14,679 research outputs found
The evolution of the high energy tail in the quiescent spectrum of the soft X-ray transient Aql X-1
A moderate level of variability has been detected in the quiescent luminosity
of several neutron star soft X-ray transients. Spectral variability was first
revealed by Chandra observations of Aql X-1 in the four months that followed
the 2000 X-ray outburst. By adopting the canonical model for quiescent spectrum
of soft X-ray transients, i.e. an absorbed neutron star atmosphere model plus a
power law tail, Rutledge et al. (2002a) concluded that the observed spectral
variations can be ascribed to temperature variations of the neutron star
atmosphere. These results can hardly be reconciled with the neutron star
cooling that is expected to take place in between outbursts (after deep crustal
heating in the accretion phase). Here we reanalyse the Chandra spectra of Aql
X-1, together with a long BeppoSAX observation in the same period, and propose
a different interpretation of the spectral variability: that this is due to
correlated variations of the power law component and the column density (>5, a
part of which might be intrinsic to the source), while the temperature and flux
of the neutron star atmospheric component remained unchanged. This lends
support to the idea that the power law component arises from emission at the
shock between a radio pulsar wind and inflowing matter from the companion star.Comment: 6 pages, 2 figures. Accepted for publication on Ap
XMM-Newton observations of two transient millisecond X-ray pulsars in quiescence
We report on XMM-Newton observations of two X-ray transient millisecond
pulsars (XRTMSPs). We detected XTE J0929-314 with an unabsorbed luminosity of
\~7x10^{31} erg/s. (0.5-10 keV) at a fiducial distance of 10 kpc. The quiescent
spectrum is consistent with a simple power law spectrum. The upper limit on the
flux from a cooling neutron star atmosphere is about 20% of the total flux. XTE
J1807-294 instead was not detected. We can put an upper limit on the source
quiescent 0.5-10 keV unabsorbed luminosity <4x10^{31} erg/s at 8 kpc. These
observations strenghten the idea that XRTMSPs have quiescent luminosities
significantly lower than classical neutron star transients.Comment: 4 pages including 1 figures. Accepted for publication in A&A Letter
Strong gravitational field light deflection in binary systems containing a collapsed star
Large light deflection angles are produced in the strong gravitational field
regions around neutron stars and black holes. In the case of binary systems,
part of the photons emitted from the companion star towards the collapsed
object are expected to be deflected in the direction of the earth. Based on a
semi-classical approach we calculate the characteristic time delays and
frequency shifts of these photons as a function of the binary orbital phase.
The intensity of the strongly deflected light rays is reduced by many orders of
magnitude, therefore making the observations of this phenomenon extremely
difficult. Relativistic binary systems containing a radio pulsar and a
collapsed object are the best available candidates for the detection of the
strongly deflected photons. Based on the accurate knowledge of their orbital
parameters, these systems allow to predict accurately the delays of the pulses
along the highly deflected path, such that the sensitivity to very weak signals
can be substantially improved through coherent summation over long time
intervals. We discuss in detail the cases of PSR 1913+16 and PSR 1534+12 and
find that the system geometry is far more promising for the latter. The
observation of the highly deflected photons can provide a test of general
relativity in an unprecedented strong field regime as well as a tight
constraint on the radius of the collapsed object.Comment: 7 pages, uuencoded, gzip'ed, postscript file with figures included.
