12,859 research outputs found
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
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
Three Additional Quiescent Low-Mass X-ray Binary Candidates in 47 Tucanae
We identify through their X-ray spectra one certain (W37) and two probable
(W17 and X4) quiescent low-mass X-ray binaries (qLMXBs) containing neutron
stars in a long Chandra X-ray exposure of the globular cluster 47 Tucanae, in
addition to the two previously known qLMXBs. W37's spectrum is dominated by a
blackbody-like component consistent with radiation from the hydrogen atmosphere
of a 10 km neutron star. W37's lightcurve shows strong X-ray variability which
we attribute to variations in its absorbing column depth, and eclipses with a
probable 3.087 hour period. For most of our exposures, W37's blackbody-like
emission (assumed to be from the neutron star surface) is almost completely
obscured, yet some soft X-rays (of uncertain origin) remain. Two additional
candidates, W17 and X4, present X-ray spectra dominated by a harder component,
fit by a power-law of photon index ~1.6-3. An additional soft component is
required for both W17 and X4, which can be fit with a 10 km hydrogen-atmosphere
neutron star model. X4 shows significant variability, which may arise from
either its power-law or hydrogen-atmosphere spectral component. Both W17 and X4
show rather low X-ray luminosities, Lx(0.5-10 keV)~5*10^{31} ergs/s. All three
candidate qLMXBs would be difficult to identify in other globular clusters,
suggesting an additional reservoir of fainter qLMXBs in globular clusters that
may be of similar numbers as the group of previously identified objects. The
number of millisecond pulsars inferred to exist in 47 Tuc is less than 10 times
larger than the number of qLMXBs in 47 Tuc, indicating that for typical
inferred lifetimes of 10 and 1 Gyr respectively, their birthrates are
comparable.Comment: Accepted for publication in ApJ. 13 pages, 7 figures (2 color
Further Constraints on Thermal Quiescent X-ray Emission from SAX J1808.4-3658
We observed SAX J1808.4-3658 (1808), the first accreting millisecond pulsar,
in deep quiescence with XMM-Newton and (near-simultaneously) Gemini-South. The
X-ray spectrum of 1808 is similar to that observed in quiescence in 2001 and
2006, describable by an absorbed power-law with photon index 1.74+-0.11 and
unabsorbed X-ray luminosity L_X=7.9+-0.7*10^{31} ergs/s, for N_H=1.3*10^{21}
cm^{-2}. Fitting all the quiescent XMM-Newton X-ray spectra with a power-law,
we constrain any thermally emitting neutron star with a hydrogen atmosphere to
have a temperature less than 30 eV and L_{NS}(0.01-10 keV)<6.2*10^{30} ergs/s.
A thermal plasma model also gives an acceptable fit to the continuum. Adding a
neutron star component to the plasma model produces less stringent constraints
on the neutron star; a temperature of 36^{+4}_{-8} eV and L_{NS}(0.01-10
keV)=1.3^{+0.6}_{-0.8}*10^{31} ergs/s. In the framework of the current theory
of neutron star heating and cooling, the constraints on the thermal luminosity
of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these
neutron stars.
We compile data from the literature on the mass transfer rates and quiescent
thermal flux of the largest possible sample of transient neutron star LMXBs. We
identify a thermal component in the quiescent spectrum of the accreting
millisecond pulsar IGR J00291+5934, which is consistent with the standard
cooling model. The contrast between the cooling rates of IGR J00291+5934 and
1808 suggests that 1808 may have a significantly larger mass. This can be
interpreted as arising from differences in the binary evolution history or
initial neutron star mass in these otherwise similar systems.Comment: ApJ in press, 7 pages, 2 color figure
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&
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
Giant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries
associated with OB supergiant companions and characterised by an X-ray flaring
behaviour whose dynamical range reaches 5 orders of magnitude on timescales of
a few hundred to thousands of seconds. Current investigations concentrate on
finding possible mechanisms to inhibit accretion in SFXTs and explain their
unusually low average X-ray luminosity. We present the Swift observations of an
exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014
October 10 when the source achieved a peak luminosity of erg
s. This extends the total source dynamic range to 10, the
largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence
for pulsations at a period of 11.6 s is also reported. We show that these
observations challenge, for the first time, the maximum theoretical luminosity
achievable by an SFXT and propose that this giant outburst was due to the
formation of a transient accretion disc around the compact object.Comment: Accepted for publication in Astronomy and Astrophysics Letters. 5
pages, 5 figures, 2 table
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
Nucleon decay and atmospheric neutrinos in the Mont Blanc experiment
In the NUSEX experiment, during 2.8 years of operation, 31 fully contained events have been collected; 3 among them are nucleon decay candidates, while the others have been attributed to upsilon interactions. Limits on nucleon lifetime and determinations of upsilon interaction rates are presented
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