12,594 research outputs found
Swift monitoring of the new accreting millisecond X-ray pulsar IGRJ17511-3057 in outburst
A new accreting millisecond X-ray pulsar, IGR J17511-3057, was discovered in
outburst on 2009 September 12 during the INTEGRAL Galactic bulge monitoring
programme. To study the evolution of the source X-ray flux and spectral
properties during the outburst, we requested a Swift monitoring of
IGRJ17511-3057. In this paper we report on the results of the first two weeks
of monitoring the source. The persistent emission of IGR J17511-3057 during the
outburst is modeled well with an absorbed blackbody (kT~0.9 keV) and a
power-law component (photon index~1-2), similar to what has been observed from
other previously known millisecond pulsars. Swift also detected three type-I
Xray bursts from this source. By assuming that the peak luminosity of these
bursts is equal to the Eddington value for a pure helium type-I X-ray burst, we
derived an upper limit to the source distance of ~10 kpc. The theoretical,
expected recurrence time of the bursts according to the helium burst hypothesis
is 0.2-0.9 days, in agreement with the observations.Comment: Accepted for publication in A&A Letters. V2: corrected some typos and
added one referenc
The discovery of 12min X-ray pulsations from 1WGA J1958.2+3232
During a systematic search for periodic signals in a sample of ROSAT PSPC
(0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray
pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the
galactic plane. The energy spectrum is well fit by a power law with a photon
index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1.
The source is probably a long period, low luminosity X-ray pulsar, similar to X
Per, or an intermediate polar.Comment: 5 pages (figures included). Accepted for publication on MNRA
Quiescent Thermal Emission from the Neutron Star in Aql X-1
We report on the quiescent spectrum measured with Chandra/ACIS-S of the
transient, type-I X-ray bursting neutron star Aql X-1, immediately following an
accretion outburst. The neutron star radius, assuming a pure hydrogen
atmosphere and hard power-law spectrum, is =13.4{+5}{-4} (d/5 \kpc)
km. Based on the historical outburst record of RXTE/ASM, the quiescent
luminosity is consistent with that predicted by Brown, Bildsten and Rutledge
from deep crustal heating, lending support to this theory for providing a
minimum quiescent luminosity of transient neutron stars. While not required by
the data, the hard power-law component can account for 18+/-8% of the 0.5-10
keV thermal flux. Short-timescale intensity variability during this observation
is less than 15% rms (3 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the
Chandra spectrum and three X-ray spectral observations made between Oct 1992
and Oct 1996 find all spectra consistent with a pure H atmosphere, but with
temperatures ranging from 145--168 eV, spanning a factor of 1.87+/-0.21 in
observed flux. The source of variability in the quiescent luminosity on long
timescales (greater than years) remains a puzzle. If from accretion, then it
remains to be explained why the quiescent accretion rate provides a luminosity
so nearly equal to that from deep crustal heating.Comment: 15 pages, 1 figure, 2 tables; ApJ, accepte
On the bolometric quiescent luminosity and luminosity swing of black hole candidate and neutron star low mass X-ray transients
Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs)
display on average a factor of ~100 larger swing in the minimum (quiescent) to
maximum (outburst) X-ray luminosity than neutron stars (NSs), despite the fact
that the swing in the mass inflow rate is likely in the same range. Advection
dominated accretion flows (ADAFs) were proposed to interpret such a difference.
