3,788 research outputs found
The puzzling symbiotic X-ray system 4U1700+24
Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems
consisting of a neutron star accreting material from a red giant donor star via
stellar wind or Roche lobe overflow. Only a few confirmed members are currently
known; 4U 1700+24 is a good candidate as it is a relatively bright X-ray
object, possibly associated with the late-type star V934 Her. We analysed the
archive {\it XMM}-Newton and Swift/XRT observations of 4U 1700+24 in order to
have a uniform high-energy ( keV) view of the source. We confirmed the
existence of a red-shifted O VIII Ly- transition (already observed in
the 2002 {\it XMM}-Newton data) in the high-resolution spectra collected via
the RGS instruments. The red-shift of the line is found in all the analysed
observations and, on average, it was estimated to be . We also
observed a modulation of the centroid energy of the line on short time scales
(a few days) and discuss the observations in the framework of different
scenarios. If the modulation is due to the gravitational red-shift of the
neutron star, it might arise from a sudden re-organization of the emitting
-ray matter on the scale of a few hundreds of km. Alternatively, we are
witnessing a uni-polar jet of matter (with typical velocity of km
s) possibly emitted by the neutron star in an almost face-on system. The
second possibility seems to be required by the apparent lack of any modulation
in the observed -ray light curve. We also note also that the low-resolution
spectra (both {\it XMM}-Newton and Swift/XRT in the keV band) show the
existence of a black body radiation emitted by a region (possibly associated
with the neutron star polar cap) with typical size from a few tens to hundreds
of meters. The size of this spot-like region reduces as the overall luminosity
of 4U 1700+24 decreases.Comment: In press on A&
A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67
The accretion-powered X-ray pulsar 4U 1626−67 is one of the few highly magnetized pulsars that accretes through Roche lobe overflow from a low-mass companion. The characteristics of its broad-band spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary system, with a broad resonant cyclotron scattering feature (CRSF) at ∼37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broad-band spectrum using data from NuSTAR and Swift. We use the Becker & Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broad-band model for the disc-reflected spectrum. In the softer X-ray band, we need to add a soft blackbody component with kT_(bb) ∼ 0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at ∼61 keV
INTEGRAL/RossiXTE high-energy observation of a state transition of GX 339-4
On 2004 August 15, we observed a fast (shorter than 10 hours) state
transition in the bright black-hole transient GX 339-4 simultaneously with
RossiXTE and INTEGRAL. This transition was evident both in timing and spectral
properties. Combining the data from PCA, HEXTE and IBIS, we obtained good
quality broad-band (3-200 keV) energy spectra before and after the transition.
These spectra indicate that the hard component steepened. Also, the high-energy
cutoff that was present at ~70 keV before the transition was not detected after
the transition. This is the first time that an accurate determination of the
broad-band spectrum across such a transition has been measured on a short time
scale. It shows that, although some spectral parameters do not change abruptly
through the transition, the high-energy cutoff increases/disappears rather
fast. These results constitute a benchmark on which to test theoretical models
for the production of the hard component in these systems.Comment: Accepted for publication in MNRAS (9 pages, 6 figures
The peculiar 2011 outburst of the black hole candidate IGR J17091−3624, a GRS 1915+105-like source?
We report on the long-term monitoring campaign of the black hole candidate IGR J17091−3624 performed with INTEGRAL and Swift during the peculiar outburst started on 2011 January. We have studied the two-month spectral evolution of the source in detail. Unlike the previous outbursts, the initial transition from the hard to the soft state in 2011 was not followed by the standard spectral evolution expected for a transient black hole binary. IGR J17091−3624 showed pseudo-periodic flare-like events in the light curve, closely resembling those observed from GRS 1915+105. We find evidence that these phenomena are due to the same physical instability process ascribed to GRS 1915+105. Finally, we speculate that the faintness of IGR J17091−3624 could be not only due to the high distance of the source but also due to the high inclination angle of the syste
Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
We present the results of multi-year gamma-ray observations by the AGILE
satellite of the black hole binary system Cygnus X-1. In a previous
investigation we focused on gamma-ray observations of Cygnus X-1 in the hard
state during the period mid-2007/2009. Here we present the results of the
gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out
for which includes a remarkably prolonged `soft state' phase (June 2010 -- May
2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a
possible existence of a non-thermal particle component with substantial
modifications of the Comptonized emission from the inner accretion disk. Our
AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1,
provide a significant upper limit for gamma-ray emission above 100 MeV of
F_soft < 20 x 10^{-8} ph/cm^2/s, excluding the existence of prominent
non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We
discuss theoretical implications of our findings in the context of high-energy
emission models of black hole accretion. We also discuss possible gamma-ray
flares detected by AGILE. In addition to a previously reported episode observed
by AGILE in October 2009 during the hard state, we report a weak but important
candidate for enhanced emission which occurred at the end of June 2010
(2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with a
hard-to-soft state transition and before an anomalous radio flare. An appendix
summarizes all previous high-energy observations and possible detections of
Cygnus X-1 above 1 MeV.Comment: 16 pages, 12 figures, 1 table, accepted for publication in Ap
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