47 research outputs found
On the Change of the Inner Boundary of an Optically Thick Accretion Disk around White Dwarfs Using the Dwarf Nova SS Cyg as an Example
We present the results of our studies of the aperiodic optical flux
variability for SS Cyg, an accreting binary systemwith a white dwarf. The main
set of observational data presented here was obtained with the ANDOR/iXon
DU-888 photometer mounted on the RTT-150 telescope, which allowed a record(for
CCD photometers) time resolution up to 8 ms to be achieved. The power spectra
of the source's flux variability have revealed that the aperiodic variability
contains information about the inner boundary of the optically thick flow in
the binary system. We show that the inner boundary of the optically thick
accretion disk comes close to the white dwarf surface at the maximum of the
source's bolometric light curve, i.e., at the peak of the instantaneous
accretion rate onto the white dwarf, while the optically thick accretion disk
is truncated at distances 8.5e9 cm ~10 R_{WD} in the low state. We suggest that
the location of the inner boundary of the accretion disk in the binary can be
traced by studying the parameters of the power spectra for accreting white
dwarfs. In particular, this allows the mass of the accreting object to be
estimated.Comment: 9 pages, 7 figures, Published in Astronomy Letter
Fast X-ray Transients and Their Connection to Gamma-Ray Bursts
Fast X-ray transients (FXTs) with timescales from seconds to hours have been
seen by numerous space instruments. We have assembled archival data from
Ariel-5, HEAO-1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global
fluence-frequency relationship for these events. Fitting the log N-log S
distribution over several orders of magnitude to simple power law we find a
slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, the -1 power
law is an approximate result of the summation of these multiple sources. Two
major contributions come from gamma-ray bursts and stellar flares.
Extrapolating from the BATSE catalog of GRBs, we find that the fraction of
X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a
function of fluence. Certainly most FXTs are not counterparts of standard
gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic
stars, is greatest for the faintest FXTs. Our understanding of the FXT
phenomenon remains limited and would greatly benefit from a large, homogeneous
data set, which requires a wide-field, sensitive instrument.Comment: 36 pages, 8 figure
4U 0115+63 from RXTE and INTEGRAL Data: Pulse Profile and Cyclotron Line Energy
We analyze the observations of the transient X-ray pulsar 4U 0115+63 with the
RXTE and INTEGRAL observatories in a wide X-ray (3-100 keV) energy band during
its intense outbursts in 1999 and 2004. The energy of the fundamental harmonic
of the cyclotron resonance absorption line near the maximum of the X-ray flux
from the source (luminosity range 5x10^{37} - 2x10^{38} erg/s) is ~11 keV. When
the pulsar luminosity falls below ~5x10^{37} erg/s, the energy of the
fundamental harmonic is displaced sharply toward the high energies, up to ~16
keV. Under the assumption of a dipole magnetic field configuration, this change
in cyclotron harmonic energy corresponds to a decrease in the height of the
emitting region by ~2 km, while other spectral parameters, in particular, the
cutoff energy, remain essentially constant. At a luminosity ~7x10^{37} erg/s,
four almost equidistant cyclotron line harmonics are clearly seen in the
spectrum. This suggests that either the region where the emission originates is
compact or the emergent spectrum from different (in height) segments of the
accretion column is uniform. We have found significant pulse profile variations
with energy, luminosity, and time. In particular, we show that the profile
variations from pulse to pulse are not reduced to a simple modulation of the
accretion rate specified by external conditions.Comment: 30 pages, 13 figures, Astronomy Letters, 33, 368 (2007
Traces of past activity in the Galactic Centre
The Milky Way centre hosts a supermassive Black Hole (BH) with a mass of
~4*10^6 M_Sun. Sgr A*, its electromagnetic counterpart, currently appears as an
extremely weak source with a luminosity L~10^-9 L_Edd. The lowest known
Eddington ratio BH. However, it was not always so; traces of "glorious" active
periods can be found in the surrounding medium. We review here our current view
of the X-ray emission from the Galactic Center (GC) and its environment, and
the expected signatures (e.g. X-ray reflection) of a past flare. We discuss the
history of Sgr A*'s past activity and its impact on the surrounding medium. The
structure of the Central Molecular Zone (CMZ) has not changed significantly
since the last active phase of Sgr A*. This relic torus provides us with the
opportunity to image the structure of an AGN torus in exquisite detail.Comment: Invited refereed review. Chapter of the book: "Cosmic ray induced
phenomenology in star forming environments" (eds. Olaf Reimer and Diego F.
