161 research outputs found
Orbit and intrinsic spin-up of the newly discovered transient X-ray pulsar Swift J0243.6+6124
We present the orbital solution for the newly discovered transient Be X-ray
binary Swift J0243.6+6124 based on the data from gamma-ray burst monitor
onboard Fermi obtained during the Oct 2017 outburst. We model the Doppler
induced and intrinsic spin variations of the neutron star assuming that the
later is driven by accretion torque and discuss the implications of the
observed spin variations for the parameters of the neutron star and the binary.
In particular we conclude that the neutron star must be strongly magnetized,
and estimate the distance to the source at 5 kpc.Comment: accepted in A&
New hard X-ray sources discovered in the ongoing INTEGRAL Galactic Plane survey after 14 years of observations
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) continues to
successfully work in orbit after its launch in 2002. The mission provides the
deepest ever survey of hard X-ray sources throughout the Galaxy at energies
above 20 keV. We report on a catalogue of new hard X-ray source candidates
based on the latest sky maps comprising 14 years of data acquired with the IBIS
telescope onboard INTEGRAL in the Galactic Plane (|b|<17.5 deg). The current
catalogue includes in total 72 hard X-ray sources detected at S/N>4.7 sigma and
not known to previous INTEGRAL surveys. Among them, 31 objects have also been
detected in the on-going all-sky survey by the BAT telescope of the Swift
observatory. For 26 sources on the list, we suggest possible identifications:
21 active galactic nuclei, two cataclysmic variables, two isolated pulsars or
pulsar wind nebulae, and one supernova remnant; 46 sources from the catalogue
remain unclassified.Comment: 7 pages, 2 figures, 2 tables. Submitted to MNRAS Letters, comments
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Optically thick envelopes around ULXs powered by accreating neutron stars
Magnetized neutron stars power at least some ultra-luminous X-ray sources.
The accretion flow in these cases is interrupted at the magnetospheric radius
and then reaches the surface of a neutron star following magnetic field lines.
Accreting matter moving along magnetic field lines forms the accretion envelope
around the central object. We show that, in case of high mass accretion rates
the envelope becomes closed and optically
thick, which influences the dynamics of the accretion flow and the
observational manifestation of the neutron star hidden behind the envelope.
Particularly, the optically thick accretion envelope results in a multi-color
black-body spectrum originating from the magnetospheric surface. The spectrum
and photon energy flux vary with the viewing angle, which gives rise to
pulsations characterized by high pulsed fraction and typically smooth pulse
profiles. The reprocessing of radiation due to interaction with the envelope
leads to the disappearance of cyclotron scattering features from the spectrum.
We speculate that the super-orbital variability of ultra-luminous X-ray sources
powered by accreting neutron stars can be attributed to precession of the
neutron star due to interaction of magnetic dipole with the accretion disc.Comment: 8 pages, 6 figures, accepted for publication in MNRA
Hard X-ray emission of Sco X-1
We study hard X-ray emission of the brightest accreting neutron star Sco X-1
with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of
deadtime corrected exposure on this source. We show that hard X-ray tail in
time average spectrum of Sco X-1 has a power law shape without cutoff up to
energies ~200-300 keV. An absence of the high energy cutoff does not agree with
the predictions of a model, in which the tail is formed as a result of
Comptonization of soft seed photons on bulk motion of matter near the compact
object. The amplitude of the tail varies with time with factor more than ten
with the faintest tail at the top of the so-called flaring branch of its
color-color diagram. We show that the minimal amplitude of the power law tail
is recorded when the component, corresponding to the innermost part of
optically thick accretion disk, disappears from the emission spectrum.
Therefore we show that the presence of the hard X-ray tail may be related with
the existence of the inner part of the optically thick disk. We estimate
cooling time for these energetic electrons and show that they can not be
thermal. We propose that the hard X-ray tail emission originates as a Compton
upscattering of soft seed photons on electrons, which might have initial
non-thermal distribution.Comment: 9 pages, 7 figures, Accepted for publication in MNRA
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