714 research outputs found
Optical variability of the accretion disk around the intermediate mass black hole ESO 243-49 HLX-1 during the 2012 outburst
We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large
Telescope (VLT), V- and R-band) observations of the intermediate mass black
hole candidate ESO 243-49 HLX-1 before and during the 2012 outburst. We show
that the V-band magnitudes vary with time, thus proving that a portion of the
observed emission originates in the accretion disk. Using the first quiescent
optical observations of HLX-1, we show that the stellar population surrounding
HLX-1 is fainter than V~25.1 and R~24.2. We show that the optical emission may
increase before the X-ray emission consistent with the scenario proposed by
Lasota et al. (2011) in which the regular outbursts could be related to the
passage at periastron of a star circling the intermediate mass black hole in an
eccentric orbit, which triggers mass transfer into a quasi-permanent accretion
disk around the black hole. Further, if there is indeed a delay in the X-ray
emission we estimate the mass-transfer delivery radius to be ~1e11 cm.Comment: 9 pages, 2 figures, accepted for publication in ApJ
VLA Limits for Intermediate Mass Black Holes in Three Globular Clusters
The observational evidence for central black holes in globular clusters has
been argued extensively, and their existence has important consequences for
both the formation and evolution of the cluster. Most of the evidence comes
from dynamical arguments, but the interpretation is difficult, given the short
relaxation times and old ages of the clusters. One of the most robust
signatures for the existence of a black hole is radio and/or X-ray emission. We
observed three globular clusters, NGC6093 (M80), NGC6266 (M62), and NGC7078
(M15), with the VLA in the A and C configuration with a 3-sigma noise of 36, 36
and 25 microJy, respectively. We find no statistically-significant evidence for
radio emission from the central region for any of the three clusters. NGC6266
shows a 2-sigma detection. It is difficult to infer a mass from these upper
limits due to uncertainty about the central gas density, accretion rate, and
accretion model.Comment: 5 pages, accepted for publication in the Astronomical Journa
Jet-Induced Nucleosynthesis in Misaligned Microquasars
The jet axes and the orbital planes of microquasar systems are usually
assumed to be approximately perpendicular, eventhough this is not currently an
observational requirement. On the contrary, in one of the few systems where the
relative orientations are well-constrained, V4641Sgr, the jet axis is known to
lie not more than ~36 degrees from the binary plane. Such a jet, lying close to
the binary plane, and traveling at a significant fraction of the speed of light
may periodically impact the secondary star initiating nuclear reactions on its
surface. The integrated yield of such nuclear reactions over the age of the
binary system (less the radiative mass loss) will detectably alter the
elemental abundances of the companion star. This scenario may explain the
anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric
value; or, equivalently, 0.1-1 times the average solar system value) seen in
the companions of some black-hole X-ray binary systems. (Such enhancements are
puzzling since Li nuclei are exceedingly fragile - being easily destroyed in
the interiors of stars - and Li would be expected to be depleted rather than
enhanced there.) Gamma-ray line signatures of the proposed process could
include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li*
de-excitation line, both of which may be discernable with the INTEGRAL
satellite if produced in an optically thin region during a large outburst. For
very energetic jets, a relatively narrow neutral pion gamma-decay signature at
67.5 MeV could also be measurable with the GLAST satellite. We argue that about
10-20% of all microquasar systems ought to be sufficiently misaligned as to be
undergoing the proposed jet-secondary impacts.Comment: ApJ, accepted. Includes referee's suggestions and some minor
clarifications over previous versio
Binary evolution with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of very faint X-ray binaries,
orbital period distribution of black hole X-ray binaries and neutron star spin
up. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
How Abundant is Iron in the Core of the Perseus Cluster?
