59 research outputs found
The ultraluminous X-ray source NGC 1313 X-2 - Its optical counterpart and environment
NGC 1313 X-2 is one of the brightest ultraluminous X-ray sources in the sky,
at both X-ray and optical wavelengths; therefore, quite a few studies of
available ESO VLT and HST data have appeared in the literature. Here, we
present our analysis of VLT/FORS1 and HST/ACS photometric data, confirming the
identification of the B ~ 23 mag blue optical counterpart. We show that the
system is part of a poor cluster with an age of 20 Myr, leading to an upper
mass limit of some 12 M_sun for the mass donor. We attribute the different
results with respect to earlier studies to the use of isochrones in the F435W
and F555W HST/ACS photometric system that appear to be incompatible with the
corresponding Johnson B and V isochrones. The counterpart exhibits significant
photometric variability of about 0.2 mag amplitude, both between the two HST
observations and during the one month of monitoring with the VLT. This includes
variability within one night and suggests that the light is dominated by the
accretion disk in the system and not by the mass donor.Comment: 13 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment
Holmberg IX X-1 is an archetypal ultraluminous X-ray source (ULX). Here we
study the properties of the optical counterpart and of its stellar environment
using optical data from SUBARU/Faint Object Camera and
Spectrograph,GEMINI/GMOS-N and Hubble Space Telescope (HST)/Advanced Camera for
Surveys, as well as simultaneous Chandra X-ray data. The V ~ 22.6
spectroscopically identified optical counterpart is part of a loose cluster
with an age <~ 20 Myr. Consequently, the mass upper limit on individual stars
in the association is about 20 M_sun. The counterpart is more luminous than the
other stars of the association, suggesting a non-negligible optical
contribution from the accretion disk. An observed UV excess also points to
non-stellar light similar to X-ray active low-mass X-ray binaries. A broad
HeII4686 emission line identified in the optical spectrum of the ULX further
suggests optical light from X-ray reprocessing in the accretion disk. Using
stellar evolutionary tracks, we have constrained the mass of the counterpart to
be >~ 10 M_sun, even if the accretion disk contributes significantly to the
optical luminosity. Comparison of the photometric properties of the counterpart
with binary models show that the donor may be more massive, >~ 25 M_sun, with
the ULX system likely undergoing case AB mass transfer. Finally, the
counterpart exhibits photometric variability of 0.14 mag between two HST
observations separated by 50 days which could be due to ellipsoidal variations
and/or disk reprocessing of variable X-ray emission.Comment: 14 pages, 14 figures, accepted for publication in Ap
Evolution of the spectral curvature in the ULX Holmberg II X-1
Ultraluminous X-ray sources (ULXs) are interesting systems as they can host
intermediate mass black holes. Alternatively, ULXs can represent stellar-mass
black holes accreting at super-Eddington rates. Recently spectral curvature or
breaks at energies above a few keV have been detected in high quality ULX
spectra. These spectral features have been taken as evidence against the
intermediate-mass black hole case. In this paper, we report on a new XMM-Newton
observation of the ULX Holmberg II X-1 that also shows a clear spectral break
at approximately 4 keV. This observation was performed during a low luminosity
state of the system and by comparing this new data to a high luminosity state
XMM-Newton observation, we can conclude that the spectral break energy
increases with luminosity. This behaviour is different to a ULX in the Holmberg
IX galaxy,where an opposite trend between the luminosity and the spectral break
energy has been claimed. We discuss mechanisms that could explain this complex
behaviour.Comment: 7 pages, 3 figures and 2 tables. Accepted for publication in MNRA
Discovery and evolution of the new black hole candidate Swift J1539.2-6227 during its 2008 outburst
We report on the discovery by the Swift Gamma-Ray Burst Explorer of the black
hole candidate Swift J1539.2-6227 and the subsequent course of an outburst
beginning in November 2008 and lasting at least seven months. The source was
discovered during normal observations with the Swift Burst Alert Telescope
(BAT) on 2008 November 25. An extended observing campaign with the Rossi X-Ray
Timing Explorer (RXTE) and Swift provided near-daily coverage over 176 days,
giving us a good opportunity to track the evolution of spectral and timing
parameters with fine temporal resolution through a series of spectral states.
