261 research outputs found
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
Chandra deep X-ray observation on the Galactic plane
Using the Chandra ACIS-I instruments, we have carried out the deepest X-ray
observation on a typical Galactic plane region at l 28.5 deg, where no discrete
X-ray sources have been known previously. We have detected, as well as strong
diffuse emission, 275 new point X-ray sources (4 sigma confidence) within two
partially overlapping fields (~250 arcmin^2 in total) down to ~3 x 10^{-15} erg
s^{-1} cm^{-2} (2 -- 10 keV) or ~ 7 x 10^{-16} erg s^{-1} cm^{-2} (0.5 -- 2
keV). We have studied spectral distribution of these point sources, and found
that very soft sources detected only below ~ 3 keV are more numerous than hard
sources detected only above ~ 3 keV. Only small number of sources are detected
both in the soft and hard bands. Surface density of the hard sources is almost
consistent with that at high Galactic regions, thus most of the hard sources
are considered to be Active Galactic Nuclei seen through the milky way. On the
other hand, some of the bright hard X-ray sources which show extremely flat
spectra and iron line or edge features are considered to be Galactic,
presumably quiescent dwarf novae. The soft sources show thermal spectra and
small interstellar hydrogen column densities, and some of them exhibit X-ray
flares. Therefore, most of the soft sources are probably X-ray active nearby
late type stars.Comment: Contribution to the proceedings of the "New Visions of the X-Ray
Universe in the XMM-Newton and Chandra Era" symposium at ESTEC, The
Netherlands. 26-30 Nov. 200
Probing large-scale wind structures in Vela X-1 using off-states with INTEGRAL
Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high
mass X-ray binaries hosting a supergiant donor. We have analyzed in a
systematic way ten years of INTEGRAL data of Vela X-1 (22-50 keV) and we found
that when outside the X-ray eclipse, the source undergoes several luminosity
drops where the hard X-rays luminosity goes below 3x10^35 erg/s, becoming
undetected by INTEGRAL. These drops in the X-ray flux are usually referred to
as "off-states" in the literature. We have investigated the distribution of
these off-states along the Vela X-1 ~8.9 d orbit, finding that their orbital
occurrence displays an asymmetric distribution, with a higher probability to
observe an off-state near the pre-eclipse than during the post-eclipse. This
asymmetry can be explained by scattering of hard X-rays in a region of ionized
wind, able to reduce the source hard X-ray brightness preferentially near
eclipse ingress. We associate this ionized large-scale wind structure with the
photoionization wake produced by the interaction of the supergiant wind with
the X-ray emission from the neutron star. We emphasize that this observational
result could be obtained thanks to the accumulation of a decade of INTEGRAL
data, with observations covering the whole orbit several times, allowing us to
detect an asymmetric pattern in the orbital distribution of off-states in Vela
X-1.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society (5 pages, 3 figures). A few typos fixed to match the published
versio
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
Probing large-scale wind structures in Vela X-1 using off-states with INTEGRAL
Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high-mass X-ray binaries hosting a supergiant donor. We have analysed in a systematic way 10years of INTEGRAL data of VelaX-1 (22-50keV) and we found that when outside the X-ray eclipse, the source undergoes several luminosity drops where the hard X-rays luminosity goes below âŒ3Ă1035ergsâ1, becoming undetected by INTEGRAL. These drops in the X-ray flux are usually referred to as âoff-states' in the literature. We have investigated the distribution of these off-states along the VelaX-1âŒ8.9d orbit, finding that their orbital occurrence displays an asymmetric distribution, with a higher probability to observe an off-state near the pre-eclipse than during the post-eclipse. This asymmetry can be explained by scattering of hard X-rays in a region of ionized wind, able to reduce the source hard X-ray brightness preferentially near eclipse ingress. We associate this ionized large-scale wind structure with the photoionization wake produced by the interaction of the supergiant wind with the X-ray emission from the neutron star. We emphasize that this observational result could be obtained thanks to the accumulation of a decade of INTEGRAL data, with observations covering the whole orbit several times, allowing us to detect an asymmetric pattern in the orbital distribution of off-states in VelaX-
NuSTAR observation of the Supergiant Fast X-ray Transient IGR J11215-5952 during its 2017 outburst
We report on the results of a NuSTAR observation of the Supergiant Fast X-ray
Transient pulsar IGRJ11215-5952 during the peak of its outburst in June 2017.
