559 research outputs found
Stable accretion from a cold disc in highly magnetized neutron stars
The aim of this paper is to investigate the transition of a strongly
magnetized neutron star into the accretion regime with very low accretion rate.
For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57
throughout a full orbital cycle. The current observational campaign was
performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV)
between two subsequent Type I outbursts in January and September 2016. The
expected transition to the propeller regime was not observed. However, the
transitions between different regimes of accretion were detected. In
particular, after an outburst the source entered a stable accretion state
characterised by the accretion rate of ~10^14-10^15 g/s. We associate this
state with accretion from a cold (low-ionised) disc of temperature below ~6500
K. We argue that a transition to such accretion regime should be observed in
all X-ray pulsars with certain combination of the rotation frequency and
magnetic field strength. The proposed model of accretion from a cold disc is
able to explain several puzzling observational properties of X-ray pulsars.Comment: 8 pages, 3 figures, 1 table, accepted by A&
Relativistic kinetic equation for Compton scattering of polarized radiation in strong magnetic field
We derive the relativistic kinetic equation for Compton scattering of
polarized radiation in strong magnetic field using the Bogolyubov method. The
induced scattering and the Pauli exclusion principle are taken into account.
The electron polarization is also considered in the general form of the kinetic
equation. The special forms of the equation for the cases of the non-polarized
electrons, the rarefied electron gas and the two polarization mode description
of radiation are found. The derived equations are valid for any photon and
electron energies and the magnetic field strength below about 10^{16} G. These
equations provide the basis for formulation of the equation for polarized
radiation transport in atmospheres and magnetospheres of strongly magnetized
neutron stars.Comment: 23 pages, accepted for publication in Phys. Rev.
Characterizing a new class of variability in GRS 1915+105 with simultaneous INTEGRAL/RXTE observations
We report on the analysis of 100 ks INTEGRAL observations of the Galactic
microquasar GRS 1915+105. We focus on INTEGRAL Revolution number 48 when the
source was found to exhibit a new type of variability as preliminarily reported
in Hannikainen et al. (2003). The variability pattern, which we name , is
characterized by a pulsing behaviour, consisting of a main pulse and a shorter,
softer, and smaller amplitude precursor pulse, on a timescale of 5 minutes in
the JEM-X 3-35 keV lightcurve. We also present simultaneous RXTE data. From a
study of the individual RXTE/PCA pulse profiles we find that the rising phase
is shorter and harder than the declining phase, which is opposite to what has
been observed in other otherwise similar variability classes in this source.
The position in the colour-colour diagram throughout the revolution corresponds
to State A (Belloni et al. 2000) but not to any previously known variability
class. We separated the INTEGRAL data into two subsets covering the maxima and
minima of the pulses and fitted the resulting two broadband spectra with a
hybrid thermal--non-thermal Comptonization model. The fits show the source to
be in a soft state characterized by a strong disc component below ~6 keV and
Comptonization by both thermal and non-thermal electrons at higher energies.Comment: Accepted for publication in A&A. 11 pages, 10 figures, 4 in colour.
Original figures can be found at
http://www.astro.helsinki.fi/~diana/grs1915_rev48. Author affiliations
correcte
Broadband X-ray spectrum of XTE J1550-564 during 2003 outburst
Results of broadband INTEGRAL and RXTE observations of the Galactic
microquasar XTE J1550-564 during outburst in spring 2003 are presented. During
the outburst the source was found in a canonical low/hard spectral state.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy Letter
Optical Photometry and Spectroscopy of the Accretion-Powered Millisecond Pulsar HETE J1900.1-2455
We present phase resolved optical photometry and spectroscopy of the
accreting millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit
a sinusoidal modulation, at close to the orbital period, which we initially
attributed to X-ray heating of the irradiated face of the secondary star.