The analysis of Perseus data collected with the Medium Energy Concentrator
Spectrometer (MECS) on board Beppo-SAX shows that the ratio of the flux of the
8 keV line complex (dominated by Fe K emission) over the 6.8 keV line
complex (dominated by Fe K emission) is significantly larger than
predicted by standard thermal emission codes. Moreover the analysis of
spatially resolved spectra shows that the above ratio decreases with increasing
cluster radius. We find that, amongst the various explanations we consider, the
most likely requires the plasma to be optically thick for resonant scattering
at the energy of the Fe K line. We argue that if this is the case,
then measures of the iron abundance made using standard thermal emission codes,
that assume optically thin emission, can significantly underestimate the true
iron abundance. In the case of the core of Perseus we estimate the true
abundance to be 0.9 solar in a circular region with radius of
kpc and centered on NGC 1275. Finally we speculate that similar results may
hold for the core of other rich clusters.Comment: 19 pages, 3 Postscript figure
A transient ultraviolet outflow in the short-period X-ray binary UW CrB
This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5–26555. These observations are associated with program(s) 12039.Accreting low mass X-ray binaries (LMXBs) are capable of launching powerful outflows such as accretion disc winds. In disc winds, vast amounts of material can be carried away, potentially greatly impacting the binary and its environment. Previous studies have uncovered signatures of disc winds in the X-ray, optical, near-infrared, and recently even the UV band, predominantly in LMXBs with large discs (Porb ≥ 20 hrs). Here, we present the discovery of transient UV outflow features in UW CrB, a high-inclination (i ≥ 77○) neutron star LMXB with an orbital period of only Porb ≈ 111 min. We present P-Cygni profiles identified for Si iv 1400Å and tentatively for N v 1240Å in one 15 min exposure, which is the only exposure covering orbital phase φ ≈ 0.7 − 0.8, with a velocity of ≈1500 km s−1. We show that due to the presence of black body emission from the neutron star surface and/or boundary layer, a thermal disc wind can be driven despite the short Porb, but explore alternative scenarios as well. The discovery that thermal disc winds may occur in NS-LMXBs with Porb as small as ≈111 min, and can potentially be transient on time scales as short as ≈15 min, warrants further observational and theoretical work.PostprintPeer reviewe
Metallicity Effect on LMXB Formation in Globular Clusters
We present comprehensive observational results of the metallicity effect on
the fraction of globular clusters (GC) that contain low-mass X-ray binaries
(LMXB), by utilizing all available data obtained with Chandra for LMXBs and HST
ACS for GCs. Our primary sample consists of old elliptical galaxies selected
from the ACS Virgo and Fornax surveys. To improve statistics at both the lowest
and highest X-ray luminosity, we also use previously reported results from
other galaxies. It is well known that the LMXB fraction is considerably higher
in red, metal-rich, than in blue, metal-poor GCs. In this paper, we test
whether this metallicity effect is X-ray luminosity-dependent, and find that
the effect holds uniformly in a wide luminosity range. This result is
statistically significant (at >= 3 sigma) in LMXBs with luminosities in the
range LX = 2 x 10^37 - 5 x 10^38 erg s-1, where the ratio of LMXB fractions in
metal-rich to metal-poor GCs is R = 3.4 +- 0.5. A similar ratio is also found
at lower (down to 10^36 erg s-1) and higher luminosities (up to the ULX
regime), but with less significance (~2 sigma confidence). Because different
types of LMXBs dominate in different luminosities, our finding requires a new
explanation for the metallicity effect in dynamically formed LMXBs. We confirm
that the metallicity effect is not affected by other factors such as stellar
age, GC mass, stellar encounter rate, and galacto-centric distance.Comment: 21 pages, 5 figures, accepted in Ap
Tracing the jet contribution to the mid-IR over the 2005 outburst of GRO J1655-40 via broadband spectral modeling
We present new results from a multi-wavelength (radio/infrared/optical/X-ray)
study of the black hole X-ray binary GRO J1655-40 during its 2005 outburst. We
detected, for the first time, mid-infrared emission at 24 um from the compact
jet of a black hole X-ray binary during its hard state, when the source shows
emission from a radio compact jet as well as a strong non-thermal hard X-ray
component. These detections strongly constrain the optically thick part of the
synchrotron spectrum of the compact jet, which is consistent with being flat
over four orders of magnitude in frequency. Moreover, using this unprecedented
coverage, and especially thanks to the new Spitzer observations, we can test
broadband disk and jet models during the hard state. Two of the hard state
broadband spectra are reasonably well fitted using a jet model with parameters
overall similar to those previously found for Cyg X-1 and GX 339-4. Differences
are also present; most notably, the jet power in GRO J1655-40 appears to be a
factor of at least ~3-5 higher (depending on the distance) than that of Cyg X-1
and GX 339-4 at comparable disk luminosities. Furthermore, a few discrepancies
between the model and the data, previously not found for the other two black
hole systems for which there was no mid-IR/IR and optical coverage, are
evident, and will help to constrain and refine theoretical models.Comment: accepted for publication in Ap
Discovery of 105 Hz coherent pulsations in the ultracompact binary IGR J16597-3704
We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the
transient X-ray binary IGR J16597-3704 using NuSTAR and Swift. The source was
discovered by INTEGRAL in the globular cluster NGC 6256 at a distance of 9.1
kpc. The X-ray pulsations show a clear Doppler modulation implying an orbital
period of ~46 minutes and a projected semi-major axis of ~5 lt-ms, which makes
IGR J16597-3704 an ultra-compact X-ray binary system. We estimated a minimum
companion mass of 0.0065 solar masses, assuming a neutron star mass of 1.4
solar masses, and an inclination angle of <75 degrees (suggested by the absence
of eclipses or dips in its light-curve). The broad-band energy spectrum of the
source is well described by a disk blackbody component (kT ~1.4 keV) plus a
comptonised power-law with photon index ~2.3 and an electron temperature of ~30
keV. Radio pulsations from the source were searched for with the Parkes
observatory and not detected.Comment: 4 pages, 4 images, Accepted for publication in A&
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