622 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
Discovery of a Second Transient Low-Mass X-ray Binary in the Globular Cluster NGC 6440
We have identified a new transient luminous low-mass X-ray binary, NGC 6440
X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular
cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at
L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Four other
outbursts (May 29-June 4, Aug. 29-Sept. 1, Oct. 1-3, and Oct. 28-31 2009) have
been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et
al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440
X-2), with similar peak luminosities and decay times. Optical and infrared
imaging did not detect a clear counterpart, with best limits of V>21, B>22 in
quiescence from archival HST imaging, g'>22 during the August outburst from
Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the July outburst from
CTIO 4-m ISPI imaging.
Archival Chandra X-ray images of the core do not detect the quiescent
counterpart, and place a bolometric luminosity limit of L_{NS}< 6*10^31 ergs/s
(one of the lowest measured) for a hydrogen atmosphere neutron star. A short
Chandra observation 10 days into quiescence found two photons at NGC 6440 X-2's
position, suggesting enhanced quiescent emission at L_X~6*10^31 ergs/s .
NGC 6440 X-2 currently shows the shortest recurrence time (~31 days) of any
known X-ray transient, although regular outbursts were not visible in the bulge
scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may
be easily missed by current X-ray monitoring.Comment: 13 pages (emulateapj), 8 (color) figures, ApJ in press. Revised
version adds 5th outburst (Oct./Nov. 2009), additional discussion of possible
causes of short outburst recurrence time
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
Low-level accretion in neutron-star X-ray binaries
We search the literature for reports on the spectral properties of
neutron-star low-mass X-ray binaries when they have accretion luminosities
between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon
index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV
range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the
spectrum softens). Such behaviour has been reported before for individual
sources, but here we demonstrate that very likely most (if not all)
neutron-star systems behave in a similar manner and possibly even follow a
universal relation. When comparing the neutron-star systems with black-hole
systems, it is clear that most black-hole binaries have significantly harder
spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data
points, there are indications that these spectral differences also extend to
the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star
binaries and black-hole ones suggests that the spectral properties (between
0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the
nature of the accretor in unclassified X-ray binaries. We discuss our results
in the context of properties of the accretion flow at low luminosities and we
suggest that the observed spectral differences likely arise from the
neutron-star surface becoming dominantly visible in the X-ray spectra. We also
suggest that both the thermal component and the non-thermal component might be
caused by low-level accretion onto the neutron-star surface for luminosities
below a few times 1E34 erg/s.Comment: Accepted for publication in MNRA
A Chandra Study of the Dense Globular Cluster Terzan 5
We report a Chandra ACIS-I observation of the dense globular cluster Terzan
5. The previously known transient low-mass x-ray binary (LMXB) EXO 1745-248 in
the cluster entered a rare high state during our August 2000 observation,
complicating the analysis. Nevertheless nine additional sources clearly
associated with the cluster are also detected, ranging from L_X(0.5-2.5
keV)=5.6*10^{32} down to 8.6*10^{31} ergs/s. Their X-ray colors and
luminosities, and spectral fitting, indicate that five of them are probably
cataclysmic variables, and four are likely quiescent LMXBs containing neutron
stars. We estimate the total number of sources between L_X(0.5-2.5 keV)=10^{32}
and 10^{33} ergs/s as 11.4^{+4.7}_{-1.8} by the use of artificial point source
tests, and note that the numbers of X-ray sources are similar to those detected
in NGC 6440. The improved X-ray position allowed us to identify a plausible
infrared counterpart to EXO 1745-248 on our 1998 Hubble Space Telescope NICMOS
images. This blue star (F110W=18.48, F187W=17.30) lies within 0.2'' of the
boresighted LMXB position. Simultaneous Rossi X-ray Timing Explorer (RXTE)
spectra, combined with the Chandra spectrum, indicate that EXO 1745-248 is an
ultracompact binary system, and show a strong broad 6.55 keV iron line and an 8
keV smeared reflection edge.Comment: 18 pages, 8 figures, accepted to Ap
Constraints on Thermal X-ray Radiation from SAX J1808.4-3658 and Implications for Neutron Star Neutrino Emission
Thermal X-ray radiation from neutron star soft X-ray transients in quiescence
provides the strongest constraints on the cooling rates of neutron stars, and
thus on the interior composition and properties of matter in the cores of
neutron stars. We analyze new (2006) and archival (2001) XMM-Newton
observations of the accreting millisecond pulsar SAX J1808.4-3658 in
quiescence, which provide the most stringent constraints to date. The X-ray
spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a
power-law of photon index 1.83\pm0.17, without requiring the presence of a
blackbody-like component from a neutron star atmosphere. Our 2006 observation
shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of
68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting
of all available XMM data allows a constraint on the quiescent neutron star
(0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some
current models of neutrino emission mediated by pion condensates, and provides
further evidence for additional cooling processes, such as neutrino emission
via direct Urca processes involving nucleons and/or hyperons, in the cores of
massive neutron stars.Comment: 5 pages, 2 figures; slight revisions, accepted by Ap
SAX J1808.4-3657 in Quiescence: A Keystone for Neutron Star Science
The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object
between accreting X-ray binaries and millisecond radio pulsars. We have
constrained the thermal radiation from its surface through XMM-Newton X-ray
observations, providing strong evidence for neutrino cooling processes from the
neutron star core. We have also undertaken simultaneous X-ray and optical
(Gemini) observations, shedding light on whether the strong heating of the
companion star in quiescence may be due to X-ray irradiation, or to a radio
pulsar turning on when accretion stops.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond
Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 4
page
- …