However, further analysis reveals that the source of the modulation is more
likely due to superhumps caused by a precessing accretion disc. Doppler
tomography of a broad Halpha emission line reveals an emission ring, consistent
with that expected from an accretion disc. Using the velocity of the emission
ring as an estimate for the projected outer disc velocity, we constrain the
maximum projected velocity of the secondary to be 200 km/s, placing a lower
limit of 0.05 Msun on the secondary mass. For a 1.4 Msun primary, this implies
that the orbital inclination is low, < 20 degrees. Utilizing the observed
relationship between the secondary mass and orbital period in short period
cataclysmic variables, we estimate the secondary mass to be ~0.085 Msun, which
implies an upper limit of ~2.4 Msun for the primary mass.Comment: 8 pages, 6 figures; Accepted for publication in MNRAS. Minor
revisions to match final published versio
The Hard X-ray Emission of Cen A
The radio galaxy Cen A has been detected all the way up to the TeV energy
range. This raises the question about the dominant emission mechanisms in the
high-energy domain. Spectral analysis allows us to put constraints on the
possible emission processes. Here we study the hard X-ray emission as measured
by INTEGRAL in the 3-1000 keV energy range, in order to distinguish between a
thermal and non-thermal inverse Compton process. The hard X-ray spectrum of Cen
A shows a significant cut-off at energies Ec = 434 (+106 -73) keV with an
underlying power law of photon index 1.73 +- 0.02. A more physical model of
thermal Comptonisation (compPS) gives a plasma temperature of kT = 206+-62 keV
within the optically thin corona with Compton parameter y = 0.42 (+0.09 -0.06).
The reflection component is significant at the 1.9 sigma level with R = 0.12
(+0.09 -0.10), and a reflection strength R>0.3 can be excluded on a 3 sigma
level. Time resolved spectral studies show that the flux, absorption, and
spectral slope varied in the range f(3-30 keV) = (1.2 - 9.2)e-10 erg/cm**2/s,
NH = (7 - 16)e22 1/cm**2, and photon index 1.75 - 1.87. Extending the cut-off
power law or the Comptonisation model to the gamma-ray range shows that they
cannot account for the high-energy emission. On the other hand, also a broken
or curved power law model can represent the data, therefore a non-thermal
origin of the X-ray to GeV emission cannot be ruled out. The analysis of the
SPI data provides no sign of significant emission from the radio lobes and
gives a 3 sigma upper limit of f(40-1000 keV) < 0.0011 ph/cm**2/s. While
gamma-rays, as detected by CGRO and Fermi, are caused by non-thermal (jet)
processes, the main process in the hard X-ray emission of Cen A is still not
unambiguously determined, being either dominated by thermal inverse Compton
emission, or by non-thermal emission from the base of the jet.Comment: 8 pages, 6 figures, accepted for publication in A&
OSSE and RXTE Observations of GRS 1915+105: Evidence for Non-thermal Comptonization
GRS 1915+105 was observed by the CGRO/OSSE 9 times in 1995-2000, and 8 of
those observations were simultaneous with those by RXTE. We present an analysis
of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and
highest X-ray fluxes. The OSSE data show a power-law--like spectrum extending
up to ~600 keV without any break. We interpret this emission as strong evidence
for the presence of non-thermal electrons in the source. The broad-band spectra
cannot be described by either thermal or bulk-motion Comptonization, whereas
they are well described by Comptonization in hybrid thermal/ non-thermal
plasmas.Comment: 4 pages in emulateapj5 style; Slightly modified version accepted to
ApJ Letter
Wind Accretion and State Transitions in Cygnus X-1
We present the results of a spectroscopic monitoring program (from 1998 to
2002) of the H-alpha emission strength in HDE 226868, the optical counterpart
of the black hole binary, Cyg X-1. The H-alpha profiles consist of (1) a P
Cygni component associated with the wind of the supergiant, (2) emission
components that attain high velocity at the conjunctions and that probably form
in enhanced outflows both towards and away from the black hole, and (3) an
emission component that moves in anti-phase with the supergiant's motion. We