The residual optical/UV emission of quiescent LMXRTs, after subtraction of the
companion star spectrum, is produced by synchrotron radiation in the (latest
version) of ADAF and therefore is part of the ADAF's luminosity budget. We
demonstrate that, once the residual optical/UV emission is taken into account,
the bolometric luminosity swing of BHCs is consistent with that of NSs. We
explore here an alternative scenario to ADAFs in which very little mass
accretion onto the collapsed star takes place in the quiescence intervals. The
residual optical/UV emission of BHCs are expected to derive from the energy
released by the matter transferred from the companion star at radii comparable
to the circularisation radius. The quiescent X-ray luminosity originates either
from accretion onto the BH at very low rates and/or from coronal activity in
the companion star or in the outer disk. For comparably small mass inflow
rates, the NSs in these systems are likely in the radio pulsar regime. In the
interaction of the radio pulsar relativistic wind with matter transferred from
the companion star, a shock forms, the power law-like emission of which powers
both the harder X-ray emission and most of the residual optical/UV. The soft,
thermal-like X-ray component may arise from the cooling of the NS surface. This
scenario matches well both the X-ray and bolometric luminosity swing of LMXRTs.
(ABRIDGED).Comment: 13 pages (including 2 postscript figures - use emulateapj macro).
Accepted for publication in Ap
The LOFT (Large Observatory for X-ray Timing) background simulations
The Large Observatory For X-ray Timing (LOFT) is an innovative medium-class
mission selected for an assessment phase in the framework of the ESA M3 Cosmic
Vision call. LOFT is intended to answer fundamental questions about the
behaviour of matter in the very strong gravitational and magnetic fields around
compact objects. With an effective area of ~10 m^2 LOFT will be able to measure
very fast variability in the X-ray fluxes and spectra. A good knowledge of the
in-orbit background environment is essential to assess the scientific
performance of the mission and to optimize the instrument design. The two main
contributions to the background are cosmic diffuse X-rays and high energy
cosmic rays; also, albedo emission from the Earth is significant. These
contributions to the background for both the Large Area Detector and the Wide
Field Monitor are discussed, on the basis of extensive Geant-4 simulations of a
simplified instrumental mass model.Comment: Proceedings of SPIE, Vol. 8443, Paper No. 8443-209, 201
XMM-Newton and Swift observations of XTE J1743-363
XTEJ1743-363 is a poorly known hard X-ray transient, that displays short and
intense flares similar to those observed from Supergiant Fast X-ray Transients.
The probable optical counterpart shows spectral properties similar to those of
an M8 III giant, thus suggesting that XTEJ1743-363 belongs to the class of the
Symbiotic X-ray Binaries. In this paper we report on the first dedicated
monitoring campaign of the source in the soft X-ray range with XMM-Newton and
Swift/XRT. T hese observations confirmed the association of XTEJ1743-363 with
the previously suggested M8 III giant and the classification of the source as a
member of the Symbiotic X-ray binaries. In the soft X-ray domain, XTEJ1743-363
displays a high absorption (~6x10^22 cm^-2 ) and variability on time scales of
hundreds to few thousand seconds, typical of wind accreting systems. A
relatively faint flare (peak X-ray flux 3x10^-11 erg/cm^2/s) lasting ~4 ks is
recorded during the XMM-Newton observation and interpreted in terms of the wind
accretion scenario.Comment: Accepted for publication on A&
The Swift X-ray Telescope Cluster Survey II. X-ray spectral analysis
(Abridged) We present a spectral analysis of a new, flux-limited sample of 72
X-ray selected clusters of galaxies identified with the X-ray Telescope (XRT)
on board the Swift satellite down to a flux limit of ~10-14 erg/s/cm2 (SWXCS,
Tundo et al. 2012). We carry out a detailed X-ray spectral analysis with the
twofold aim of measuring redshifts and characterizing the properties of the
Intra-Cluster Medium (ICM). Optical counterparts and spectroscopic or
photometric redshifts are obtained with a cross-correlation with NED.