Accepted for pubblication in MNRA
The optical counterpart of IGR J00291+5934 in quiescence
The recent (December 2004) discovery of the sixth accretion-powered
millisecond X-ray pulsar IGR J00291+5934 provides a very good chance to deepen
our knowledge of such systems. Although these systems are well studied at high
energies, poor informations are available for their optical/NIR counterparts
during quiescence. Up to now, only for SAX J1808.4-3658, the first discovered
system of this type, we have a secure multiband detection of its optical
counterpart in quiescence. Among the seven known system IGR J00291+5934 is the
one that resembles SAX J1808.4-3658 more closely. With the Italian 3.6 m TNG
telescope, we have performed deep optical and NIR photometry of the field of
IGR J00291+5934 during quiescence in order to look for the presence of a
variable counterpart. We present here the first multiband () detection
of the optical and NIR counterpart of IGR J00291+5934 in quiescence as well as
a deep upper limit in the band. We obtain an optical light curve that shows
variability consistent with a sinusoidal modulation at the known 2.46 hr
orbital period and present evidence for a strongly irradiated companion.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysic
Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence
We present the first optical and infrared polarimetric study of the low mass
transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed
to search for an intrinsic linear polarisation component in the system emitted
radiation that might be due, e.g., to synchrotron emission from a compact jet,
or to Thomson scattering with free electrons in an accretion disc. Multiband
(BVRI) optical polarimetric observations were obtained during two nights in
2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These
observations cover about the 30% of the 15.1 hours orbital period. J-band
observations were obtained in 2007 with the NICS (TNG) instrument at La Palma,
for a totality of 1 hour observation. We obtained 3-sigma upper limits to the
polarisation degree in all the optical bands, with the most constraining one
being in the I-band (P<0.5%). No phase-correlated variability has been noticed
in all the filters. The J-band observations provided a 6% upper limit on the
polarisation level. The constraining upper limits to the polarisation in the
optical allowed us to evaluate the contribution of the possible emission of a
relativistic particles jet to the total system radiation to be less then the
10%. This is in agreement with the observation of a spectral energy
distribution typical of a single black body of a K-spectral type main sequence
star irradiated from the compact object. Due to the low S/N ratio it was not
possible to investigate the possible dependency of the polarisation degree from
the wavelength, that could be suggestive of polarisation induced by Thomson
scattering of radiation with free electrons in the outer part of the accretion
disc. Observations with higher S/N ratio are required to examine in depth this
hypothesis, searching for significant phase-correlated variability.Comment: 7 pages, 9 figures, accepted for publication in section 7. Stellar
structure and evolution of Astronomy and Astrophysic
The quiescent X-ray emission of three transient X-ray pulsars
We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients
in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U
0115+63 and V 0332+53. These observations allowed us to study these transients
at the faintest flux levels thus far. Spectra are remarkably different from the
ones obtained at luminosities a factor >10 higher, testifying that the
quiescent emission mechanism is different. Pulsations were not detected in any
of the sources, indicating that accretion of matter down to the neutron star
surface has ceased. We conclude that the quiescent emission of the three X-ray
transients likely originates from accretion onto the magnetospheric boundary in
the propeller regime and/or from deep crustal heating resulting from
pycnonuclear reactions during the outbursts.Comment: Accepted for publication on ApJ (5 pages and 2 figures
Missing cosmic metals revealed by X-ray absorption towards distant sources
The census of heavy elements (metals) produced by all stars through cosmic
times up to present-day is limited to ~50%; of these only half are still found
within their parent galaxy. The majority of metals is expelled from galaxies
into the circumgalactic (or even more distant, intergalactic) space by powerful
galactic winds, leaving unpleasant uncertainty on the amount, thermal
properties and distribution of these key chemical species. These dispersed
metals unavoidably absorb soft X-ray photons from distant sources. We show that
their integrated contribution can be detected in the form of increasing X-ray
absorption with distance, for all kinds of high-energy cosmic sources. Based on
extensive cosmological simulations, we assess that 10\% of all cosmic
metals reside in the intergalactic medium. Most of the X-ray absorption arises
instead from a few discrete structures along the line of sight. These extended
structures, possibly pin-pointing galaxy groups, contain million degree,
metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional
~10% of cosmic metals could reside in this phase.Comment: Accepted for publication in Astronomy & Astrophysics. 9 pages, 4
figures, 1 tabl
On the detection of very high redshift Gamma Ray Bursts with Swift
We compute the probability to detect long Gamma Ray Bursts (GRBs) at z>5 with
Swift, assuming that GRBs form preferentially in low-metallicity environments.
The model fits well both the observed BATSE and Swift GRB differential peak
flux distribution and is consistent with the number of z>2.5 detections in the
2-year Swift data. We find that the probability to observe a burst at z>5
becomes larger than 10% for photon fluxes P<1 ph s^{-1} cm^{-2}, consistent
with the number of confirmed detections. The corresponding fraction of z>5
bursts in the Swift catalog is ~10%-30% depending on the adopted metallicity
threshold for GRB formation. We propose to use the computed probability as a
tool to identify high redshift GRBs. By jointly considering promptly-available
information provided by Swift and model results, we can select reliable z>5
candidates in a few hours from the BAT detection. We test the procedure against
last year Swift data: only three bursts match all our requirements, two being
confirmed at z>5. Other three possible candidates are picked up by slightly
relaxing the adopted criteria. No low-z interloper is found among the six
candidates.Comment: 5 pages, 2 figures, MNRAS in pres
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