Mon.Not.Roy.Astron.Soc. 348 (2004) L28 We examine the spatial offsets between X-ray point sources and star clusters
in three starburst galaxies. We find that the X-ray sources are preferentially
...located near the star clusters. Because the star clusters are very good tracers
of the star formation activity in the galaxies, this indicates that the X-ray
sources are young objects associated with current star formation. We find
significant displacements of the X-ray sources from the clusters. These
displacements are likely due to motion of the X-ray sources and indicates that
they are X-ray binaries. We find that brighter X-ray sources preferentially
occur closer to clusters. The absence of very bright sources at large
displacements from clusters may help constrain models of the sources.
We characterized the evolution of spectral and temporal properties of several
Galactic black hole transients observed between 1996-2001 using the data from
well sampled PCA observations close to the ...transition to the low/hard state. We
showed that the changes in temporal properties are much sharper than the
changes in the spectral properties, and it is much easier to identify a state
transition with the temporal properties. The ratio of the power-law flux to the
total flux in the 3-25 keV band increases close to the transition, and the
power-law flux shows a sharp increase along with the changes in temporal
properties during the transitions. In this work we concentrate on the decay of
two recent outbursts, from 4U 1543$-$47, and H1743$-$322 and discuss the state
transitions by tracking their daily, and sometimes hourly evolution, and
interpret results based on the expectations from our earlier observations.
Astrophys.J.603:231-241,2004 We characterize the evolution of spectral and temporal properties of several
Galactic black hole transients during outburst decay using the data from well
sampled ...PCA/\rxte observations close to the transition to the low/hard state.
We find several global patterns of evolution for spectral and temporal
parameters before, during, and after the transition. We show that the changes
in temporal properties (sudden increase or decrease in the rms amplitude of
variability) are much sharper than the changes in the spectral properties, and
it is much easier to identify a state transition with the temporal properties.
The spectral index shows a drop 3-5 days before the transition for some of
our sources. The ratio of the power-law flux to the total flux in the 3-25 keV
band increases close to the transition, which may mean that the system must be
dominated by the coronal emission for the transition to occur. We also show
that the power-law flux shows a sharp change along with the temporal properties
during the transitions which may indicate a threshold transition volume for the
corona. The evolution of the spectral and temporal properties after the
transition is consistent with the idea that the inner accretion disk moves away
from the black hole. Based on the evolution of spectral and temporal parameters
and changes during the transitions, we discuss possible scenarios of how the
transition is happening.
We present new results from a multi-wavelength (radio/infrared/optical/X-ray) study of the black hole X-ray binary GRO J1655-40 during its 2005 outburst. We detected, for the first time, mid-infrared ...emission at 24 um from the compact jet of a black hole X-ray binary during its hard state, when the source shows emission from a radio compact jet as well as a strong non-thermal hard X-ray component. These detections strongly constrain the optically thick part of the synchrotron spectrum of the compact jet, which is consistent with being flat over four orders of magnitude in frequency. Moreover, using this unprecedented coverage, and especially thanks to the new Spitzer observations, we can test broadband disk and jet models during the hard state. Two of the hard state broadband spectra are reasonably well fitted using a jet model with parameters overall similar to those previously found for Cyg X-1 and GX 339-4. Differences are also present; most notably, the jet power in GRO J1655-40 appears to be a factor of at least ~3-5 higher (depending on the distance) than that of Cyg X-1 and GX 339-4 at comparable disk luminosities. Furthermore, a few discrepancies between the model and the data, previously not found for the other two black hole systems for which there was no mid-IR/IR and optical coverage, are evident, and will help to constrain and refine theoretical models.