IGRJ11215-5952 is the only SFXT undergoing strictly periodic outbursts, every
165 days. NuSTAR caught several X-ray flares, spanning a dynamic range of 100,
and detected X-ray pulsations at 187.0 s, consistent with previous
measurements. The spectrum from the whole observation is well described by an
absorbed power-law (with a photon index of 1.4) modified, above 7 keV, by a
cutoff with an e-folding energy of 24 keV. A weak emission line is present at
6.4 keV, consistent with Kalpha emission from cold iron in the supergiant wind.
The time-averaged flux is 1.5E-10 erg/cm2/s (3-78 keV, corrected for the
absorption), translating into an average luminosity of about 9E35 erg/s (1-100
keV, assuming a distance of 6.5 kpc). The NuSTAR observation allowed us to
perform the most sensitive search for cyclotron resonant scattering features in
the hard X-ray spectrum, resulting in no significant detection in any of the
different spectral extractions adopted (time-averaged, temporally-selected,
spin-phase-resolved and intensity-selected spectra). The pulse profile showed
an evolution with both the energy (3-12 keV energy range compared with 12-78
keV band) and the X-ray flux: a double peaked profile was evident at higher
fluxes (and in both energy bands), while a single peaked, sinusoidal profile
was present at the lowest intensity state achieved within the NuSTAR
observations (in both energy bands). The intensity-selected analysis allowed us
to observe an anti-correlation of the pulsed fraction with the X-ray
luminosity. The pulse profile evolution can be explained by X-ray photon
scattering in the accreting matter above magnetic poles of a neutron star at
the quasi-spherical settling accretion stage.Comment: Accepted for publication in Astronomy & Astrophysics (Received 3
April 2020 / Accepted 17 April 2020). 14 pages, 5 Tables, 9 Figure
Resolving the Hard X-ray Emission of GX 5-1 with INTEGRAL
We present the study of one year of INTEGRAL data on the neutron star low
mass X-ray binary GX 5-1. Thanks to the excellent angular resolution and
sensitivity of INTEGRAL, we are able to obtain a high quality spectrum of GX
5-1 from ~5 keV to ~100 keV, for the first time without contamination from the
nearby black hole candidate GRS 1758-258 above 20 keV. During our observations,
GX 5-1 is mostly found in the horizontal and normal branch of its hardness
intensity diagram. A clear hard X-ray emission is observed above ~30 keV which
exceeds the exponential cut-off spectrum expected from lower energies. This
spectral flattening may have the same origin of the hard components observed in
other Z sources as it shares the property of being characteristic to the
horizontal branch. The hard excess is explained by introducing Compton
up-scattering of soft photons from the neutron star surface due to a thin hot
plasma expected in the boundary layer. The spectral changes of GX 5-1 downward
along the "Z" pattern in the hardness intensity diagram can be well described
in terms of monotonical decrease of the neutron star surface temperature. This
may be a consequence of the gradual expansion of the boundary layer as the mass
accretion rate increases.Comment: 10 pages, 17 figures, accepted for publication in A&
The acute effects of different high-intensity conditioning activities on sprint performance differ between sprinters of different strength and power characteristics
The purpose of the present study was to examine the effect of different conditioning activities (CAs) on short-term increase in sprint performance. In twelve male sprinters (21.1±2.6 years, 100 m performance: 11.5±0.6 s) their body composition, half squat maximum strength, 100 m sprinting and countermovement jump performances were evaluated. The performance of a 50 m sprint (splits at 10 m, 30 m and 50 m) was evaluated before and 5, 10 and 15 min after four postactivation performance enhancement CAs on different occasions: [1] 3 sets x 4 s maximum isometric half squat (IHF), [2] 3 sets x 3 consecutive countermovement jumps (cCMJs), [3] 3 repetitions x 30 m overspeed sprinting (OVSP) and [4] dynamic submaximal half squat (2 sets x 2 reps x 90% of 1-RM half squat; HSQ). Significant improvements of sprinting performance were found 10 and 15 min following the cCMJs, OVSP and HSQâs interventions, in all distances (p.05). Significant inter-individual differences were found in the magnitude of sprint performance improvements as well as in the optimal time window (p<.05), with the stronger sprinters responding better after HSQs, while the more powerful sprinters after cCMJs and OVSPs. In conclusion, it seems that cCMJs, OVSP and HSQ can acutely increase sprinting performance after 10 min, but CAâs induced increases in sprinting performance
are highly related to the strength and power characteristics of each sprinter
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