Additional photometric redshifts are computed with a dedicated follow-up
program with the TNG and a cross-correlation with the SDSS. We also detect the
iron emission lines in 35% of the sample, and hence obtain a robust measure of
the X-ray redshift zX. We use zX whenever the optical redshift is not
available. Finally, for all the sources with measured redshift,
background-subtracted spectra are fitted with a mekal model. We perform
extensive spectral simulations to derive an empirical formula to account for
fitting bias. The bias-corrected values are then used to investigate the
scaling properties of the X-ray observables. Overall, we are able to
characterize the ICM of 46 sources. The sample is mostly constituted by
clusters with temperatures between 3 and 10 keV, plus 14 low-mass clusters and
groups with temperatures below 3 keV. The redshift distribution peaks around
z~0.25 and extends up to z~1, with 60% of the sample at 0.1<z<0.4. We derive
the Luminosity-Temperature relation for these 46 sources, finding good
agreement with previous studies. The quality of the SWXCS sample is comparable
to other samples available in the literature and obtained with much larger
X-ray telescopes. Our results have interesting implications for the design of
future X-ray survey telescopes, characterised by good-quality PSF over the
entire field of view and low background.Comment: 27 pages, 15 figures; minor typos corrected. To be published in A&A,
Volume 567, July 2014. Websites of the SWXCS project:
http://www.arcetri.astro.it/SWXCS/ and http://swxcs.ustc.edu.cn
On the Chandra X-ray Sources in the Galactic Center
Recent deep Chandra surveys of the Galactic center region have revealed the
existence of a faint, hard X-ray source population. While the nature of this
population is unknown, it is likely that several types of stellar objects
contribute. For sources involving binary systems, accreting white dwarfs and
accreting neutron stars with main sequence companions have been proposed. Among
the accreting neutron star systems, previous studies have focused on stellar
wind-fed sources. In this paper, we point out that binary systems in which mass
transfer occurs via Roche lobe overflow (RLOF) can also contribute to this
X-ray source population.
A binary population synthesis study of the Galactic center region has been
carried out, and it is found that evolutionary channels for neutron star
formation involving the accretion induced collapse of a massive ONeMg white
dwarf, in addition to the core collapse of massive stars, can contribute to
this population. The RLOF systems would appear as transients with quiescent
luminosities, above 2 keV, in the range from 10^31-10^32 ergs/s. The results
reveal that RLOF systems primarily contribute to the faint X-ray source
population in the Muno et al. (2003) survey and wind-fed systems can contribute
to the less sensitive Wang et al. (2002) survey. However, our results suggest
that accreting neutron star systems are not likely to be the major contributor
to the faint X-ray source population in the Galactic center.Comment: 12 pages, 3 figures, 1 table ApJ in press (Dec 2004). Substantial
change
The supergiant fast X-ray transients XTE J1739-302 and IGR J08408-4503 in quiescence with XMM-Newton
Context. Supergiant fast X-ray transients are a subclass of high mass X-ray
binaries that host a neutron star accreting mass from the wind of its OB
supergiant companion. They are characterized by an extremely pronounced and
rapid variability in X-rays, which still lacks an unambiguous interpretation. A
number of deep pointed observations with XMM-Newton have been carried out to
study the quiescent emission of these sources and gain insight into the
mechanism that causes their X-ray variability. Aims. We continued this study by
using three XMM-Newton observations of the two supergiant fast X-ray transient
prototypes XTEJ1739-302 and IGR J08408-4503 in quiescence. Methods. An in-depth
timing and spectral analysis of these data have been carried out. Results. We
found that the quiescent emission of these sources is characterized by both
complex timing and spectral variability, with multiple small flares occurring
sporadically after periods of lower X-ray emission. Some evidence is found in
the XMM-Newton spectra of a soft component below ~2 keV, similar to that
observed in the two supergiant fast X-ray transients AXJ1845.0-0433 and
IGRJ16207-5129 and in many other high mass X-ray binaries. Conclusions.We
suggest some possible interpretations of the timing and spectral properties of
the quiescent emission of XTEJ1739- 302 and IGR J08408-4503 in the context of
the different theoretical models proposed to interpret the behavior of the
supergiant fast X-ray transients.Comment: 13 pages, 14 figures. Accepted for publication in A&A. V2: Corrected
few typo
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