582 research outputs found
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
The magnetospheric radius of an inclined rotator in the magnetically threaded disk model
The estimate of the magnetospheric radius in a disk-fed neutron star X-ray
binary is a long standing problem in high energy Astrophysics. We review the
magnetospheric radius calculations in the so-called magnetically threaded disk
model, comparing the simplified approach originally proposed by Ghosh & Lamb
(1979) with the revised version proposed by Wang (1987), Wang (1995), and Wang
(1997). We show that for a given set of fixed parameters (assuming also a
comparable screening factor of the neutron star magnetic field by the currents
induced on the disk surface) the revised magnetically threaded disk model
predicts a magnetospheric radius that is significantly smaller than that
derived from the Ghosh & Lamb (1979) treatment. For a fixed value of the
neutron star magnetic field and a wide range of mass accretion rates, the
inclusion of a large inclination angle between the neutron star rotation and
magnetic field axes (60 deg) leads to a further decrease of the
magnetospheric radius. To illustrate the relevance of these calculations, we
consider, as an example, the case of the transitional pulsars. During the
so-called "high mode" of their sub-luminous accretion disk state, these sources
have shown X-ray pulsations interpreted as due to accretion at an unprecedented
low luminosity level compared to other neutron stars in X-ray binaries. In the
context of the magnetic threaded disk model, we show that accretion at
luminosities of 10 erg s (and thus accretion-driven X-ray
pulsations) can be more easily explained when the prescription of the
magnetospheric radius provided by Wang (1997) is used. This avoids the need of
invoking very strong propeller outflows in the transitional pulsars, as
proposed in other literature works.Comment: Accepted for publication in A&
Glancing through the accretion column of EXO 2030+375
We took advantage of the large collecting area and good timing capabilities
of the EPIC cameras on-board XMM-Newton to investigate the accretion geometry
onto the magnetized neutron star hosted in the high mass X-ray binary EXO
2030+375 during the rise of a source Type-I outburst in 2014. We carried out a
timing and spectral analysis of the XMM-Newton observation as function of the
neutron star spin phase. We used a phenomenological spectral continuum model
comprising the required fluorescence emission lines. Two neutral absorption
components are present: one covering fully the source and one only partially.
The same analysis was also carried out on two Suzaku observations of the source
performed during outbursts in 2007 and 2012, to search for possible spectral
variations at different luminosities. The XMM-Newton data caught the source at
an X-ray luminosity of erg s and revealed the presence
of a narrow dip-like feature in its pulse profile that was never reported
before. The width of this feature corresponds to about one hundredth of the
neutron star spin period. From the results of the phase-resolved spectral
analysis we suggest that this feature can be ascribed to the self-obscuration
of the accretion stream passing in front of the observer line of sight. We
inferred from the Suzaku observation carried out in 2007 that the
self-obscuration of the accretion stream might produce a significantly wider
feature in the neutron star pulsed profile at higher luminosities
( erg s).Comment: Accepted for publication on A&
Swift, NuSTAR, and INTEGRAL observations of the symbiotic X-ray binary IGR J16194-2810
We report on a simultaneous observational campaign with both Swift/XRT and
NuSTAR targeting the symbiotic X-ray binary IGR J16194-2810. The main goal of
the campaign was to investigate the possible presence of cyclotron scattering
absorption features in the broad-band spectrum of the source, and help advance
our understanding of the process of neutron star formation via the
accretion-induced collapse of a white dwarf. The 1-30 keV spectrum of the
source, as measured during our campaign, did not reveal the presence of any
statistically significant absorption feature. The spectrum could be well
described using a model comprising a thermal black-body hot component, most
likely emerging from the surface of the accreting neutron star, and a power-law
with no measurable cut-off energy (and affected by a modest absorption column
density). Compared to previous analyses in the literature, we could rule out
the presence of a colder thermal component emerging from an accretion disk,
compatible with the idea that IGR J16194-2810 is a wind-fed binary (as most of
the symbiotic X-ray binaries). Our results were strengthened by exploiting the
archival XRT and INTEGRAL data, extending the validity of the spectral model
used up to 0.3-40 keV and demonstrating that IGR J16194-2810 is unlikely to
undergo significant spectral variability over time in the X-ray domain.Comment: Accepted for publication on MNRA
Swift/XRT orbital monitoring of the candidate supergiant fast X-ray transient IGR J17354-3255
We report on the Swift/X-ray Telescope (XRT) monitoring of the field of view
around the candidate supergiant fast X-ray transient (SFXT) IGR J17354-3255,
which is positionally associated with the AGILE/GRID gamma-ray transient AGL
J1734-3310. Our observations, which cover 11 days for a total on-source
exposure of about 24 ks, span 1.2 orbital periods (P_orb=8.4474 d) and are the
first sensitive monitoring of this source in the soft X-rays. These new data
allow us to exploit the timing variability properties of the sources in the
field to unambiguously identify the soft X-ray counterpart of IGR J17354-3255.
The soft X-ray light curve shows a moderate orbital modulation and a dip. We
investigated the nature of the dip by comparing the X-ray light curve with the
prediction of the Bondi-Hoyle-Lyttleton accretion theory, assuming both
spherical and nonspherical symmetry of the outflow from the donor star. We
found that the dip cannot be explained with the X-ray orbital modulation. We
propose that an eclipse or the onset of a gated mechanism is the most likely
explanation for the observed light curve.Comment: Accepted for publication in Astronomy and Astrophysics. 9 page
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
Supergiant fast X-ray transients as an under-luminous class of supergiant X-ray binaries
The usage of cumulative luminosity distributions, constructed thanks to the
long-term observations available through wide field hard X-ray imagers, has
been recently exploited to study the averaged high energy emission (>17 keV)
from Supergiant Fast X-ray Transients (SFXTs) and classical Supergiant High
Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term
monitorings now available with Swift/XRT to construct for the first time the
cumulative luminosity distributions of a number of SFXTs and the classical SgXB
IGR J18027-2016 in the soft X-ray domain with a high sensitivity focusing X-ray
telescope (0.3-10 keV). By complementing previous results obtained in the hard
X-rays, we found that classical SgXBs are characterized by cumulative
distributions with a single knee around 10-10 erg/s, while
SFXTs are found to be systematically sub-luminous and their distributions are
shifted at significantly lower luminosities (a factor of 10-100). As the
luminosity states in which these sources spend most of their time are typically
below the sensitivity limit of large field of view hard X-ray imagers, we
conclude that soft X-ray monitorings carried out with high sensitivity
telescopes are particularly crucial to reconstruct the complete profile of the
SFXT cumulative luminosity distributions. The difference between the cumulative
luminosity distributions of classical SgXBs and SFXTs is interpreted in terms
of accretion from a structured wind in the former sources and the presence of
magnetic/centrifugal gates or a quasi-spherical settling accretion regime in
the latter.Comment: Accepted for publication in Advances in Space Researc
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