3,338 research outputs found
Optical studies of the ultraluminous X-ray source NGC1313 X-2
NGC1313 X-2 was among the first ultraluminous X-ray sources discovered, and
has been a frequent target of X-ray and optical observations. Using the HST/ACS
multi-band observations, this source is identified with a unique counterpart
within an error circle of 0\farcs2. The counterpart is a blue star on the
edge of a young cluster of years amid a dominant old stellar
population. Its spectral energy distribution is consistent with that for a
Z=0.004 star with 8.5 about years old, or for an O7 V
star at solar metallicity. The counterpart exhibited significant variability of
mag between two F555W observations separated by
three months, reminiscent of the ellipsoidal variability due to the orbital
motion of this ULX binary.Comment: 21 pages, 7 figures, scheduled for the ApJ June 10, 2007, v662n 1
issu
On the evolution of the inner disk radius with flux in the neutron star low-mass X-ray binary Serpens X-1
We analyze the latest \emph{Suzaku} observation of the bright neutron star
low-mass X-ray binary Serpens X-1 taken in 2013 October and 2014 April. The
observation was taken using the burst mode and only suffered mild pile-up
effects. A broad iron line is clearly detected in the X-ray spectrum. We test
different models and find that the iron line is asymmetric and best interpreted
by relativistic reflection. The relativistically broadened iron line is
generally believed to originate from the innermost regions of the accretion
disk, where strong gravity causes a series of special and general relativistic
effects. The iron line profile indicates an inner radius of , which gives an upper limit on the size of the neutron star. The asymmetric
iron line has been observed in a number of previous observations, which gives
several inner radius measurements at different flux states. We find that the
inner radius of Serpens X-1 does not evolve significantly over the range of
, and the lack of flux dependence of the inner radius
implies that the accretion disk may be truncated outside the innermost stable
circular orbit by the boundary layer rather than the stellar magnetic field.Comment: 8 pages, 6 figures, accepted for publication in Ap
Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During its 2016 Outburst
We present a spectral and timing study of the NuSTAR and Swift observations
of the black hole candidate IGR J17091-3624 in the hard state during its
outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra
taken in three epochs. Fitting with relativistic reflection models reveals that
the accretion disk is truncated during all epochs with , with the data favoring a low disk inclination of . The steepening of the continuum spectra between epochs
is accompanied by a decrease in the high energy cut-off: the electron
temperature drops from keV to keV, changing
systematically with the source flux. We detect type-C QPOs in the power spectra
with frequency varying between 0.131 Hz and 0.327 Hz. In addition, a secondary
peak is found in the power spectra centered at about 2.3 times the QPO
frequency during all three epochs. The nature of this secondary frequency is
uncertain, however a non-harmonic origin is favored. We investigate the
evolution of the timing and spectral properties during the rising phase of the
outburst and discuss their physical implications.Comment: 11 pages, 9 figures, accepted by Ap
On the Progenitor System of the Type Iax Supernova 2014dt in M61
We present pre-explosion and post-explosion Hubble Space Telescope images of
the Type Iax supernova (SN Iax) 2014dt in M61. After astrometrically aligning
these images, we do not detect any stellar sources at the position of the SN in
the pre-explosion images to relatively deep limits (3 sigma limits of M_F438W >
-5.0 mag and M_F814W > -5.9 mag). These limits are similar to the luminosity of
SN 2012Z's progenitor system (M_F435W = -5.43 +/- 0.15 and M_F814W = -5.24 +/-
0.16 mag), the only probable detected progenitor system in pre-explosion images
of a SN Iax, and indeed, of any white dwarf supernova. SN 2014dt is consistent
with having a C/O white-dwarf primary/helium-star companion progenitor system,
as was suggested for SN 2012Z, although perhaps with a slightly smaller or
hotter donor. The data are also consistent with SN 2014dt having a low-mass red
giant or main-sequence star companion. The data rule out main-sequence stars
with M_init > 16 M_sun and most evolved stars with M_init > 8 M_sun as being
the progenitor of SN 2014dt. Hot Wolf-Rayet stars are also allowed, but the
lack of nearby bright sources makes this scenario unlikely. Because of its
proximity (D = 12 Mpc), SN 2014dt is ideal for long-term monitoring, where
images in ~2 years may detect the companion star or the luminous bound remnant
of the progenitor white dwarf.Comment: 5 pages, 3 figures, submitted to ApJ
Suzaku Observations of 4U 1957+11: Potentially the Most Rapidly Spinning Black Hole in (the Halo of) the Galaxy
We present three Suzaku observations of the black hole candidate 4U 1957+11
(V1408 Aql) --- a source that exhibits some of the simplest and cleanest
examples of soft, disk-dominated spectra. 4U 1957+11 also presents among the
highest peak temperatures found from disk-dominated spectra. Such temperatures
may be associated with rapid black hole spin. The 4U 1957+11 spectra also
require a very low normalization, which can be explained by a combination of
