927 research outputs found
The accretion environment of Supergiant Fast X-ray Transients probed with XMM-Newton
Supergiant fast X-ray transients (SFXTs) are characterized by a remarkable
variability in the X-ray domain, widely ascribed to the accretion from a clumpy
stellar wind. In this paper we performed a systematic and homogeneous analysis
of sufficiently bright X-ray flares from the SFXTs observed with XMM-Newton to
probe spectral variations on timescales as short as a few hundred of seconds.
Our ultimate goal is to investigate if SFXT flares and outbursts are triggered
by the presence of clumps and eventually reveal whether strongly or mildly
dense clumps are required. For all sources, we employ a technique developed by
our group, making use of an adaptive rebinned hardness ratio to optimally
select the time intervals for the spectral extraction. A total of twelve
observations performed in the direction of five SFXTs are reported. We show
that both strongly and mildly dense clumps can trigger these events. In the
former case, the local absorption column density may increase by a factor of
>>3, while in the latter case, the increase is only by a factor of 2-3 (or
lower). Overall, there seems to be no obvious correlation between the dynamic
ranges in the X-ray fluxes and absorption column densities in SFXTs, with an
indication that lower densities are recorded at the highest fluxes. This can be
explained by the presence of accretion inhibition mechanism(s). We propose a
classification of the flares/outbursts from these sources to drive future
observational investigations. We suggest that the difference between the
classes of flares/outbursts is related to the fact that the mechanism(s)
inhibiting accretion can be overcome more easily in some sources compared to
others. We also investigate the possibility that different stellar wind
structures, rather than clumps, could provide the means to temporarily overcome
the inhibition of accretion in SFXTs.Comment: Accepted for publication on A&
Swift J1734.5-3027: a new long type-I X-ray bursting source
Swift J1734.5-3027 is a hard X-ray transient discovered by Swift while
undergoing an outburst in September 2013. Archival observations showed that
this source underwent a previous episode of enhanced X-ray activity in May-June
2013. In this paper we report on the analysis of all X-ray data collected
during the outburst in September 2013, the first that could be intensively
followed-up by several X-ray facilities. Our data-set includes INTEGRAL, Swift,
and XMM-Newton observations. From the timing and spectral analysis of these
observations, we show that a long type-I X-ray burst took place during the
source outburst, making Swift J1734.5-3027 a new member of the class of
bursting neutron star low-mass X-ray binaries. The burst lasted for about 1.9
ks and reached a peak flux of (6.01.8)10 erg cm
s in the 0.5-100 keV energy range. The estimated burst fluence in the
same energy range is (1.100.10)10 erg cm. By
assuming that a photospheric radius expansion took place during the first
200 s of the burst and that the accreted material was predominantly
composed by He, we derived a distance to the source of 7.21.5 kpc.Comment: Accepted for publication on A&
XMM-Newton and Swift observations of XTE J1743-363
XTEJ1743-363 is a poorly known hard X-ray transient, that displays short and
intense flares similar to those observed from Supergiant Fast X-ray Transients.
The probable optical counterpart shows spectral properties similar to those of
an M8 III giant, thus suggesting that XTEJ1743-363 belongs to the class of the
Symbiotic X-ray Binaries. In this paper we report on the first dedicated
monitoring campaign of the source in the soft X-ray range with XMM-Newton and
Swift/XRT. T hese observations confirmed the association of XTEJ1743-363 with
the previously suggested M8 III giant and the classification of the source as a
member of the Symbiotic X-ray binaries. In the soft X-ray domain, XTEJ1743-363
displays a high absorption (~6x10^22 cm^-2 ) and variability on time scales of
hundreds to few thousand seconds, typical of wind accreting systems. A
relatively faint flare (peak X-ray flux 3x10^-11 erg/cm^2/s) lasting ~4 ks is
recorded during the XMM-Newton observation and interpreted in terms of the wind
accretion scenario.Comment: Accepted for publication on A&
The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
We present for the first time a study of the 3-200 keV broad band spectra of
the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity
diagram. The analysis was done using the INTEGRAL public and Galactic Center
deep exposure data ranging from February 2003 to October 2004. The spectra are
well described by a thermal Comptonization model with an electron temperature
from 35 keV to 3 keV and Thomson optical depth, tau_T, from 0.5 to 5 in a slab
geometry. The source undergoes a transition from an intermediate/hard to a soft
state where the source luminosity increases from 2 to 12% of Eddington. We have
also detected 36 type I X-ray bursts two of which show photospheric radius
expansion. The energetic bursts with photospheric radius expansion occurred at
an inferred low mass accretion rate per unit area of \dot m ~ 1.7x10E3 g/cm2/s,
while the others at a higher one between 2.4x10E3 - 9.4x10E3 g/cm2/s. For
4U1728-34 the bursts' total fluence, and the bursts' peak flux are
anti-correlated with the mass accretion rate. The type I X-ray bursts involve
pure helium burning either during the hard state, or during the soft state of
the source.Comment: 11 pages, 7 figures, and 2 tables. Accepted for publication in A&
General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*
Near-IR and X-ray flares have been detected from the supermassive black hole
Sgr A* at the center of our Galaxy with a (quasi)-period of ~17-20 minutes,
suggesting an emission region only a few Schwarzschild radii above the event
horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its
detailed lightcurve, yielding not only the highest quality period thus far, but
also important structure reflecting the geometry of the emitting region. Recent
MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave
instability, that enhances the accretion rate for several hours, possibly
accounting for the observed flares. In this Letter, we carry out ray-tracing
calculations in a Schwarzschild metric to determine as accurately as possible
the lightcurve produced by general relativistic effects during such a
disruption. We find that the Rossby wave induced spiral pattern in the disk is
an excellent fit to the data, implying a disk inclination angle of ~77 deg.
