9,079 research outputs found
The Galactic Center GeV Excess from a Series of Leptonic Cosmic-Ray Outbursts
It has been proposed that a recent outburst of cosmic-ray electrons could
account for the excess of GeV-scale gamma rays observed from the region
surrounding the Galactic Center. After studying this possibility in some
detail, we identify scenarios in which a series of leptonic cosmic-ray
outbursts could plausibly generate the observed excess. The morphology of the
emission observed outside of from the Galactic Center
can be accommodated with two outbursts, one which took place approximately
years ago, and another (injecting only about 10\% as much energy as
the first) about years ago. The emission observed from the innermost
requires one or more additional recent outbursts
and/or a contribution from a centrally concentrated population of unresolved
millisecond pulsars. In order to produce a spectrum that is compatible with the
measured excess (whose shape is approximately uniform over the region of the
excess), the electrons from the older outburst must be injected with
significantly greater average energy than those injected more recently,
enabling their spectra to be similar after years of energy losses.Comment: 28 pages, 7 figures, 3 tables, 1 appendi
Combined Spectral and Timing Analysis of the Black Hole Candidate MAXI J1659-152 Discovered by MAXI and Swift
We report on X-ray spectral and timing results of the new black hole
candidate (BHC) MAXI J1659-152 with the orbital period of 2.41 hours (shortest
among BHCs) in the 2010 outburst from 65 Rossi X-ray Timing Explorer (RXTE)
observations and 8 simultaneous Swift and RXTE observations. According to the
definitions of the spectral states in Remillard & McClintock (2006), most of
the observations have been classified into the intermediate state. All the
X-ray broadband spectra can be modeled by a multi-color disk plus a power-law
with an exponential cutoff or a multi-color disk plus a Comptonization
component. During the initial phase of the outburst, a high energy cutoff was
visible at 30-40 keV. The innermost radius of the disk gradually decreased by a
factor of more than 3 from the onset of the outburst and reached a constant
value of 35 d_10 cos i^-1/2 km, where d_10 is the distance in units of 10 kpc
and is the inclination. The type-C quasi-periodic oscillation (QPO)
frequency varied from 1.6 Hz to 7.3 Hz in association with a change of the
innermost radius, while the innermost radius remained constant during the
type-B QPO detections at 1.6-4.1 Hz. Hence, we suggest that the origin of the
type-B QPOs is different from that of type-C QPOs, the latter of which would
originate from the disk truncation radius. Assuming the constant innermost
radius in the latter phase of the outburst as the innermost stable circular
orbit, the black hole mass in MAXI J1659-152 is estimated to be 3.6-8.0 M_solar
for a distance of 5.3-8.6 kpc and an inclination angle of 60-75 degrees.Comment: 27 pages, 14 figures, accepted for publication in PAS
Discovery of photon index saturation in the black hole binary GRS 1915+105
We present a study of the correlations between spectral, timing properties
and mass accretion rate observed in X-rays from the Galactic Black Hole (BH)
binary GRS 1915+105 during the transition between hard and soft states. We
analyze all transition episodes from this source observed with RXTE,
coordinated with Ryle Radio Telescope (RT) observations. We show that
broad-band energy spectra of GRS 1915+105 during all these spectral states can
be adequately presented by two Bulk Motion Comptonization (BMC) components: a
hard component (BMC1, photon index Gamma_1=1.7-3.0) with turnover at high
energies and soft thermal component (BMC2, Gamma_2=2.7-4.2) with characteristic
color temperature <1 keV, and the redskewed iron line (LAOR) component. We also
present observable correlations between the index and the normalization of the
disk "seed" component. The use of "seed" disk normalization, which is
presumably proportional to mass accretion rate in the disk, is crucial to
establish the index saturation effect during the transition to the soft state.
