ABSTRACT: A subset of ultraluminous X-ray sources (those with luminosities of less than 10(40) erg s(-1); ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of ∼5-20, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 10(39) erg s(-1). The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.
Nature 12/2012; · 36.28 Impact Factor
ABSTRACT: The Large Magellanic Cloud (LMC) hosts a rich and varied population of
supernova remnants (SNRs). Optical, X-ray, and radio observations are required
to identify these SNRs, as well as to ascertain the various processes
responsible for the large array of physical characteristics observed. In this
paper we attempted to confirm the candidate SNR [HP99] 1234, identified in
X-rays with ROSAT, as a true SNR by supplementing these X-ray data with optical
and radio observations. Optical data from the Magellanic Cloud Emission Line
Survey (MCELS) and new radio data from the Molonglo Observatory Synthesis
Telescope (MOST), in addition to the ROSAT X-ray data, were used to perform a
morphological analysis of this candidate SNR. An approximately ellipsoidal
shell of enhanced [SII], typical of an SNR ([SII]/Halpha > 0.4), was detected
in the optical. This enhancement is coincident with faint radio emission at 36
cm. Using the available data we estimated the size of the remnant to be ~5.1' x
4.0' (~75 pc x 59 pc). However, the measurement along the major-axis was
somewhat uncertain due to a lack of optical and radio emission at its
extremities and the poor resolution of the X-ray data. Assuming this SNR is in
the Sedov phase and adopting the ambient mass density of 1.2x10^-25 g cm^-3
measured in a nearby HII region, an age estimate of ~25 kyr was calculated for
a canonical initial explosion energy of 10^51 erg. However, this age estimate
should be treated cautiously due to uncertainties on the adopted parameters.
Analysis of the local stellar population suggested a type Ia event as a
precursor to this SNR, however, a core-collapse mechanism could not be ruled
out due to the possibility of the progenitor being a runaway massive star. With
the detection of X-ray, radio and optical line emission with enhanced [SII],
this object was confirmed as an SNR and we assign the identifier MCSNR
ABSTRACT: We present the analysis of supernova remnants (SNRs) and candidates in M31
identified in the XMM-Newton large programme survey of M31. SNRs are among the
bright X-ray sources in a galaxy. They are good indicators of recent star
formation activities of a galaxy and of the interstellar environment in which
they evolve. By combining the X-ray data of sources in M31 with optical data as
well as with optical and radio catalogues, we aim to compile a complete,
revised list of SNRs emitting X-rays in M31 detected with XMM-Newton, study
their luminosity and spatial distribution, and understand the X-ray spectrum of
the brightest SNRs. We analysed the X-ray spectra of the twelve brightest SNRs
and candidates using XMM-Newton data. The four brightest sources allowed us to
perform a more detailed spectral analysis and the comparison of different
models to describe their spectrum. For all M31 large programme sources we
searched for optical counterparts on the Ha, [Sii], and [Oiii] images of the
Local Group Galaxy Survey. We confirm 21 X-ray sources as counterparts of known
SNRs. In addition, we identify five new X-ray sources as X-ray and optically
emitting SNRs. Seventeen sources are no longer considered as SNR candidates. We
have thus created a list of 26 X-ray SNRs and 20 candidates in M31 based on
their X-ray, optical, and radio emission, which is the most recent complete
list of X-ray SNRs in M31. The brightest SNRs have X-ray luminosities of up to
8 x 10^36 erg/s in the 0.35 - 2.0 keV band.
ABSTRACT: The Rapid Burster (MXB 1730-335) is a unique object, showing both type I and
type II X-ray bursts. A type I burst of the Rapid Burster was observed with
Swift/XRT on 2009 March 5, showing photospheric radius expansion for the first
time in this source. We report here on the mass and radius determination from
this photospheric radius expansion burst using a Bayesian approach. After
marginalization over the likely distance of the system (5.8-10 kpc) we obtain
M=1.1+/-0.3 M_sun and R=9.6+/-1.5 km (1-sigma uncertainties) for the compact
object, ruling out the stiffest equations of state for the neutron star. We
study the sensitivity of the results to the distance, the color correction
factor, and the hydrogen mass fraction in the envelope. We find that only the
distance plays a crucial role.