Note, however, that if this association is correct, the observed period is not
due to the underlying Keplerian motion but, rather, to the pattern speed. The
favorable comparison between the observed and simulated lightcurves provides
important additional evidence that the flares are produced in Sgr A*'s inner
disk.Comment: 5 Pages, 3 Figures, accepted for publication in ApJ Lette
Analytical time-like geodesics
Time-like orbits in Schwarzschild space-time are presented and classified in
a very transparent and straightforward way into four types. The analytical
solutions to orbit, time, and proper time equations are given for all orbit
types in the form r=r(\lambda), t=t(\chi), and \tau=\tau(\chi), where \lambda\
is the true anomaly and \chi\ is a parameter along the orbit. A very simple
relation between \lambda\ and \chi\ is also shown. These solutions are very
useful for modeling temporal evolution of transient phenomena near black holes
since they are expressed with Jacobi elliptic functions and elliptic integrals,
which can be calculated very efficiently and accurately.Comment: 15 pages, 10 figures, accepted by General Relativity and Gravitatio
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
Multi-wavelength observations of IGR J17544-2619 from quiescence to outburst
In this paper we report on a long multi-wavelength observational campaign of
the supergiant fast X-ray transient prototype IGR J17544-2619. A 150 ks-long
observation was carried out simultaneously with XMM-Newton and NuSTAR, catching
the source in an initial faint X-ray state and then undergoing a bright X-ray
outburst lasting about 7 ks. We studied the spectral variability during
outburst and quiescence by using a thermal and bulk Comptonization model that
is typically adopted to describe the X-ray spectral energy distribution of
young pulsars in high mass X-ray binaries. Although the statistics of the
collected X-ray data were relatively high we could neither confirm the presence
of a cyclotron line in the broad-band spectrum of the source (0.5-40 keV), nor
detect any of the previously reported tentative detection of the source spin
period. The monitoring carried out with Swift/XRT during the same orbit of the
system observed by XMM-Newton and NuSTAR revealed that the source remained in a
low emission state for most of the time, in agreement with the known property
of all supergiant fast X-ray transients being significantly sub-luminous
compared to other supergiant X-ray binaries. Optical and infrared observations
were carried out for a total of a few thousands of seconds during the
quiescence state of the source detected by XMM-Newton and NuSTAR. The measured
optical and infrared magnitudes were slightly lower than previous values
reported in the literature, but compatible with the known micro-variability of
supergiant stars. UV observations obtained with the UVOT telescope on-board
Swift did not reveal significant changes in the magnitude of the source in this
energy domain compared to previously reported values.Comment: Accepted for publication on A&A. V2: few typos correcte
A large spin-up rate measured with INTEGRAL in the High Mass X-ray Binary Pulsar SAXJ2103.5+4545
The High Mass X-ray Binary Pulsar SAXJ2103.5+4545 has been observed with
INTEGRAL several times during the last outburst in 2002-2004. We report a
comprehensive study of all INTEGRAL observations, allowing a study of the pulse
period evolution during the recent outburst. We measured a very rapid spin-up
episode, lasting 130days, which decreased the pulse period by 1.8s. The spin-up
rate, pdot=-1.5e-7 s/s, is the largest ever measured for SAXJ2103.5+4545, and
it is among the fastest for an accreting pulsar. The pulse profile shows
evidence for temporal variability, apparently not related to the source flux or
to the orbital phase. The X-ray spectrum is hard and there is significant
emission up to 150keV. A new derivation of the orbital period, based on RXTE
data, is also reported.Comment: 8 pages, 7 figures, accepted for publication in A&
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