2,095 research outputs found
RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1500 Bright X-ray Sources
We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA
Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray
sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that
are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic
sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS
sources is low enough that they can be reliably identified with RBSC sources
having average rms positional uncertainties = 10 arcsec. We used the more
accurate radio positions to make reliable X-ray/radio/optical identifications
down to the POSS plate limits. We obtained optical spectra for many of the
bright identifications lacking published redshifts. The resulting X-ray/radio
sample is unique in its size (N ~ 1500 objects), composition (a mixture of
nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low
average redshift ( ~ 0.1).Comment: 35 LaTeX pages including 6 eps figures + 40 LaTeX page table2
(landscape) w/ AASTeX 5.0; accepted to ApJ
Chandra and HST Confirmation of the Luminous and Variable X-ray Source IC 10 X-1 as a Possible Wolf-Rayet, Black-Hole Binary
We present a Chandra and HST study of IC 10 X-1, the most luminous X-ray
binary in the closest starburst galaxy to the Milky Way. Our new hard X-ray
observation of X-1 confirms that it has an average 0.5-10 keV luminosity of
1.5e38 erg/s, is strongly variable (a factor of ~2 in >3 ks), and is spatially
coincident (within 0.'23 +/-0.'30) with the Wolf-Rayet (WR) star [MAC92] 17A in
IC 10. The spectrum of X-1 is best fit by a power law with photon index of ~1.8
and a thermal plasma with kT~1.5 keV, although systematic residuals hint at
further complexity. Taken together, these facts suggest that X-1 may be a black
hole belonging to the rare class of WR binaries; it is comparable in many ways
to Cyg X-3. The Chandra observation also finds evidence for extended X-ray
emission co-spatial with the large non-thermal radio superbubble surrounding
X-1.Comment: ApJL in press (Oct 2003), 4 pages, 4 figures (w/ fig1 at severely
reduced quality), latest emulateapj.cls use
Probing the Complex and Variable X-ray Absorption of Markarian 6 with XMM-Newton
We report on an X-ray observation of the Seyfert 1.5 galaxy Mrk 6 obtained
with the EPIC instruments onboard XMM-Newton. Archival BeppoSAX PDS data from
18-120 keV were also used to constrain the underlying hard power-law continuum.
The results from our spectral analyses generally favor a double
partial-covering model, although other spectral models such as absorption by a
mixture of partially ionized and neutral gas cannot be firmly ruled out. Our
best-fitting model consists of a power law with a photon index of 1.81+/-0.20
and partial covering with large column densities up to 10^{23} cm**-2. We also
detect a narrow emission line consistent with Fe Kalpha fluorescence at
6.45+/-0.04 keV with an equivalent width of ~93+/-25 eV. Joint analyses of
XMM-Newton, ASCA, and BeppoSAX data further provide evidence for both spectral
variability (a factor of ~2 change in absorbing column) and
absorption-corrected flux variations (by ~60%) during the ~4 year period probed
by the observations.Comment: 7 pages, 2 figures. accepted for publication in the Astronomical
Journa
NuSTAR Spectroscopy of Multi-Component X-ray Reflection from NGC 1068
We report on observations of NGC1068 with NuSTAR, which provide the best constraints to date on its > 10 keV spectral shape. The NuSTAR data are consistent with past instruments, with no strong continuum or line variability over the past two decades, consistent with its classification as a Compton-thick AGN. The combined NuSTAR, Chandra, XMM-Newton, and Swift BAT spectral dataset offers new insights into the complex secondary emission seen instead of the completely obscured transmitted nuclear continuum. The critical combination of the high signal-to-noise NuSTAR data and the decomposition of the nuclear and extranuclear emission with Chandra allow us to break several model degeneracies and greatly aid physical interpretation. When modeled as a monolithic (i.e., a single N_H) reflector, none of the common Compton-reflection models are able to match the neutral fluorescence lines and broad spectral shape of the Compton reflection without requiring unrealistic physical parameters (e.g., large Fe overabundances, inconsistent viewing angles, poor fits to the spatially resolved spectra). A multi-component reflector with three distinct column densities (e.g., with best-fit values of N_H = 1.5×10^(23), 5×10^(24), and 10^(25) cm^(-2)) provides a more reasonable fit to the spectral lines and Compton hump, with near-solar Fe abundances. In this model, the higher N_H component provides the bulk of the flux to the Compton hump while the lower N_H component produces much of the line emission, effectively decoupling two key features of Compton reflection. We also find that ≈ 30% of the neutral Fe Kɑ line flux arises from >2" (≈140 pc) and is clearly extended, implying that a significant fraction of the <10 keV reflected component arises from regions well outside of a parsec-scale torus. These results likely have ramifications for the interpretation of Compton-thick spectra from observations with poorer signal-to-noise and/or
