface. Two of the SSSs were first
detected and identified as such through ROSAT observations. One SSS and one QSS
may be identified with symbiotics, and 2 SSSs with supernova remnants. Both
SSSs and QSSs in the disk are found near star-forming regions, possibly
indicating that they are young. VSSs in the outer disk and halo are likely to
be old systems; in these regions, there are more QSSs than SSSs, which is
opposite to what is found in fields closer to the galaxy center. The largest
density of bright VSSs is in the bulge; some of the bulge sources are close
enough to the nucleus to be remnants of the tidal disruption of a giant by the
massive central black hole. By using Chandra data in combination with ROSAT and
XMM observations, we find most VSSs to be highly variable, fading from or
brightening toward detectability on time scales of months. There is evidence
for VSSs with low luminosities ( erg s).Comment: 14 pages, 4 figures. Accepted for publication in ApJ. Classification
of supersoft and quasisoft sources is clarifie
Optical Constraints on an X-ray Transient Source in M31
We have detected a transient X-ray source in the M31 bulge through a
continuing monitoring campaign with the Chandra ACIS-I camera. The source was
detected at R.A.=00:42:33.428 +/- 0.11'', Dec.=+41:17:03.37 +/- 0.11'' in only
a single observation taken 2004 May 23. Fortuitous optical HST/ACS imaging of
the transient location prior to the X-ray outburst, along with follow-up
HST/ACS imaging during and after the outburst, reveals no transient optical
source brighter than B (equivalent) = 25.5. The location of the source and its
X-ray properties suggest it is a low mass X-ray binary (LMXB). Assuming the
transient is similar to many Galactic X-ray novae, the X-ray luminosity of (3.9
+/- 0.5) X 10 erg s and the upper-limit on the optical luminosity
provide a prediction of <1.6 days for the orbital period of the binary system.Comment: 15 pages, 3 figures, accepted for publication in Ap
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