Torres
On the nature of XTE J0421+560/CI Cam
We present the results of the analysis of RXTE, BATSE and optical/infrared
data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam).
The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days,
slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex,
softening considerably during decay and with strongly variable intrinsic
absorption. A strong iron emission line is observed. No fast time variability
is detected (<0.5 % rms in the 1-4096 Hz band at the outburst peak). The
analysis of the optical/IR data suggests that the secondary is a B[e] star
surrounded by cool dust and places the system at a distance of >~ 2 kpc. At
this distance the peak 2-25 keV luminosity is ~4 x 10^37 erg/s. We compare the
properties of this peculiar system with those of the Be/NS LMC transient A
0538-66 and suggest that CI Cam is of similar nature. The presence of strong
radio emission during outburst indicates that the compact object is likely to
be a black hole or a weakly magnetized neutron star.Comment: Accepted for publication on The Astrophysical Journal, July 199
On the Possibility of Observing the Shapiro Effect for Pulsars in Globular Clusters
For pulsars in globular clusters, we suggest using observations of the
relativistic time delay of their radiation in the gravitational eld of a
massive body (the Shapiro effect) located close to the line of sight to detect
and identify invisible compact objects and to study the distribution of both
visible and dark matter in globular clusters and various components of the
Galaxy. We have derived the dependences of the event probability on the
Galactic latitude and longitude of sources for two models of the mass
distribution in the Galaxy: the classical Bahcall-Soneira model and the more
recent Dehnen-Binney model. Using three globular clusters (M15, 47 Tuc, Terzan
5) as an example, we show that the ratios of the probability of the events due
to the passages of massive Galactic objects close to the line of sight to the
parameter f2 for pulsars in the globular clusters 47 Tuc and M15 are comparable
to those for close passages of massive objects in the clusters themselves and
are considerably higher than those for the cluster Terzan 5. We have estimated
the rates of such events. We have determined the number of objects near the
line of sight toward the pulsar that can produce a modulation of its pulse
arrival times characteristic of the effect under consideration; the population
of brown dwarfs in the Galactic disk, whose concentration is comparable to that
of the disk stars, has been taken into account for the first time.Comment: 26 pages, 9 figure
Diagnostics of the Early Explosion Phase of a Classical Nova Using Its X-ray Emission: A Model for the X-ray Outburst of CI Camelopardalis in 1998
We have computed a spherically symmetric model for the interaction of matter
ejected during the outburst of a classical nova with the stellar wind from its
optical component.This model is used to describe the intense X-ray outburst
(the peak 3-20 keV flux was ~2 Crab) of the binary system CI Camelopardalis in
1998. According to our model, the stellar wind from the optical component
heated by a strong shock wave produced when matter is ejected from the white
dwarf as the result of a thermonuclear explosion on its surface is the emission
source in the standard X-ray band. Comparison of the calculated and observed
time dependences of the mean radiation temperature and luminosity of the binary
system during its outburst has yielded very important characteristics of the
explosion.We have been able to measure the velocity of the ejected matter
immediately after the onset of the explosion for the first time: it follows
from our model that the ejected matter had a velocity of ~2700 km/s even on
0.1-0.5 day after the outburst onset and it flew with such a velocity for the
first 1-1.5 day under an external force, possibly, the radiation pressure from
the white dwarf. Subsequently, the matter probably became transparent and began
to decelerate. The time dependence of the mean radiation temperature at late
expansion phases has allowed us to estimate the mass of the ejected matter,
~10^{-7}-10^{-6} Msun. The mass loss rate in the stellar wind required to
explain the observed peak luminosity of the binary system during its outburst