We discovered the photon index saturation of the soft and hard spectral
components at values of 4.2 and 3 respectively. We present a physical model
which explains the index-seed photon normalization correlations. We argue that
the index saturation effect of the hard component (BMC1) is due to the soft
photon Comptonization in the converging inflow close to BH and that of soft
component is due to matter accumulation in the transition layer when mass
accretion rate increases. In addition to our spectral model components we also
find a strong feature of "blackbody-like" bump which color temperature is about
4.5 keV in eight observations of the intermediate and soft states. We discuss a
possible origin of this "blackbody-like" emission.Comment: 33 pages, 16 figures, accepted for publication in ApJ, on December
10, 2009, v. 70
Innermost Stable Circular Orbits and Epicyclic Frequencies Around a Magnetized Neutron Star
A full-relativistic approach is used to compute the radius of the innermost
stable circular orbit (ISCO), the Keplerian, frame-dragging, precession and
oscillation frequencies of the radial and vertical motions of neutral test
particles orbiting the equatorial plane of a magnetized neutron star. The
space-time around the star is modelled by the six parametric solution derived
by Pachon et al. It is shown that the inclusion of an intense magnetic field,
such as the one of a neutron star, have non-negligible effects on the above
physical quantities, and therefore, its inclusion is necessary in order to
obtain a more accurate and realistic description of the physical processes
occurring in the neighbourhood of this kind of objects such as the dynamics of
accretion disk. The results discussed here also suggest that the consideration
of strong magnetic fields may introduce non-negligible corrections in, e.g.,
the relativistic precession model and therefore on the predictions made on the
mass of neutron stars.Comment: LaTeX file, 13 pages, 4 figure
The X-ray spectral evolution of Cyg X-2 in the framework of bulk Comptonization
We used the newly developed thermal plus bulk Comptonization model comptb to
investigate the spectral evolution of the neutron star LMXB Cyg X-2 along its
Z-track. We selected a single source in order to trace in a quantitative way
the evolution of the physical parameters of the model. We analyzed archival
broad-band BeppoSAX spectra of Cyg X-2. Five broad-band spectra have been newly
extracted according to the source position in the Z-track described in the
colour-colour and colour-intensity diagrams. We have fitted the spectra of the
source with two comptb components. The first one, with bulk parameter delta=0,
dominates the overall source broad-band spectrum and its origin is related to
thermal upscattering (Comptonization) of cold seed photons off warm electrons
in high-opacity enviroment. We attribute the origin of these seed photons to
the part of the disk which illuminates the outer coronal region (transition
layer) located between the accretion disk itself and the neutron star surface.
This thermal component is roughly constant with time and with inferred mass
accretion rate. The second comptb model describes the overall Comptonization
(thermal plus bulk, delta > 0) of hotter seed photons which come from both the
inner transition layer and from the neutron star surface. The appearance of
this component in the colour-colour or hardness-intensity diagram is more
pronounced in the horizontal branch and is progressively disappearing towards
the normal branch, where a pure blackbody spectrum is observed. The spectral
evolution of Cyg X-2 is studied and interpreted in terms of changes in the
innermost environmental conditions of the system, leading to a variable
thermal-bulk Comptonization efficiency.Comment: 10 pages, 7 figures, accepted for publication in A&
Investigating the Uniformity of the Excess Gamma rays towards the Galactic Center Region
We perform a composite likelihood analysis of subdivided regions within the
central of the Milky Way, with the aim of
characterizing the spectrum of the gamma-ray galactic center excess in regions
of varying galactocentric distance. Outside of the innermost few degrees, we
find that the radial profile of the excess is background-model dependent and
poorly constrained. The spectrum of the excess emission is observed to extend
upwards of 10 GeV outside in radius, but cuts off steeply between
10--20 GeV only in the innermost few degrees. If interpreted as a real feature
of the excess, this radial variation in the spectrum has important implications
for both astrophysical and dark matter interpretations of the galactic center
excess. Single-component dark matter annihilation models face challenges in
reproducing this variation; on the other hand, a population of unresolved
millisecond pulsars contributing both prompt and secondary inverse Compton
emission may be able to explain the spectrum as well as its spatial dependency.
We show that the expected differences in the photon-count distributions of a
smooth dark matter annihilation signal and an unresolved point source
population are an order of magnitude smaller than the fluctuations in residuals
after fitting the data, which implies that mismodeling is an important
systematic effect in point source analyses aimed at resolving the gamma-ray
excess.Comment: 27 pages, 9 figures. Matches accepted version: references added, typo
corrected in Sec. 4.2, some additional discussion added (results unchanged
Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources
Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and
Galactic superluminal jet sources share the common spectral characteristic that
they have unusually high disk temperatures which cannot be explained in the
framework of the standard optically thick accretion disk in the Schwarzschild
metric. On the other hand, the standard accretion disk around the Kerr black
hole might explain the observed high disk temperature, as the inner radius of
the Kerr disk gets smaller and the disk temperature can be consequently higher.