more distant objects
Hard X-ray Morphological and Spectral Studies of The Galactic Center Molecular Cloud Sgr B2: Constraining Past Sgr A* Flaring Activity
Galactic Center (GC) molecular cloud Sgr B2 is the best manifestation of an
X-ray reflection nebula (XRN) reprocessing a past giant outburst from the
supermassive black hole Sgr A*. Alternatively, Sgr B2 could be illuminated by
low-energy cosmic ray electrons (LECRe) or protons (LECRp). In 2013, NuSTAR for
the first time resolved Sgr B2 hard X-ray emission on sub-arcminute scales. Two
prominent features are detected above 10 keV - a newly emerging cloud
G0.66-0.13 and the central 90" radius region containing two compact cores Sgr
B2(M) and Sgr B2(N) surrounded by diffuse emission. It is inconclusive whether
the remaining level of Sgr B2 emission is still decreasing or has reached a
constant background level. A decreasing Fe K emission can be best
explained by XRN while a constant background emission can be best explained by
LECRp. In the XRN scenario, the 3-79 keV Sgr B2 spectrum can well constrain the
past Sgr A* outburst, resulting in an outburst spectrum with a peak luminosity
of derived from the
maximum Compton-scattered continuum and the Fe K emission consistently.
The XRN scenario is preferred by the fast variability of G0.66-0.13, which
could be a molecular clump located in the Sgr B2 envelope reflecting the same
Sgr A* outburst. In the LECRp scenario, we derived the required CR ion power
and the CR ionization rate
. The Sgr B2 background level
X-ray emission will be a powerful tool to constrain GC CR population.Comment: 17 pages, 6 figures, submitted to Ap
Determining the nature of the faint X-ray source population near the Galactic Centre
We present results of a multi-wavelength program to study the faint discrete
X-ray source population discovered by Chandra in the Galactic Centre (GC). From
IR imaging obtained with the VLT we identify candidate K-band counterparts to
75% of the X-ray sources in our sample. By combining follow-up VLT K-band
spectroscopy of a subset of these candidate counterparts with the magnitude
limits of our photometric survey, we suggest that only a small percentage of
the sources are HMXBs, while the majority are likely to be canonical LMXBs and
CVs at the distance of the GC. In addition, we present our discovery of highly
structured small-scale (5-15") extinction towards the Galactic Centre. This is
the finest-scale extinction study of the Galactic Centre to date. Finally, from
these VLT observations we are able to place constraints on the stellar
counterpart to the ``bursting pulsar'' GRO J1744-28.Comment: 9 pages, in Proceedings of "VI Microquasar Workshop: Microquasars and
Beyond, 18-22 September 2006, Como, Italy; paper in PDF format with
full-resolution figures available at
http://www.astro.ufl.edu/~reba/mqw_rmb.pd
Resonant elastic X-ray scattering of antiferromagnetic superstructures in EuPtSi
We report resonant elastic X-ray scattering (REXS) of long-range magnetic
order in EuPtSi, combining different scattering geometries with
full linear polarization analysis to unambiguously identify magnetic scattering
contributions. At low temperatures, EuPtSi stabilizes type A
antiferromagnetism featuring various long-wavelength modulations. For magnetic
fields applied in the hard magnetic basal plane, well-defined regimes of
cycloidal, conical, and fan-like superstructures may be distinguished that
encompass a pocket of commensurate type A order without superstructure. For
magnetic field applied along the easy axis, the phase diagram comprises the
cycloidal and conical superstructures only. Highlighting the power of polarized
REXS, our results reveal a combination of magnetic phases that suggest a highly
unusual competition between antiferromagnetic exchange interactions with
Dzyaloshinsky--Moriya spin--orbit coupling of similar strength
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