ABSTRACT: We report on follow-up observations of candidate X-ray-bright, radio-quiet isolated neutron stars (INSs) identified from correlations of the ROSAT All-Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS) Data Release 4 in Agüeros et al. We obtained Chandra X-ray Observatory exposures for 13 candidates in order to pinpoint the source of X-ray emission in optically blank RASS error circles. These observations eliminated 12 targets as good INS candidates. We discuss subsequent observations of the remaining candidate with XMM-Newton, the Gemini North Observatory, and the Apache Point Observatory. We identify this object as a likely extragalactic source with an unusually high log (f X/f opt) ~ 2.4. We also use an updated version of the population synthesis models of Popov et al. to estimate the number of RASS-detected INSs in the SDSS Data Release 7 footprint. We find that these models predict ~3-4 INSs in the 11,000 deg2 imaged by SDSS, which is consistent with the number of known INSs that fall within the survey footprint. In addition, our analysis of the four new INS candidates identified in the SDSS footprint implies that they are unlikely to be confirmed as INSs; together, these results suggest that new INSs are not likely to be found from further correlations of the RASS and SDSS.
The Astronomical Journal 04/2011; 141(6):176. · 4.03 Impact Factor
ABSTRACT: We report on follow-up observations of candidate X-ray bright, radio-quiet
isolated neutron stars (INSs) identified from correlations of the ROSAT All-Sky
Survey (RASS) and the Sloan Digital Sky Survey (SDSS) Data Release 4 in
Ag\"ueros et al. (2006). We obtained Chandra X-ray Telescope exposures for 13
candidates in order to pinpoint the source of X-ray emission in optically blank
RASS error circles. These observations eliminated 12 targets as good INS
candidates. We discuss subsequent observations of the remaining candidate with
the XMM-Newton X-ray Observatory, the Gemini North Observatory, and the Apache
Point Observatory. We identify this object as a likely extragalactic source
with an unusually high log(fX/fopt) ~ 2.4. We also use an updated version of
the population synthesis models of Popov et al. (2010) to estimate the number
of RASS-detected INSs in the SDSS Data Release 7 footprint. We find that these
models predict ~3-4 INSs in the 11,000 square deg imaged by SDSS, which is
consistent with the number of known INSs that fall within the survey footprint.
In addition, our analysis of the four new INS candidates identified by Turner
et al. (2010) in the SDSS footprint implies that they are unlikely to be
confirmed as INSs; together, these results suggest that new INSs are not likely
to be found from further correlations of the RASS and SDSS.
ABSTRACT: Aims: We study the diffuse X-ray emission observed in the field of view of
the pulsar B 0540-69 in the Large Magellanic Cloud (LMC) by XMM-Newton. We want
to understand the nature of this soft diffuse emission, which coincides with
the superbubble in the HII region N 158, and improve our understanding of the
evolution of superbubbles. Methods: We analyse the XMM-Newton spectra of the
diffuse emission. Using the parameters obtained from the spectral fit, we
perform calculations of the evolution of the superbubble. The mass loss and
energy input rates are based on the initial mass function (IMF) of the observed
OB association inside the superbubble. Results: The analysis of the spectra
shows that the soft X-ray emission arises from hot shocked gas surrounded by a
thin shell of cooler, ionised gas. We show that the stellar winds alone cannot
account for the energy inside the superbubble, but the energy release of 2 - 3
supernova explosions in the past ~1 Myr provides a possible explanation.
Conclusions: The combination of high sensitivity X-ray data, allowing spectral
analysis, and analytical models for superbubbles bears the potential to reveal
the evolutionary state of interstellar bubbles, if the stellar content is
ABSTRACT: Chandra data of the X-ray source [PMH2004] 47 were obtained in the ACIS Survey of M 33 (ChASeM33) in 2006. During one of the observations, the source varied from a high state to a low state and back, in two other observations it varied from a low state to respectively intermediate states. These transitions are interpreted as eclipse ingresses and egresses of a compact object in a high-mass X-ray binary (HMXB) system. The phase of mideclipse is given by HJD 245 3997.476 ± 0.006, the eclipse half angle is 306 ± 12. Adding XMM-Newton observations of [PMH2004] 47 in 2001 we determine the binary period to be 1.732479 ± 0.000027 days. This period is also consistent with ROSAT HRI observations of the source in 1994. No short-term periodicity compatible with a rotation period of the compact object is detected. There are indications for a long-term variability similar to that detected for Her X-1. During the high state the spectrum of the source is hard (power-law spectrum with photon index ~0.85) with an unabsorbed luminosity of 2 ×1037 erg s–1 (0.2-4.5 keV). We identify as an optical counterpart a V ~ 21.0 mag star with T eff>19000 K, log(g)>2.5. The Canada-France-Hawaii Telescope optical light curves for this star show an ellipsoidal variation with the same period as the X-ray light curve. The optical light curve together with the X-ray eclipse can be modeled by a compact object with a mass consistent with a neutron star or a black hole in an HMXB. However, the hard power-law X-ray spectrum favors a neutron star as the compact object in this second eclipsing X-ray binary in M 33. Assuming a neutron star with a canonical mass of 1.4 M ☉ and the best-fit companion temperature of 33,000 K, a system inclination i = 72° and a companion mass of 10.9 M ☉ are implied.