However, we point out that the observable Kerr disk spectra becomes
significantly harder than Schwarzschild disk spectra only when the disk is
highly inclined. This is because the emission from the innermost part of the
accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen
for a face-on disk. The Galactic superluminal jet sources are known to be
highly inclined systems, thus their energy spectra may be explained with the
standard Kerr disk with known black hole masses. For ULXs, on the other hand,
the standard Kerr disk model seems implausible, since it is highly unlikely
that their accretion disks are preferentially inclined, and, if edge-on Kerr
disk model is applied, the black hole mass becomes unreasonably large (> 300
M_solar). Instead, the slim disk (advection dominated optically thick disk)
model is likely to explain the observed super-Eddington luminosities, hard
energy spectra, and spectral variations of ULXs. We suggest that ULXs are
accreting black holes with a few tens of solar mass, which is not unexpected
from the standard stellar evolution scenario, and that their X-ray emission is
from the slim disk shining at super-Eddington luminosities.Comment: ApJ, accepte
A Chandra View of the Normal SO Galaxy NGC 1332: II: Solar Abundances in the Hot Gas and Implications for SN Enrichment
We present spectral analysis of the diffuse emission in the normal, isolated,
moderate-Lx S0 NGC 1332, constraining both the temperature profile and the
metal abundances in the ISM. The characteristics of the point source population
and the gravitating matter are discussed in two companion papers. The diffuse
emission comprises hot gas, with an ~isothermal temperature profile (~0.5 keV),
and emission from unresolved point-sources. In contrast with the cool cores of
many groups and clusters, we find a small central temperature peak. We obtain
emission-weighted abundance contraints within 20 kpc for several key elements:
Fe, O, Ne, Mg and Si. The measured iron abundance (Z_Fe=1.1 in solar units;
>0.53 at 99% confidence) strongly excludes the very sub-solar values often
historically reported for early-type galaxies but agrees with recent
observations of brighter galaxies and groups. The abundance ratios, with
respect to Fe, of the other elements were also found to be ~solar, although
Z_o/Z_Fe was significantly lower (<0.4). Such a low O abundance is not
predicted by simple models of ISM enrichment by Type Ia and Type II supernovae,
and may indicate a significant contribution from primordial hypernovae.
Revisiting Chandra observations of the moderate-Lx, isolated elliptical NGC
720, we obtain similar abundance constraints. Adopting standard SNIa and SNII
metal yields, our abundance ratio constraints imply 73+/-5% and 85+/-6% of the
Fe enrichment in NGC 1332 and NGC 720, respectively, arises from SNIa. Although
these results are sensitive to the considerable systematic uncertainty in the
SNe yields, they are in good agreement with observations of more massive
systems. These two moderate-Lx early-type galaxies reveal a consistent pattern
of metal enrichment from cluster scales to moderate Lx/Lb galaxies. (abridged)Comment: 12 pages, 4 figures, accepted for publication in ApJ. Minor changes
to match published versio
The X-ray luminous cluster underlying the bright radio-quiet quasar H1821+643
We present a Chandra observation of the only low redshift, z=0.299, galaxy
cluster to contain a highly luminous radio-quiet quasar, H1821+643. By
simulating the quasar PSF, we subtract the quasar contribution from the cluster
core and determine the physical properties of the cluster gas down to 3 arcsec
(15 kpc) from the point source. The temperature of the cluster gas decreases
from 9.0\pm0.5 keV down to 1.3\pm0.2 keV in the centre, with a short central
radiative cooling time of 1.0\pm0.1 Gyr, typical of a strong cool-core cluster.
The X-ray morphology in the central 100 kpc shows extended spurs of emission
from the core, a small radio cavity and a weak shock or cold front forming a
semi-circular edge at 15 arcsec radius. The quasar bolometric luminosity was
estimated to be 2 x 10^{47} erg per sec, requiring a mass accretion rate of 40
Msolar per yr, which corresponds to half the Eddington accretion rate. We
explore possible accretion mechanisms for this object and determine that Bondi
accretion, when boosted by Compton cooling of the accretion material, could
provide a significant source of the fuel for this outburst. We consider
H1821+643 in the context of a unified AGN accretion model and, by comparing
H1821+643 with a sample of galaxy clusters, we show that the quasar has not
significantly affected the large-scale cluster gas properties.Comment: 20 pages, 19 figures, accepted by MNRA
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