56 research outputs found
The Discovery of Soft X-ray Loud Broad Absorption Line Quasars
It is been known for more than a decade that BALQSOs (broad absorption line
quasars) are highly attenuated in the X-ray regime compared to other quasars,
especially in the soft band ( 1 keV). Using X-ray selection techniques we
have found "soft X-ray loud" BALQSOs that, by definition, have soft X-ray (0.3
keV) to UV () flux density ratios that are higher than typical nonBAL
radio quiet quasars. Our sample of 3 sources includes one LoBALQSO (low
ionization BALQSO) which are generally considered to be the most highly
attenuated in the X-rays. The three QSOs are the only known BALQSOs that have
X-ray observations that are consistent with no intrinsic soft X-ray absorption.
The existence of a large X-ray luminosity and the hard ionizing continuum that
it presents to potential UV absorption gas is in conflict with the ionization
states that are conducive to line driving forces within BAL winds (especially
for the LoBALs).Comment: To appear in ApJ Letter
Discovery of a 3.6-hr Eclipsing Luminous X-Ray Binary in the Galaxy NGC 4214
We report the discovery of an eclipsing X-ray binary with a 3.62-hr period
within 24" of the center of the dwarf starburst galaxy NGC 4214. The orbital
period places interesting constraints on the nature of the binary, and allows
for a few very different interpretations. The most likely possibility is that
the source lies within NGC 4214 and has an X-ray luminosity of up to 7 e38
ergs/s. In this case the binary may well be comprised of a naked He-burning
donor star with a neutron-star accretor, though a stellar-mass black-hole
accretor cannot be completely excluded. There is no obvious evidence for a
strong stellar wind in the X-ray orbital light curve that would be expected
from a massive He star; thus, the mass of the He star should be <3-4 solar
masses. If correct, this would represent a new class of very luminous X-ray
binary -- perhaps related to Cyg X-3. Other less likely possibilities include a
conventional low-mass X-ray binary that somehow manages to produce such a high
X-ray luminosity and is apparently persistent over an interval of years; or a
foreground AM Her binary of much lower luminosity that fortuitously lies in the
direction of NGC 4214. Any model for this system must accommodate the lack of
an optical counterpart down to a limiting magnitude of 22.6 in the visible.Comment: 7 pages, ApJ accepted versio
UGC 7069: The largest ring galaxy
We find that UGC 7069 is the largest ring galaxy known to date. In this
Letter, we present a multiwavelength study of this galaxy (combining radio,
2MASS, optical and ultraviolet data). The ring of UGC 7069, whose diameter
measures ~115 kpc, is also warped at its edges. The nucleus appears
double-peaked and hosts a possible LINER. The ultraviolet data indicate a
strong blue colour and suggest that UGC 7069 is a starburst galaxy. We also
present N-body simulation results, which indicate that galaxy collisions can
produce such huge rings. Large inclination angles between the target and the
intruder galaxy may account for the formation of warped rings. Multiwavelength
observations are highly essential to constrain our simulation results, which
will address the formation and evolution of such a rare galaxy.Comment: 6 pages, 5 figures, MNRAS Letters, in pres
Optical Spectroscopy of the environment of a ULX in NGC 7331
Optical photometric and spectroscopic data are presented that show an
association of an ultraluminous X-ray source in NGC 7331 with a young star
cluster of mass 1.1e5 solar masses and age 4.25 Myr. If the ULX is part of the
bright stellar cluster, then the mass of the progenitor of the compact accretor
must have been greater than about 40-50 solar masses in order to already have
evolved through the supernova stage to a compact object. The companion star is
also likely an evolved massive star. The emission line spectrum of the nebula
surrounding the cluster can be interpreted as a result of photoionization by
the cluster OB stars with an additional source of shock excitation producing
strong [SII], [OI] and NII lines. This additional source appears to be as much
as five times more powerful than the supernovae and stellar winds in the
cluster can provide. Additional mechanical energy input associated with the ULX
itself can help explain the residual shock excited line luminosities of the
emission region.Comment: 17 pages, accepted to Ap
On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)
We combine 9 ROSAT, 9 Chandra, and 2 XMM-Newton observations of the Cen~A
galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU
J132519.9430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4
keV flux F_X>10^{-12} ergs cm^{-2} s^{-1} during a 10~day span in 1995 July.
The inferred peak isotropic luminosity of the source therefore exceeded 3
10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous
X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second
bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) in 1999 December.
The source is detected and varies significantly within three additional
observations but is below the detection threshold in 7 observations. The X-ray
spectrum in 1999 December is best described as a cut-off power law or a
disk-blackbody (multi-colored disk). We also detect an optical source, m_F555W
~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST
images taken 195~days before the nearest X-ray observation. The optical
brightness of this source is consistent with a late O or early B star at the
distance of Cen A. If the optical source is the counterpart, then the X-ray and
optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray
pulsar A 0538-66.Comment: 7 pages, 8 figures. ApJ (accepted
Properties of the Chandra Sources in M81
The Chandra X-ray Observatory obtained a 50-ks observation of the central
region of M81 using the ACIS-S in imaging mode. The global properties of the 97
x-ray sources detected in the inner 8.3x8.3 arcmin field of M81 are examined.
Roughly half the sources are concentrated within the central bulge. The
remainder are distributed throughout the disk with the brightest disk sources
lying preferentially along spiral arms. The average hardness ratios of both
bulge and disk sources are consistent with power law spectra of index Gamma~1.6
indicative of a population of x-ray binaries. A group of much softer sources
are also present. The background source-subtracted logN-logS distribution of
the disk follows a power law of index ~ -0.5 with no change in slope over three
decades in flux. The logN-logS distribution of the bulge follows a similar
shape but with a steeper slope above ~4.0e+37 ergs/s. There is unresolved x-ray
flux from the bulge with a radial profile similar to that of the bulge sources.
This unresolved flux is softer than the average of the bulge sources and
extrapolating the bulge logN-logS distribution towards weaker sources can only
account for 20% of the unresolved flux. No strong time variability was observed
for any source with the exception of one bright, soft source.Comment: 5 pages, 3 color PS figures, to appear in ApJ
On the Nature of the Bright Short-Period X-ray Source in the Circinus Galaxy Field
The spectrum and light curve of the bright X-ray source CG X-1 in the field
of the Circinus galaxy are re-examined. Previous analyses have concluded that
the source is an accreting black hole of about 50 solar masses although it was
noted that the light curve resembles that of an AM Her system. Here we show
that the short period and an assumed main sequence companion constrain the mass
of the companion to less than one solar mass. Further a possible eclipse seen
during one of the Chandra observations and a subsequent XMM-Newton observation
constrains the mass of the compact object to less than about 60 solar masses.
If such a system lies in the Circinus galaxy, then the accreting object must
either radiate anisotropically or strongly violate the Eddington limit. Even if
the emission is beamed, then the companion star which intercepts this flux
during eclipse will be driven out of thermal equilibrium and evaporate within
about 1000 years. We find that the observations cannot rule out an AM Her
system in the Milky Way and that such a system can account for the variations
seen in the light curve.Comment: 25 pages, 8 figures, accepted for publication in the Astrophysical
Journa
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