13 research outputs found
A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
We developed a new model for the X-ray spectral fitting \xspec package which
takes into account the effects of both thermal and dynamical (i.e. bulk)
Comptonization. The model consists of two components: one is the direct
blackbody-like emission due to seed photons which are not subjected to
effective Compton scattering, while the other one is a convolution of the
Green's function of the energy operator with a blackbody-like seed photon
spectrum. When combined thermal and bulk effects are considered, the analytic
form of the Green's function may be obtained as a solution of the diffusion
Comptonization equation. Using data from the BeppoSAX, INTEGRAL and RXTE
satellites, we test our model on the spectra of a sample of six persistently
low magnetic field bright neutron star Low Mass X-ray Binaries, covering three
different spectral states. Particular attention is given to the transient
powerlaw-like hard X-ray (> 30 keV) tails that we interpret in the framework of
the bulk motion Comptonization process. We show that the values of the best-fit
delta-parameter, which represents the importance of bulk with respect to
thermal Comptonization, can be physically meaningful and can at least
qualitatively describe the physical conditions of the environment in the
innermost part of the system. Moreover, we show that in fitting the thermal
Comptonization spectra to the X-ray spectra of these systems, the best-fit
parameters of our model are in excellent agreement with those of COMPTT, a
broadly used and well established XSPEC model.Comment: 15 pages, 8 figures, accepted for publication in Ap
Hard X-ray colours of Neutron Star and Black Hole Low Mass X-ray Binaries with INTEGRAL
The X-ray spectra of Low Mass X-ray Binaries (LMXB) can change on short
time-scales, making it difficult to follow their spectral characteristics in
detail through model fitting. Colour-colour (C-C) diagrams are therefore often
used as alternative, model independent, tools to study the spectral variability
of these sources. The INTEGRAL mission, with its high sensitivity, large field
of view and good angular resolution, is well suited to study the hard X-ray
properties of LMXBs. In particular the ISGRI imager on board of INTEGRAL allows
the regular monitoring of the sources in the less frequently studied domain
above 20 keV. In this proceeding, C-C diagrams have been made with data from
the INTEGRAL public archive; a search is made for systematic differences in the
C-C diagrams between black hole candidates (BH) and neutron stars (NS) in LMXBs
using a moments analysis method.Comment: Paper from poster presentation at COSPAR meeting, Beijing, 2006. in
press: Advances in Space Research, Editor: Wynn H
A new explanation for the SFXTs outbursts
The physical mechanism responsible for the short outbursts in a recently
recognized class of High Mass X-ray Binaries, the Supergiant Fast X-ray
Transients (SFXTs), is still unknown. Recent observations performed with
Swift/XRT, XMM-Newton and INTEGRAL of the 2007 outburst from IGRJ11215-5952,
the only SFXT known to exhibit periodic outbursts, suggest a new explanation
for the outburst mechanism in this class of transients, linked to the possible
presence of a second wind component in the supergiant companion, in the form of
an equatorial wind. The applicability of the model to the short outburst
durations of all other SFXTs, where a clear periodicity in the outbursts has
not been found yet, is discussed. The scenario we are proposing also includes
the persistently accreting supergiant High Mass X-ray Binaries.Comment: Contributed talk at the conference "A population explosion: the
nature and evolution of X-ray binaries in diverse environments", held in
St.Petersburg Beach, Florida (28 October-2 November 2007); R.M.Bandyopadhyay,
S.Wachter, D.Gelino, C.R.Gelino, ed
The Dynamic X-ray Sky of the Local Universe
Over the next decade, we can expect time domain astronomy to flourish at
optical and radio wavelengths. In parallel with these efforts, a dedicated
transient "machine" operating at higher energies (X-ray band through soft
gamma-rays) is required to reveal the unique subset of events with variable
emission predominantly visible above 100 eV. Here we focus on the transient
phase space never yet sampled due to the lack of a sensitive, wide-field and
triggering facility dedicated exclusively to catching high energy transients
and enabling rapid coordinated multi-wavelength follow-up. We first describe
the advancements in our understanding of known X-ray transients that can only
be enabled through such a facility and then focus on the classes of transients
theoretically predicted to be out of reach of current detection capabilities.
Finally there is the exciting opportunity of revealing new classes of X-ray
transients and unveiling their nature through coordinated follow-up
observations at longer wavelengths.Comment: 8 pages, 2 figures; White Paper submitted to the Astro2010 SSE pane