argue that the third component forms in accreted gas near the black hole, and
the radial velocity curve of the emission is consistent with a mass ratio of
M_X / M_opt = 0.36 +/- 0.05. We find that there is a general anti-correlation
between the H-alpha emission strength and X-ray flux in the sense that when the
H-alpha emission is strong (W_\lambda < -0.5 Angstroms) the X-ray flux is
weaker and the spectrum harder. On the other hand, there is no correlation
between H-alpha emission strength and X-ray flux when H-alpha is weak. During
the low/hard X-ray state, the strong wind is fast and the accretion rate is
relatively low, while in the high/soft state the weaker, highly ionized wind
attains only a moderate velocity and the accretion rate increases. We argue
that the X-ray transitions from the normal low/hard to the rare high/soft state
are triggered by episodes of decreased mass loss rate in the supergiant donor
star.Comment: 45 pages, 16 figures, ApJ, in pres
SANEPIC: A Map-Making Method for Timestream Data From Large Arrays
We describe a map-making method which we have developed for the Balloon-borne
Large Aperture Submillimeter Telescope (BLAST) experiment, but which should
have general application to data from other submillimeter arrays. Our method
uses a Maximum Likelihood based approach, with several approximations, which
allows images to be constructed using large amounts of data with fairly modest
computer memory and processing requirements. This new approach, Signal And
Noise Estimation Procedure Including Correlations (SANEPIC), builds upon
several previous methods, but focuses specifically on the regime where there is
a large number of detectors sampling the same map of the sky, and explicitly
allowing for the the possibility of strong correlations between the detector
timestreams. We provide real and simulated examples of how well this method
performs compared with more simplistic map-makers based on filtering. We
discuss two separate implementations of SANEPIC: a brute-force approach, in
which the inverse pixel-pixel covariance matrix is computed; and an iterative
approach, which is much more efficient for large maps. SANEPIC has been
successfully used to produce maps using data from the 2005 BLAST flight.Comment: 27 Pages, 15 figures; Submitted to the Astrophysical Journal; related
results available at http://blastexperiment.info/ [the BLAST Webpage
The efficient low-mass Seyfert MCG-05-23-016
The Seyfert 1.9 galaxy MCG-05-23-016 has been shown to exhibit a complex
X-ray spectrum. This source has moderate X-ray luminosity, hosts a comparably
low-mass black hole, but accretes at a high Eddington rate, and allows us to
study a super massive black hole in an early stage. Three observations of the
INTEGRAL satellite simultaneous with pointed Swift/XRT observations performed
from December 2006 to June 2007 are used in combination with public data from
the INTEGRAL archive to study the variability of the hard X-ray components and
to generate a high-quality spectrum from 1 to 150 keV. The AGN shows little
variability in the hard X-ray spectrum, with some indication of a variation in
the high-energy cut-off energy ranging from 50 keV to >>100 keV, with an
electron plasma temperature in the 10 - 90 keV range. The reflection component
is not evident and, if present, the reflected fraction can be constrained to R
< 0.3 for the combined data set. Comparison to previous observations shows that
the reflection component has to be variable. No variability in the UV and
optical range is observed on a time scale of 1.5 years. The hard X-ray spectrum
of MCG-05-23-016 appears to be stable with the luminosity and underlying power
law varying moderately and the optical/UV flux staying constant. The spectral
energy distribution appears to be similar to that of Galactic black hole
systems, e.g. XTE 1118+480 in the low state. The AGN exhibits a remarkably high
Eddington ratio of L(bol)/L(Edd)> 0.8 (or L(bol)/L(Edd) > 0.1, if we consider a
higher mass of the central engine) and, at the same time, a low cut-off energy
around 70 keV. Objects like MCG-05-23-016 might indicate the early stages of
super massive black holes, in which a strong accretion flow feeds the central
engine.Comment: 8 pages, 5 figures, accepted for publication in A&
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