small inner disk radius and a large distance (>10 kpc) which places 4U 1957+11
well into the Galactic halo. We perform joint fits to the Suzaku spectra with
both relativistic and Comptonized disk models. Assuming a low mass black hole
and the nearest distance (3 M_sun, 10 kpc), the dimensionless spin parameter a*
= Jc/GM^2 >~ 0.9. Higher masses and farther distances yield a*~1. Similar
conclusions are reached with Comptonization models; they imply a combination of
small inner disk radii (or, equivalently, rapid spin) and large distance. Low
spin cannot be recovered unless 4U 1957+11 is a low mass black hole that is at
the unusually large distance of >~ 40 kpc. We speculate whether the suggested
maximal spin is related to how the system came to reside in the halo.Comment: To be published in the Astrophysical Journa
An X-Ray View of the Jet-Cycle in the Radio Loud AGN 3C120
We present a study of the central engine in the broad-line radio galaxy 3C120
using a multi-epoch analysis of a deep XMM-Newton observation and two deep
Suzaku pointings (in 2012). In order to place our spectral data into the
context of the disk-disruption/jet-ejection cycles displayed by this object, we
monitor the source in the UV/X-ray bands, and in the radio band. We find three
statistically acceptable spectral models, a disk-reflection model, a jet-model
and a jet+disk model. Despite being good descriptions of the data, the
disk-reflection model violates the radio constraints on the inclination, and
the jet-model has a fine-tuning problem, requiring a jet contribution exceeding
that expected. Thus, we argue for a composite jet+disk model. Within the
context of this model, we verify the basic predictions of the jet-cycle
paradigm, finding a truncated/refilling disk during the Suzaku observations and
a complete disk extending down to the innermost stable circular orbit (ISCO)
during the XMM-Newton observation. The idea of a refilling disk is further
supported by the detection of the ejection of a new jet knot approximately one
month after the Suzaku pointings. We also discover a step-like event in one of
the Suzaku pointings in which the soft band lags the hard band. We suggest that
we are witnessing the propagation of a disturbance from the disk into the jet
on a timescale set by the magnetic field.Comment: 14 pages, 10 figures, accepted for publication in Ap
An Extreme Black Hole in the Recurrent X-ray Transient XTE J2012+381
The black hole candidate XTE J2012+381 underwent an outburst at the end of
2022. We analyzed 105 NICER observations and 2 NuSTAR observations of the
source during the outburst. The NuSTAR observations of the
black hole indicate clear signs of relativistic disk reflection, which we
modeled to measure a BH spin of and an inclination
of degrees ( statistical errors). In our
analysis, we test an array of models and examine the effect of fitting NuSTAR
spectra alone versus fitting simultaneously with NICER. We find that when the
underlying continuum emission is properly accounted for, the reflected emission
is similarly characterized by multiple models. We combined 52 NICER spectra to
obtain a spectrum with an effective exposure of 190 ks in order to probe the
presence of absorption lines that would be suggestive of disk winds, but the
resulting features were not statistically significant. We discuss the
implications of this measurement in relation to the overall BH spin
distribution in X-ray binary systems.Comment: 17 pages, 7 figures. Accepted for publication in Ap
Prescribing “placebo treatments”: results of national survey of US internists and rheumatologists
Objective To describe the attitudes and behaviours regarding placebo treatments, defined as a treatment whose benefits derive from positive patient expectations and not from the physiological mechanism of the treatment itself
Requirements for Pseudomonas aeruginosa Type I-F CRISPR-Cas Adaptation Determined Using a Biofilm Enrichment Assay
CRISPR (clustered regularly interspaced short palindromic repeat)-Cas (CRISPR-associated protein) systems are diverse and found in many archaea and bacteria. These systems have mainly been characterized as adaptive immune systems able to protect against invading mobile genetic elements, including viruses. The first step in this protection is acquisition of spacer sequences from the invader DNA and incorporation of those sequences into the CRISPR array, termed CRISPR adaptation. Progress in understanding the mechanisms and requirements of CRISPR adaptation has largely been accomplished using overexpression of cas genes or plasmid loss assays; little work has focused on endogenous CRISPR-acquired immunity from viral predation. Here, we developed a new biofilm-based assay system to enrich for Pseudomonas aeruginosa strains with new spacer acquisition. We used this assay to demonstrate that P. aeruginosa rapidly acquires spacers protective against DMS3vir, an engineered lytic variant of the Mu-like bacteriophage DMS3, through primed CRISPR adaptation from spacers present in the native CRISPR2 array. We found that for the P. aeruginosa type I-F system, the cas1 gene is required for CRISPR adaptation, recG contributes to (but is not required for) primed CRISPR adaptation, recD is dispensable for primed CRISPR adaptation, and finally, the ability of a putative priming spacer to prime can vary considerably depending on the specific sequences of the spacer
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