The Astrophysical Journal 03/2009; 694(1):449. · 6.02 Impact Factor
ABSTRACT: The Chandra ACIS Survey of M33 (ChASeM33) has acquired seven fields of ACIS data covering M33 with 200 ks of exposure in each field. A catalog from the first 10 months of data, along with archival Chandra observations dating back to the year 2000, is currently available. We have searched these data for transient sources that are measured to have a 0.35-8.0 keV unabsorbed luminosity of at least 4 × 1035 ergs s−1 in one epoch and are not detected in another epoch. This set of the survey data has yielded seven such sources, including one previously known supersoft source. We analyzed XMM-Newton data from the archive distributed over the years 2000-2003 to search for recurrent outbursts and to get a spectrum for the supersoft transient. We find only one recurrent transient in our sample. The X-ray spectra, light curves, and optical counterpart candidates of two of the other sources suggest that they are high-mass X-ray binaries. Archival Spitzer photometry and high X-ray absorption suggest that one of the sources is a highly variable background active galactic nucleus. The other three sources are more difficult to classify. The bright transient population of M33 appears to contain a large fraction of high-mass X-ray binaries compared with the transient populations of M31 and the Galaxy, reflecting the later morphology of M33.
The Astrophysical Journal 12/2008; 680(2):1120. · 6.02 Impact Factor
ABSTRACT: We present an overview of the Chandra ACIS Survey of M33 (ChASeM33): A Deep Survey of the Nearest Face-on Spiral Galaxy. The 1.4 Ms survey covers the galaxy out to R ≈ 18'(≈ 4 kpc) . These data provide the most intensive, high spatial resolution assessment of the X-ray source populations available for the confused inner regions of M33. Mosaic images of the ChASeM33 observations show several hundred individual X-ray sources as well as soft diffuse emission from the hot interstellar medium. Bright, extended emission surrounds the nucleus and is also seen from the giant H II regions NGC 604 and IC 131. Fainter extended emission and numerous individual sources appear to trace the inner spiral structure. The initial source catalog, arising from ~2/3 of the expected survey data, includes 394 sources significant at the 3 σ confidence level or greater, down to a limiting luminosity (absorbed) of ~1.6 × 1035 ergs s−1 (0.35-8.0 keV). The hardness ratios of the sources separate those with soft, thermal spectra such as supernova remnants from those with hard, nonthermal spectra such as X-ray binaries and background active galactic nuclei. Emission extended beyond the Chandra point-spread function is evident in 23 of the 394 sources. Cross-correlation of the ChASeM33 sources against previous catalogs of X-ray sources in M33 results in matches for the vast majority of the brighter sources and shows 28 ChASeM33 sources within 10'' of supernova remnants identified by prior optical and radio searches. This brings the total number of such associations to 31 out of 100 known supernova remnants in M33.
