529 research outputs found
Near-infrared counterparts of three transient very faint neutron star X-ray binaries
We present near-infrared (NIR) imaging observations of three transient
neutron star X-ray binaries, SAX J1753.5-2349, SAX J1806.5-2215 and AX
J1754.2-2754. All three sources are members of the class of `very faint' X-ray
transients which exhibit X-ray luminosities erg s.
The nature of this class of sources is still poorly understood. We detect NIR
counterparts for all three systems and perform multi-band photometry for both
SAX J1753.5-2349 and SAX J1806.5-2215, including narrow-band Br
photometry for SAX J1806.5-2215. We find that SAX J1753.5-2349 is significantly
redder than the field population, indicating that there may be absorption
intrinsic to the system, or perhaps a jet is contributing to the infrared
emission. SAX J1806.5-2215 appears to exhibit absorption in Br,
providing evidence for hydrogen in the system. Our observations of AX
J1754.2--2754 represent the first detection of a NIR counterpart for this
system. We find that none of the measured magnitudes are consistent with the
expected quiescent magnitudes of these systems. Assuming that the infrared
radiation is dominated by either the disc or the companion star, the observed
magnitudes argue against an ultracompact nature for all three systems.Comment: 10 pages, 10 figures, accepted for publication in MNRA
Limits on thermal variations in a dozen quiescent neutron stars over a decade
In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the
origin of the thermal X-ray component may be either release of heat from the
core of the neutron star, or continuing low-level accretion. In general, heat
from the core should be stable on timescales years, while continuing
accretion may produce variations on a range of timescales. While some quiescent
neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal
components on a range of timescales, several others, particularly those in
globular clusters with no detectable nonthermal hard X-rays (fit with a
powerlaw), have shown no measurable variations. Here, we constrain the spectral
variations of 12 low mass X-ray binaries in 3 globular clusters over
years. We find no evidence of variations in 10 cases, with limits on
temperature variations below 11% for the 7 qLMXBs without powerlaw components,
and limits on variations below 20% for 3 other qLMXBs that do show non-thermal
emission. However, in 2 qLMXBs showing powerlaw components in their spectra
(NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in
temperature, suggesting the presence of continuing low-level accretion. This
work adds to the evidence that the thermal X-ray component in quiescent neutron
stars without powerlaw components can be explained by heat deposited in the
core during outbursts. Finally, we also investigate the correlation between
hydrogen column density (N) and optical extinction (A) using our sample
and current models of interstellar X-ray absorption, finding .Comment: 16 pages, 5 figures, MNRAS, in pres
On the coexistence of stellar-mass and intermediate-mass black holes in globular clusters
In this paper, we address the question: What is the probability of
stellar-mass black hole (BH) binaries co-existing in a globular cluster with an
intermediate-mass black hole (IMBH)? Our results suggest that the detection of
one or more BH binaries can strongly constrain the presence of an IMBH in most
Galactic globular clusters. More specifically, the detection of one or more BH
binaries could strongly indicate against the presence of an IMBH more massive
than M in roughly 80\% of the clusters in our
sample. To illustrate this, we use a combination of N-body simulations and
analytic methods to weigh the rate of formation of BH binaries against their
ejection and/or disruption rate via strong gravitational interactions with the
central (most) massive BH.
The eventual fate of a sub-population of stellar-mass BHs (with or without
binary companions) is for all BHs to be ejected from the cluster by the central
IMBH, leaving only the most massive stellar-mass BH behind to form a close
binary with the IMBH. During each phase of evolution, we discuss the rate of
inspiral of the central BH-BH pair as a function of both the properties of the
binary and its host cluster.Comment: 16 pages, 8 figures, 1 table, accepted for publication in MNRA
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
We present radio continuum light curves of the magnetar SGR J17452900 and
Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations
between 2012 and 2014. During this period, a powerful X-ray outburst from SGR
J17452900 occurred on 2013-04-24. Enhanced radio emission is delayed with
respect to the X-ray peak by about seven months. In addition, the flux density
of the emission from the magnetar fluctuates by a factor of 2 to 4 at
frequencies between 21 and 41 GHz and its spectral index varies erratically.
Here we argue that the excess fluctuating emission from the magnetar arises
from the interaction of a shock generated from the X-ray outburst with the
orbiting ionized gas at the Galactic center. In this picture, variable
synchrotron emission is produced by ram pressure variations due to
inhomogeneities in the dense ionized medium of the Sgr A West bar. The pulsar
with its high transverse velocity is moving through a highly blue-shifted
ionized medium. This implies that the magnetar is at a projected distance of
pc from Sgr A* and that the orbiting ionized gas is partially or
largely responsible for a large rotation measure detected toward the magnetar.
Despite the variability of Sgr A* expected to be induced by the passage of the
G2 cloud, monitoring data shows a constant flux density and spectral index
during this periodComment: 12 pages, 3 figures, ApJL (in press
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