1,764 research outputs found
Dynamical Comptonization in spherical flows: black hole accretion and stellar winds
The transport of photons in steady, spherical, scattering flows is
investigated. The moment equations are solved analytically for accretion onto a
Schwarzschild black hole, taking into full account relativistic effects. We
show that the emergent radiation spectrum is a power law at high frequencies
with a spectral index smaller (harder spectrum) than in the non--relativistic
case. Radiative transfer in an expanding envelope is also analyzed. We find
that adiabatic expansion produces a drift of injected monochromatic photons
towards lower frequencies and the formation of a power--law, low--energy tail
with spectral index .Comment: 11 pages with 3 ps figures, MNRAS to appea
Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We
analysed a sample of 20 Swift observations, taken between 2013 April and
August, and two Chandra observations taken in 2001 September and 2004 June. We
found only marginal hints of a limited number of dips in the light curve,
previously reported to occur in this source, although the uncertainties due to
the low counting statistics of the data are large. The Chandra and Swift
spectra showed clearly spectral variability which resembles those observed in
other ULXs. We can account for this spectral variability in terms of changes in
both the normalization and intrinsic column density of a two-components model
consisting of a blackbody (for the soft component) and a multicolour accretion
disc (for the hard component). We discuss the possibility that strong outflows
ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA
Discovery of a 6.4 h black hole binary in NGC 4490
We report on the discovery with Chandra of a strong modulation (~90% pulsed
fraction) at ~6.4 h from the source CXOU J123030.3+413853 in the star-forming,
low-metallicity spiral galaxy NGC 4490, which is interacting with the irregular
companion NGC 4485. This modulation, confirmed also by XMM-Newton observations,
is interpreted as the orbital period of a binary system. The spectra from the
Chandra and XMM-Newton observations can be described by a power-law model with
photon index ~1.5. During these observations, which span from 2000 November to
2008 May, the source showed a long-term luminosity variability by a factor of
~5, between ~2E+38 and 1.1E+39 erg/s (for a distance of 8 Mpc). The maximum
X-ray luminosity, exceeding by far the Eddington limit of a neutron star,
indicates that the accretor is a black hole. Given the high X-ray luminosity,
the short orbital period and the morphology of the orbital light curve, we
favour an interpretation of CXOU J123030.3+413853 as a rare high-mass X-ray
binary system with a Wolf-Rayet star as a donor, similar to Cyg X-3. This would
be the fourth system of this kind known in the local Universe. CXOU
J123030.3+413853 can also be considered as a transitional object between high
mass X-ray binaries and ultraluminous X-ray sources (ULXs), the study of which
may reveal how the properties of persistent black-hole binaries evolve entering
the ULX regime.Comment: Fig. 1 in reduced quality; minor changes to match the MNRAS versio
VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774
X-ray observations performed with ROSAT led to the discovery of a group
(seven to date) of X-ray dim and radio-silent middle-aged isolated neutron
stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra
(kT~40-100 eV), long X-ray pulsations (P=3-12 s), and appear to be endowed with
relatively high magnetic fields, (B~10d13-14 G). RBS 1774 is one of the few
XDINSs with a candidate optical counterpart, which we discovered with the VLT.
We performed deep observations of RBS 1774 in the R band with the VLT to
disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose
contributions are expected to be very much different in the red part of the
spectrum. We did not detect the RBS 1774 candidate counterpart down to a 3
sigma limiting magnitude of R~27. The constraint on its colour, (B-R)<0.6,
rules out that it is a background object, positionally coincident with the
X-ray source. Our R-band upper limit is consistent with the extrapolation of
the B-band flux (assuming a 3 sigma uncertainty) for a set of power-laws F_nu
~nu^alpha with spectral indeces alpha<0.07. If the optical spectrum of RBS 1774
were non-thermal, its power-law slope would be very much unlike those of all
isolated neutron stars with non-thermal optical emission, suggesting that it is
most likely thermal. For instance, a Rayleigh-Jeans with temperature T_O = 11
eV, for an optically emitting radius r_O=15 km and a source distance d=150 pc,
would be consistent with the optical measurements. The implied low distance is
compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is
nearly aligned with the magnetic axis or with the line of sight, or it is
slightly misaligned with respect to both the magnetic axis and the line of
sight by 5-10 degreesComment: 8 pages, 8 postscript figures, accepted for publication in Astronomy
& Astrophysic
LOFT as a discovery machine for jetted Tidal Disruption Events
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of jetted tidal disruption events.
For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
The two Ultraluminous X-ray sources in the galaxy NGC 925
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC
925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality,
simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first
glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because
of its high X-ray luminosity ( erg s) and its
spectral/temporal features, a closer inspection shows that its properties are
more similar to those of a typical super-Eddington accreting stellar black hole
and we classify it as a `broadened disc' ultraluminous X-ray source. Based on
the physical interpretation of this spectral state, we suggest that ULX-1 is
seen at small inclination angles, possibly through the evacuated cone of a
powerful wind originating in the accretion disc. The spectral classification of
ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington
rates as none of its spectral/temporal properties can be associated to either
the soft or hard state of Galactic accreting black hole binaries.Comment: Accepted on MNRAS with very minor comments, 7 pages, 5 figures, 1
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