The Astrophysical Journal Supplement Series 12/2008; 174(2):366. · 13.46 Impact Factor
ABSTRACT: The flight calibration of the spectral response of CCD instruments below 1.5 keV is difficult in general because of the lack of strong lines in the on-board calibration sources typically available. We have been using 1E 0102.2-7219, the brightest supernova remnant in the Small Magellanic Cloud, to evaluate the response models of the ACIS CCDs on the Chandra X-ray Observatory (CXO), the EPIC CCDs on the XMM-Newton Observatory, the XIS CCDs on the Suzaku Observatory, and the XRT CCD on the Swift Observatory. E0102 has strong lines of O, Ne, and Mg below 1.5 keV and little or no Fe emission to complicate the spectrum. The spectrum of E0102 has been well characterized using high-resolution grating instruments, namely the XMM-Newton RGS and the CXO HETG, through which a consistent spectral model has been developed that can then be used to fit the lower-resolution CCD spectra. We have also used the measured intensities of the lines to investigate the consistency of the effective area models for the various instruments around the bright O (~570 eV and 654 eV) and Ne (~910 eV and 1022 eV) lines. We find that the measured fluxes of the O VII triplet, the O VIII Ly-alpha line, the Ne IX triplet, and the Ne X Ly-alpha line generally agree to within +/-10 % for all instruments, with 28 of our 32 fitted normalizations within +/-10% of the RGS-determined value. The maximum discrepancies, computed as the percentage difference between the lowest and highest normalization for any instrument pair, are 23% for the O VII triplet, 24% for the O VIII Ly-alpha line, 13% for the Ne IX triplet, and 19% for the Ne X Ly-alpha line. If only the CXO and XMM are compared, the maximum discrepancies are 22% for the O VII triplet, 16% for the O VIII Ly-alpha line, 4% for the Ne IX triplet, and 12% for the Ne X Ly-alpha line. Comment: 16 pages, 11 figures, to be published in Proceedings of the SPIE 7011: Space Telescopes and Instrumentation II: Ultraviolet to Gamma Ray 2008
ABSTRACT: We report on the latest (2007 Jan) observations of supernova remnant (SNR) 1987A from the XMM-Newton mission. Since the 2003 May observations of Haberl et al. (2006), 11 emission lines have experienced increases in flux by factors ~ 3 to 10, with the 775 eV line of O VIII showing the greatest increase; we have observed 6 lines of Fe XVII and Fe XVIII previously unreported by XMM-Newton. A two-shock model representing plasmas in non-equilibrium ionization is fitted to the EPIC-pn spectra, yielding temperatures of ~ 0.4 and ~ 3 keV, as well as elemental abundances for N, O, Ne, Mg, Si, S and Fe. We demonstrate that the abundance ratio of N and O can be constrained to less than ~20% accuracy. Within the same confidence interval, the same analysis suggests that the C+N+O abundance varies from ~ 1.1 to 1.4 X 10^-4. Normalizing our obtained abundances by the Large Magellanic Cloud (LMC) values of Hughes, Hayashi & Koyama (1998), we find that O, Ne, Mg and Fe are under-abundant, while Si and S are over-abundant, consistent with the findings of Aschenbach (2007). Such a result has implications for both the single-star and binary accretion/merger models for the progenitor of SNR 1987A. In the context of the binary merger scenario proposed by Morris & Podsiadlowski (2006, 2007), material forming the inner, equatorial ring was expelled after the merger, implying that either our derived Fe abundance is inconsistent with typical LMC values or that iron is under-abundant at the site of the progenitor star of SNR 1987A. Comment: 14 pages, 10 diagrams (2 omitted). Accepted by ApJ
ABSTRACT: The black hole candidate XTE J1817-330 was discovered in outburst on 26 January 2006 with RXTE/ASM. One year later, on 28 February 2007, another X-ray transient discovered in 1996, XTE J1856+053, was detected by RXTE during a new outburst. We report on the spectra obtained by XMM-Newton of these two black hole candidates.
ABSTRACT: The new black hole candidate XTEJ1817-330, discovered on 26 January 2006 with RXTE, was observed with XMM-Newton and INTEGRAL
in February and March 2006, respectively. The X-ray spectrum is dominated by the thermal emission of the accretion disk in
the soft band, with a low absorption column density (N
H=1.77(±0.01)×1021cm−2) and a maximum disk temperature kT
max=0.68(±0.01)keV, plus a power law component, with the photon index decreasing from 2.66±0.02 to 1.98±0.07 between the two
observations. Several interstellar absorption lines are detected in the X-ray spectrum, corresponding to OI, OII, OIII,
OVII and FeXXIV. We constrain the distance to the system to be in the range 1–5kpc.
Astrophysics and Space Science 05/2007; 309(1):315-319. · 1.69 Impact Factor
ABSTRACT: We present a first analysis of a deep X-ray spectrum of the isolated neutron star RBS1223 obtained with XMM-Newton. Spectral
data from four new monitoring observations in 2005/2006 were combined with archival observations obtained in 2003 and 2004
to form a spin-phase averaged spectrum containing 290 000 EPIC-pn photons. This spectrum shows higher complexity than its
predecessors, and can be parameterised with two Gaussian absorption lines superimposed on a blackbody. The line centers, E
1, could be regarded as supporting the cyclotron interpretation of the absorption features in a field B∼4×1013G. The flux ratio of those lines does not support this interpretation. Hence, either feature might be of truly atomic origin.
