682 research outputs found
Swift monitoring of IGR J16418-4532
We report on the Swift observations of the candidate supergiant fast X-ray
transient (SFXT) IGR J16418-4532, which has an orbital period of ~3.7 d. Our
monitoring, for a total of ~43 ks, spans over three orbits and represents the
most intense and complete sampling along the orbital period of the light curve
of this source. If one assumes a circular orbit, the X-ray emission from this
source can be explained by accretion from a spherically symmetric clumpy wind
from a blue supergiant, composed of clumps with different masses, ranging from
~5x10^16 g to 10^21g.Comment: 4 pages; Proceedings, 5th International Symposium on High-Energy
Gamma-Ray Astronomy, (Gamma2012) Heidelberg, Germany, July 9-13th, 201
Soft X-ray characterisation of the long term properties of Supergiant Fast X-ray Transients
We perform the first high-sensitivity soft X-ray long-term monitoring with
Swift/XRT of three relatively unexplored Supergiant Fast X-ray Transients
(SFXTs), IGR J08408-4503, IGR J16328-4726, and IGR J16465-4507, whose hard
X-ray duty cycles are the lowest measured among the SFXT sample, and compare
their properties with those of the prototypical SFXTs. The behaviour of J08408
and J16328 resembles that of other SFXTs, and it is characterized by a
relatively high inactivity duty cycle (IDC) and pronounced dynamic range (DR)
in the X-ray luminosity. Like the SFXT prototypes, J08408 shows two distinct
populations of flares, the first one associated with the brightest outbursts
( erg s), the second one comprising less
bright events with 10 erg s. This
double-peaked distribution seems to be a ubiquitous feature of the extreme
SFXTs. The lower DR of J16328 suggests it is an intermediate SFXT. We find
J16465 is characterized by IDC5% and DR40, reminiscent of classical
supergiant HMXBs. The duty cycles measured with XRT are found to be comparable
with those reported previously by BAT and INTEGRAL, when the higher limiting
sensitivities of these instruments are taken into account and sufficiently long
observational campaigns are available. We prove that no clear correlation
exists between the duty cycles of the SFXTs and their orbital periods, which
makes it difficult to interpret the SFXT peculiar variability by only using
arguments related to the properties of supergiant star winds. Our findings
favour the idea that a correct interpretation of the SFXT phenomenology
requires a mechanism to strongly reduce the mass accretion rate onto the
compact object during most of its orbit around the companion, as proposed in a
number of theoretical works. [Abridged]Comment: Accepted for publication in Astronomy and Astrophysics. 18 pages, 8
figures, 8 table
3C454.3 reveals the structure and physics of its 'blazar zone'
Recent multi-wavelength observations of 3C454.3, in particular during its
giant outburst in 2005, put severe constraints on the location of the 'blazar
zone', its dissipative nature, and high energy radiation mechanisms. As the
optical, X-ray, and millimeter light-curves indicate, significant fraction of
the jet energy must be released in the vicinity of the millimeter-photosphere,
i.e. at distances where, due to the lateral expansion, the jet becomes
transparent at millimeter wavelengths. We conclude that this region is located
at ~10 parsecs, the distance coinciding with the location of the hot dust
region. This location is consistent with the high amplitude variations observed
on ~10 day time scale, provided the Lorentz factor of a jet is ~20. We argue
that dissipation is driven by reconfinement shock and demonstrate that X-rays
and gamma-rays are likely to be produced via inverse Compton scattering of
near/mid IR photons emitted by the hot dust. We also infer that the largest
gamma-to-synchrotron luminosity ratio ever recorded in this object - having
taken place during its lowest luminosity states - can be simply due to weaker
magnetic fields carried by a less powerful jet.Comment: 19 pages, 3 figures, accepted for publication in Ap
Spectral and temporal properties of the supergiant fast X-ray transient IGR J18483-0311 observed by INTEGRAL
IGR J18483-0311 is a supergiant fast X-ray transient whose compact object is
located in a wide (18.5 d) and eccentric (e~0.4) orbit, which shows sporadic
outbursts that reach X-ray luminosities of ~1e36 erg/s. We investigated the
timing properties of IGR J18483-0311 and studied the spectra during bright
outbursts by fitting physical models based on thermal and bulk Comptonization
processes for accreting compact objects. We analysed archival INTEGRAL data
collected in the period 2003-2010, focusing on the observations with IGR
J18483-0311 in outburst. We searched for pulsations in the INTEGRAL light
curves of each outburst. We took advantage of the broadband observing
capability of INTEGRAL for the spectral analysis. We observed 15 outbursts,
seven of which we report here for the first time. This data analysis almost
doubles the statistics of flares of this binary system detected by INTEGRAL. A
refined timing analysis did not reveal a significant periodicity in the
INTEGRAL observation where a ~21s pulsation was previously detected. Neither
did we find evidence for pulsations in the X-ray light curve of an archival
XMM-Newton observation of IGR J18483-0311. In the light of these results the
nature of the compact object in IGR J18483-0311 is unclear. The broadband X-ray
spectrum of IGR J18483-0311 in outburst is well fitted by a thermal and bulk
Comptonization model of blackbody seed photons by the infalling material in the
accretion column of a neutron star. We also obtained a new measurement of the
orbital period using the Swift/BAT light curve.Comment: Accepted for publication in Astronomy and Astrophysics. 8 page
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