Indirect dark matter searches with ground-based gamma-ray observatories provide an alternative for identifying the particle nature of dark matter that is complementary to that of direct search or ...accelerator production experiments. We present the results of observations of the dwarf spheroidal galaxies Draco, Ursa Minor, Bootes 1, and Willman 1 conducted by VERITAS. These galaxies are nearby dark matter dominated objects located at a typical distance of several tens of kiloparsecs for which there are good measurements of the dark matter density profile from stellar velocity measurements. Since the conventional astrophysical background of very high energy gamma rays from these objects appears to be negligible, they are good targets to search for the secondary gamma-ray photons produced by interacting or decaying dark matter particles. No significant gamma-ray flux above 200 GeV was detected from these four dwarf galaxies for a typical exposure of ~20 hours. The 95% confidence upper limits on the integral gamma-ray flux are in the range 0.4-2.2x10^-12 photons cm^-2s^-1. We interpret this limiting flux in the context of pair annihilation of weakly interacting massive particles and derive constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the WIMPs. The limits are obtained under conservative assumptions regarding the dark matter distribution in dwarf galaxies and are approximately three orders of magnitude above the generic theoretical prediction for WIMPs in the minimal supersymmetric standard model framework. However significant uncertainty exists in the dark matter distribution as well as the neutralino cross sections which under favorable assumptions could further lower the limits.
This is a report on the findings of the SNR/cosmic-ray working group for the white paper on the status and future of ground-based gamma-ray astronomy. The white paper is an APS commissioned document, ...and the overall version has also been released and can be found on astro-ph. This detailed section of the white paper discusses the status of past and current attempts to observe shell-type supernova remnants and diffuse emission from cosmic rays at GeV-TeV energies. We concentrate on the potential of future ground-based gamma-ray experiments to study the acceleration of relativistic charged particles which is one of the main unsolved, yet fundamental, problems in modern astrophysics. The acceleration of particles relies on interactions between energetic particles and magnetic turbulence. In the case of SNRs we can perform spatially resolved studies in systems with known geometry, and the plasma physics deduced from these observations will help us to understand other systems where rapid particle acceleration is believed to occur and where observations as detailed as those of SNRs are not possible.
Astron. Astrophys. 349, L77 (1999) We report the first simultaneous measurement of the broad band X-ray (0.3-150
keV) spectrum of the neutron star x-ray binary 4U0614+091. Our data confirm the
...presence of a hard x-ray tail that can be modeled as thermal Comptonization of
low-energy photons on electrons having a very high temperature, greater than
220 keV, or as a non-thermal powerlaw. We detected a spectral feature that can
be interpreted as reprocessing, via Compton reflection, of the direct emission
by an optically-thick disk and found a correlation between the photon index of
the power-law tail and the fraction of radiation reflected which is similar to
the correlation found for black hole candidate x-ray binaries and Seyfert
galaxies.
POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and ...500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment -- GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest.
Astron.Astrophys.407:L41,2003 The hard X-ray sensitivity and arcminute position accuracy of the recently
launched International Gamma-Ray Laboratory (INTEGRAL) has lead to the
(re-)discovery of a ...class of heavily absorbed hard X-ray sources lying in the
Galactic plane. We report on the analysis of an XMM observation of such a
source IGR J16320-4751 = AX J1631.9-4752. Our analysis allowed us to obtain the
most accurate X-ray position to date (Rodriguez et al. 2003), and to identify a
likely infrared counterpart (Tomsick et al. 2003). We present the detailed
analysis of the IGR J1632-4751 XMM spectra. The PN spectrum can be well
represented by a single powerlaw or a comptonized spectrum with a high
equivalent absorption column density of ~ 2 x10^{23} cm^{-2}. The current
analysis and the comparison with the properties of other sources favor the
possibility that the source is a Galactic X-Ray Binary (XRB). The
identification of two candidate IR counterparts is in good agreement with this
identification. The hard spectrum previously seen with ASCA, and the brightness
of the candidate counterparts indicate that IGR J1632-4751 is most probably a
highly absorbed High Mass X-ray Binary, hosting a neutron star.
The cores of neutron stars harbor the highest matter densities known to occur in nature, up to several times the densities in atomic nuclei. Similarly, magnetic field strengths can exceed the ...strongest fields generated in terrestrial laboratories by ten orders of magnitude. Hyperon-dominated matter, deconfined quark matter, superfluidity, even superconductivity are predicted in neutron stars. Similarly, quantum electrodynamics predicts that in strong magnetic fields the vacuum becomes birefringent. The properties of matter under such conditions is governed by Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED), and the close study of the properties of neutron stars offers the unique opportunity to test and explore the richness of QCD and QED in a regime that is utterly beyond the reach of terrestrial experiments. Experimentally, this is almost virgin territory.