Astrophysics and Space Science 03/2007; 308(1):619-623. · 1.69 Impact Factor
ABSTRACT: The excellent spatial resolution of the Chandra observatory offers the unprecedented possibility to measure proper motions
at X-ray wavelength with relatively high accuracy using as reference the background of extragalactic or remote galactic X-ray
sources. We took advantage of this capability to constrain the proper motion of RXJ0806.4-4123 and RXJ0420.0-5022, two X-ray
bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and lacking an optical counterpart. In this paper,
we present results from a preliminary analysis from which we derive 2σ upper limits of 76 mas/yr and 138 mas/yr on the proper motions of RXJ0806.4-4123 and RXJ0420.0-5022 respectively. We use
these values together with those of other ROSAT discovered INSs to constrain the origin, distance and evolutionary status
of this particular group of objects. We find that the tangential velocities of radio quiet ROSAT neutron stars are probably
consistent with those of ‘normal’ pulsars. Their distribution on the sky and, for those having accurate proper motion vectors,
their possible birth places, all point to a local population, probably created in the part of the Gould Belt nearest to the
Astrophysics and Space Science 01/2007; 308(1):217-224. · 1.69 Impact Factor
ABSTRACT: A growing number of early Be stars exhibit unusually hard X-ray spectra and luminosities intermediate between those typical of early type stars and those emitted by most Be/X-ray binaries in quiescence. We report on XMM-Newton and optical observations of two such Be stars, SAO 49725 and HD 161103. The nature of the hard-thermal X-ray emission is discussed in the light of the models proposed for Gamma Cas, magnetic disc-star interaction or accretion onto a compact companion object - neutron star or white dwarf. These two new objects added to similar cases discovered in XMM-Newton surveys point at the emergence of a new class of Gamma Cas analogs. Comment: 14 pages, 9 figs, accepted for publication in A&A
ABSTRACT: A growing number of early Be stars discovered in X-ray surveys exhibit X-ray luminosities intermediate between those of normal stars and those of most Be/X-ray binaries in quiescence. Their X-ray spectra are also much harder than those of shocked wind OB stars and can be best fitted by a thin thermal plasma with T ~ 10^8 K, added to a cooler and much fainter thermal component. An iron line complex including a fluorescence component is detected in many cases. There is no evidence for coherent pulsations in any of these systems but strong variability on time scales as short as 100 s is usually observed. Large oscillations with quasi-periods of the order of one hour or more are detected in the X-ray light curves of several sources, but have so far failed to prove to be strictly periodic. The optical and X-ray properties of these new objects strikingly resemble those of the so far unique and enigmatic Be star Gamma-Cas and define a new class of X-ray emitters. We discuss the possible origin of the X-ray emission in the light of the models proposed for Gamma-Cas, magnetic disc-star interaction or accretion onto a compact companion object -- neutron star or white dwarf.
ABSTRACT: We present results of a sequence of XMM-Newton observations of the two microquasars GRO J1655-40 and GRS 1915+105. The observations were preformed using the EPIC-pn camera in the Burst mode. The observations of GRO J1655-40 in a bright state have made possible a substantial improvement in the calibration of the Burst mode, with determination of the rate dependence of the Charge Transfer Efficiency (CTE). We detect He-like Fe K-shell absorption features in the EPIC-pn spectrum of GRO J1655-40, indicating the presence of a highly ionized absorber, and clear absorption features at 0.71 and 0.72 keV in the RGS spectrum, most probably identified as blueshifted Fe XVIII.
ABSTRACT: Only seven radio-quiet isolated neutron stars (INSs) emitting thermal X rays are known, a sample that has yet to definitively address such fundamental issues as the equation of state of degenerate neutron matter. We describe a selection algorithm based on a cross-correlation of the ROSAT All-Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS) that identifies X-ray error circles devoid of plausible optical counterparts to the SDSS g~22 magnitudes limit. We quantitatively characterize these error circles as optically blank; they may host INSs or other similarly exotic X-ray sources such as radio-quiet BL Lacs, obscured AGN, etc. Our search is an order of magnitude more selective than previous searches for optically blank RASS error circles, and excludes the 99.9% of error circles that contain more common X-ray-emitting subclasses. We find 11 candidates, nine of which are new. While our search is designed to find the best INS candidates and not to produce a complete list of INSs in the RASS, it is reassuring that our number of candidates is consistent with predictions from INS population models. Further X-ray observations will obtain pinpoint positions and determine whether these sources are entirely optically blank at g~22, supporting the presence of likely isolated neutron stars and perhaps enabling detailed follow-up studies of neutron star physics.