The source was first detected in a hard state during which strong low-frequency
quasi-periodic oscillations (QPOs) were detected. The QPOs persisted for about
35 days and a signature of the transition from the hard to soft intermediate
states was seen in the timing data. The source entered a short-lived thermal
state about 40 days after the start of the outburst. There were variations in
spectral hardness as the source flux declined and returned to a hard state at
the end of the outburst. The progression of spectral states and the nature of
the timing features provide strong evidence that Swift J1539.2-6227 is a
candidate black hole in a low-mass X-ray binary system.Comment: Accepted by the Astrophysical Journa
The evolution of a jet ejection of the ultraluminous X-ray source Holmberg II X-1
We present quasi-simultaneous, multi-epoch radio and X-ray measurements of Holmberg II X-1 using the European VLBI Network (EVN), the Karl G. Jansky Very Large Array (VLA), and the Chandra and Swift X-ray telescopes. The X-ray data show apparently hard spectra with steady X-ray luminosities four months apart from each other. In the high-resolution EVN radio observations, we have detected an extended milliarcsecond scale source with unboosted radio emission. The source emits non-thermal, likely optically thin synchrotron emission, and its morphology is consistent with a jet ejection. The 9-GHz VLA data show an arcsecond-scale triple structure of Holmberg II X-1 similar to that seen at lower frequencies. However, we find that the central ejection has faded by at least a factor of 7.3 over 1.5 yr. We estimate the dynamical age of the ejection to be higher than 2.1 yr. We show that such a rapid cooling can be explained with simple adiabatic expansion losses. These properties of Holmberg II X-1 imply that ULX radio bubbles may be inflated by ejecta instead of self-absorbed compact jets
The evolution of a jet ejection of the ultraluminous X-ray source Holmberg II X-1
We present quasi-simultaneous, multi-epoch radio and X-ray measurements of Holm- berg II X-1 using the European VLBI Network (EVN), the Karl G. Jansky Very Large Array (VLA), and the Chandra and Swift X-ray telescopes. The X-ray data show apparently hard spectra with steady X-ray luminosities 4 months apart from each other. In the high-resolution EVN radio observations, we have detected an ex- tended milli-arcsecond scale source with unboosted radio emission. The source emits non-thermal, likely optically thin synchrotron emission and its morphology is consis- tent with a jet ejection. The 9-GHz VLA data show an arcsecond-scale triple structure of Holmberg II X-1 similar to that seen at lower frequencies. However, we find that the central ejection has faded by at least a factor of 7.3 over 1.5 years. We estimate the dynamical age of the ejection to be higher than 2.1 years. We show that such a rapid cooling can be explained with simple adiabatic expansion losses. These properties of Holmberg II X-1 imply that ULX radio bubbles may be inflated by ejecta instead of self-absorbed compact jets
The XMM-Newton serendipitous survey. VII. The third XMM-Newton serendipitous source catalogue
Thanks to the large collecting area (3 x ~1500 cm at 1.5 keV) and wide
field of view (30' across in full field mode) of the X-ray cameras on board the
European Space Agency X-ray observatory XMM-Newton, each individual pointing
can result in the detection of hundreds of X-ray sources, most of which are
newly discovered. Recently, many improvements in the XMM-Newton data reduction
algorithms have been made. These include enhanced source characterisation and
reduced spurious source detections, refined astrometric precision, greater net
sensitivity and the extraction of spectra and time series for fainter sources,
with better signal-to-noise. Further, almost 50\% more observations are in the
public domain compared to 2XMMi-DR3, allowing the XMM-Newton Survey Science
Centre (XMM-SSC) to produce a much larger and better quality X-ray source
catalogue. The XMM-SSC has developed a pipeline to reduce the XMM-Newton data
automatically and using improved calibration a new catalogue version has been
produced from XMM-Newton data made public by 2013 Dec. 31 (13 years of data).
Manual screening ensures the highest data quality. This catalogue is known as
3XMM. In the latest release, 3XMM-DR5, there are 565962 X-ray detections
comprising 396910 unique X-ray sources. For the 133000 brightest sources,
spectra and lightcurves are provided. For all detections, the positions on the
sky, a measure of the quality of the detection, and an evaluation of the X-ray
variability is provided, along with the fluxes and count rates in 7 X-ray
energy bands, the total 0.2-12 keV band counts, and four hardness ratios. To
identify the detections, a cross correlation with 228 catalogues is also
provided for each X-ray detection. 3XMM-DR5 is the largest X-ray source
catalogue ever produced. Thanks to the large array of data products, it is an
excellent resource in which to find new and extreme objects.Comment: 23 pages, version accepted for publication in A&
A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
Most ultraluminous X-ray sources have a typical set of properties not seen in Galactic stellar-mass black holes. They have luminosities of more than 3 × 10 39 ergs per second, unusually soft X-ray components (with a typical temperature of less than about 0.3 kiloelectronvolts) and a characteristic downturn in their spectra above about 5 kiloelectronvolts. Such puzzling properties have been interpreted either as evidence of intermediate-mass black holes or as emission from stellar-mass black holes accreting above their Eddington limit, analogous to some Galactic black holes at peak luminosity. Recently, a very soft X-ray spectrum was observed in a rare and transient stellar-mass black hole. Here we report that the X-ray source P13 in the galaxy NGC 7793 is in a binary system with a period of about 64 days and exhibits all three canonical properties of ultraluminous sources. By modelling the strong optical and ultraviolet modulations arising from X-ray heating of the B9Ia donor star, we constrain the black hole mass to be less than 15 solar masses. Our results demonstrate that in P13, soft thermal emission and spectral curvature are indeed signatures of supercritical accretion. By analogy, ultraluminous X-ray sources with similar X-ray spectra and luminosities of up to a few times 10 40 ergs per second can be explained by supercritical accretion onto massive stellar-mass black holes
Spectroscopy of Optical Counterparts of Ultraluminous X-ray Sources
Here we present the results of panoramic and long-slit observations of eight
ULX nebular counterparts held with the 6m SAO telescope. In two ULXNe we
detected for the first time signatures of high excitation ([OIII]5007 / H\beta
> 5). Two of the ULXs were identified with young (T ~ 5-10 Myr) massive star
clusters. Four of the eight ULX Nebulae (ULXNe) show bright high-excitation
lines. This requires existence of luminous (~ 10^{38} .. 10^{40} erg/s) UV/EUV
sources coinciding with the X-ray sources. Other 4 ULXNe require shock
excitation of the gas with shock velocities of 20-100km/s. However, all the
studied ULXN spectra show signatures of shock excitation, but even those ULXNe
where the shocks are prevailing show presence of a hard ionizing source with
the luminosity at least ~10^{38} erg/s. Most likely shock waves, X-ray and EUV
ionization act simultaneously in all the ULXNe, but they may be roughly
separated in two groups, shock-dominated and photoionization-dominated ULXNe.
The ULXs have to produce strong winds and/or jets powering their nebulae with
\~10^{39} erg/s. Both the wind/jet activity and the EUV source needed are
consistent with the suggestion that ULXs are high-mass X-ray binaries with the
supercritical accretion disks of the SS433 type.Comment: submitted to Astrophysical Bulletin (Bull. Special Astrophys. Obs.
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