has been estimated to be dM/dt ~(1-2)x10^{-6} Msun/yr.Comment: 19 pages, 24 figures; unimportant correction of the formul
A measurement of the broad-band spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications
We report on results of a target of opportunity observation of the X-ray
transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field
Instruments (0.1-200 keV) of the SAX satellite. The measured spectrum is a
power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and
a high-energy cutoff above ~100 keV. The soft excess is consistent with a
blackbody with temperature of ~40 eV and a low flux, while the cut-off power
law is well fitted by thermal Comptonization in a plasma with an electron
temperature of 100 keV and an optical depth of order of unity. Consistent with
the weakness of the blackbody, Compton reflection is weak. Though the data are
consistent with various geometries of the hot and cold phases of the accreting
gas, we conclude that a hot accretion disk is the most plausible model. The
Eddington ratio implied by recent estimates of the mass and the distance is
about 10^{-3}, which may indicate that advection is probably not the dominant
cooling mechanism. We finally suggest that the reflecting medium has a low
metallicity, consistent with location of the system in the halo.Comment: 14 pages, 8 figures, accepted for publication in The Astrophysical
Journa
Fitting the Gamma-Ray Spectrum from Dark Matter with DMFIT: GLAST and the Galactic Center Region
We study the potential of GLAST to unveil particle dark matter properties
with gamma-ray observations of the Galactic center region. We present full
GLAST simulations including all gamma-ray sources known to date in a region of
4 degrees around the Galactic center, in addition to the diffuse gamma-ray
background and to the dark matter signal. We introduce DMFIT, a tool that
allows one to fit gamma-ray emission from pair-annihilation of generic particle
dark matter models and to extract information on the mass, normalization and
annihilation branching ratios into Standard Model final states. We assess the
impact and systematic effects of background modeling and theoretical priors on
the reconstruction of dark matter particle properties. Our detailed simulations
demonstrate that for some well motivated supersymmetric dark matter setups with
one year of GLAST data it will be possible not only to significantly detect a
dark matter signal over background, but also to estimate the dark matter mass
and its dominant pair-annihilation mode.Comment: 37 pages, 16 figures, submitted to JCA
Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation
The problem of disk accretion onto the surface of a neutron star with a weak
magnetic field at a luminosity exceeding several percent of Eddington is
reduced to the problem of the braking of a hypersonic flow with a velocity that
is 0.4-0.5 of the speed of light above the base of the spreading layer -- a
dense atmosphere made up of previously fallen matter. We show that turbulent
braking in the Prandtl-Karman model with universally accepted coefficients for
terrestrial conditions and laboratory experiments and a ladder of interacting
gravity waves in a stratified quasi-exponential atmosphere at standard
Richardson numbers lead to a spin-up of the massive zone that extends to the
ocean made up of a plasma with degenerate electrons. Turbulent braking in the
ocean at the boundary with the outer solid crust reduces the rotation velocity
to the solid-body rotation velocity of the star. This situation should lead to
strong heating of deep atmospheric layers and to the switch-off of the
explosive helium burning mechanism. Obviously, a more efficient mechanism for
the dissipation of a fast azimuthal flow in the atmosphere should operate in
X-ray bursters. We show that a giant solitary gravity wave in the atmosphere
can lead to energy dissipation and to a sharp decrease in azimuthal velocity in
fairly rarefied atmospheric layers above the zone of explosive helium burning
nuclear reactions. We discuss the reasons why this wave, that has no direct
analog in the Earth's atmosphere or ocean, appears and its stability. We pose
the question as to whether neutron stars with massive atmospheres, spun up to
high velocities by accreting matter from a disk, can exist among the observed
Galactic X-ray sources.Comment: 12 pages, 12 figure