1,164 research outputs found
Supergiant Fast X-ray Transients: a review
Supergiant Fast X-ray Transients are a class of Galactic High Mass X-ray
Binaries with supergiant companions. Their extreme transient X-ray flaring
activity was unveiled thanks to INTEGRAL/IBIS observations. The SFXTs dynamic
range, with X-ray luminosities from 1E32 erg/s to 1E37 erg/s, and long time
intervals of low X-ray emission, are puzzling, given that both their donor star
properties and their orbital and spin periodicities seem very similar to those
displayed by massive binaries with persistent X-ray emission. Clumpy supergiant
winds, accretion barriers, orbital geometries and wind anisotropies are often
invoked to explain their behavior, but still several open issues remain. A
review of the main recent observational results will be outlined, together with
a summary of the new scenarios proposed to explain their bright flaring X-ray
activity. The main result of a long Suzaku observation of the SFXT
IGRJ16479-4514 with the shortest orbital period is also briefly summarized. The
observation of the X-ray eclipse in this source allowed us to directly probe
the supergiant wind density at the orbital separation, leading to the
conclusion that it is too large to justify the low X-ray luminosity. A
mechanism reducing the accretion rate onto the compact object is required.Comment: Solicited talk at the 9th INTEGRAL Workshop "An INTEGRAL view of the
high-energy sky (the first 10 years)", held in Paris, France, on 15-19
October 2012; accepted for publication in Proceedings of Scienc
Supergiant Fast X-ray Transients - a short review
I present a brief up-to-date review of the current understanding of
Supergiant Fast X-ray Transients, with an emphasis on the observational point
of view. After more than a decade since their discovery, a remarkable progress
has been made in getting the picture of their phenomenology at X-ray energies.
However, a similar in-depth investigation of the properties of the supergiant
companions is needed, but has started more recently. A multifrequency approach
is the key to fully understand the physical mechanism driving the SFXT
behaviour, still under debate.Comment: 14 pages 4 figures. Accepted for publication in the Proceedings of
the "XII Multifrequency Behaviour of High Energy Cosmic Sources Workshop",
held on 12-17 June 2017, Palermo, Italy (PoS(MULTIF2017)052). The figures
have been modified following the referee repor
An INTEGRAL overview of High Mass X-ray Binaries: classes or transitions?
We analyzed in a systematic way the public INTEGRAL observations spanning
from December 2002 to September 2016, to investigate the hard X-ray properties
of about 60 High Mass X-ray Binaries (HMXBs). We considered both persistent and
transient sources, hosting either a Be star (Be/XRBs) or a blue supergiant
companion (SgHMXBs, including Supergiant Fast X-ray Transients, SFXTs), a
neutron star or a black hole. INTEGRAL X-ray light curves (18-50 keV), sampled
at a bin time of about 2 ks, were extracted for all HMXBs to derive the
cumulative distribution of their hard X-ray luminosity, their duty cycle, the
range of variability of their hard X-ray luminosity. This allowed us to obtain
an overall and quantitative characterization of the long-term hard X-ray
activity of the HMXBs in our sample. Putting the phenomenology observed with
INTEGRAL into context with other known source properties (e.g. orbital
parameters, pulsar spin periods) together with observational constraints coming
from softer X-rays (1-10 keV), enabled the investigation of the way the
different HMXB sub-classes behave (and sometimes overlap). For given source
properties, the different sub-classes of massive binaries seem to cluster in a
suggestive way. However, for what concerns supergiant systems (SgHMXBs versus
SFXTs), several sources with intermediate properties exist, suggesting a smooth
transition between the two sub-classes.Comment: 27 pages, 17 figures, 3 tables; accepted for publication in Monthly
Notices of the Royal Astronomical Society (accepted 2018 August 30. Received
2018 August 22; in original form 2018 May 16
The longest observation of a low intensity state from a Supergiant Fast X-ray Transient: Suzaku observes IGRJ08408-4503
We report here on the longest deep X-ray observation of a SFXT outside
outburst, with an average luminosity level of 1E33 erg/s (assuming 3 kpc
distance). This observation was performed with Suzaku in December 2009 and was
targeted on IGRJ08408-4503, with a net exposure with the X-ray imaging
spectrometer (XIS, 0.4-10 keV) and the hard X-ray detector (HXD, 15-100 keV) of
67.4 ks and 64.7 ks, respectively, spanning about three days. The source was
caught in a low intensity state characterized by an initially average X-ray
luminosity level of 4E32 erg/s (0.5-10 keV) during the first 120 ks, followed
by two long flares (about 45 ks each) peaking at a flux a factor of about 3
higher than the initial pre-flare emission. Both XIS spectra (initial emission
and the two subsequent long flares) can be fitted with a double component
spectrum, with a soft thermal plasma model together with a power law,
differently absorbed. The spectral characteristics suggest that the source is
accreting matter even at this very low intensity level. From the HXD
observation we place an upper limit of 6E33 erg/s (15-40 keV; 3 kpc distance)
to the hard X-ray emission, which is the most stringent constrain to the hard
X-ray emission during a low intensity state in a SFXT, to date. The timescale
observed for the two low intensity long flares is indicative of an orbital
separation of the order of 1E13 cm in IGRJ08408-4503.Comment: Accepted for publication in MNRAS. Accepted 2010 July 6. Received
2010 July 6; in original form 2010 June 9. The paper contains 5 figures and 3
table
INTEGRAL study of temporal properties of bright flares in Supergiant Fast X-ray Transients
We have characterized the typical temporal behaviour of the bright X-ray
flares detected from the three Supergiant Fast X-ray Transients showing the
most extreme transient behaviour (XTEJ1739-302, IGRJ17544-2619,
SAXJ1818.6-1703). We focus here on the cumulative distributions of the
waiting-time (time interval between two consecutive X-ray flares), and the
duration of the hard X-ray activity (duration of the brightest phase of an SFXT
outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting
the cumulative distribution of waiting-times, it is possible to identify the
typical timescale that clearly separates different outbursts, each composed by
several single flares at ks timescale. This allowed us to measure the duration
of the brightest phase of the outbursts from these three targets, finding that
they show heavy-tailed cumulative distributions. We observe a correlation
between the total energy emitted during SFXT outbursts and the time interval
covered by the outbursts (defined as the elapsed time between the first and the
last flare belonging to the same outburst as observed by INTEGRAL). We show
that temporal properties of flares and outbursts of the sources, which share
common properties regardless different orbital parameters, can be interpreted
in the model of magnetized stellar winds with fractal structure from the
OB-supergiant stars.Comment: 10 pages, 8 figures, 1 table. Accepted for publication in MNRAS
(Accepted 2016 January 26. Received 2016 January 25 ; in original form 2015
December 15
- …