2,021 research outputs found
Modeling SN 1996cr's X-ray lines at high-resolution: Sleuthing the ejecta/CSM geometry
SN 1996cr, located in the Circinus Galaxy (3.7 Mpc, z ~ 0.001) was
non-detected in X-rays at ~ 1000 days yet brightened to ~ 4 x 10^{39} erg/s
(0.5-8 keV) after 10 years (Bauer et al. 2008). A 1-D hydrodynamic model of the
ejecta-CSM interaction produces good agreement with the measured X-ray light
curves and spectra at multiple epochs. We conclude that the progenitor of SN
1996cr could have been a massive star, M > 30 M_solar, which went from an RSG
to a brief W-R phase before exploding within its ~ 0.04 pc wind-blown shell
(Dwarkadas et al. 2010). Further analysis of the deep Chandra HETG observations
allows line-shape fitting of a handful of bright Si and Fe lines in the
spectrum. The line shapes are well fit by axisymmetric emission models with an
axis orientation ~ 55 degrees to our line-of-sight. In the deep 2009 epoch the
higher ionization Fe XXVI emission is constrained to high lattitudes: the
Occam-est way to get the Fe H-like emission coming from high latitude/polar
regions is to have more CSM at/around the poles than at mid and lower
lattitudes, along with a symmetric ejecta explosion/distribution. Similar
CSM/ejecta characterization may be possible for other SNe and, with
higher-throughput X-ray observations, for gamma-ray burst remnants as well.Comment: 4 pages, 5 figures. To appear in the Proceedings of the Gamma Ray
Bursts 2010 Conference, Annapolis, USA. Editors: McEnery, Racusin, Gehrel
Modeling the Galactic center gamma-ray emission with more realistic cosmic-ray dynamics
Very-high-energy gamma-ray observations of the Galactic center (GC) show
extended emission that is strongly correlated with the morphology of the
central molecular zone (CMZ). The best explanation for that emission is a
hadronic interaction between cosmic rays (CRs) and ambient gas, where a CR
central and continuous source accelerates protons up to 1 PeV ("PeVatron").
However, current models assume very simplistic CR dynamics. Our goal is to
verify if more realistic CR dynamics for the GC environment are consistent with
current gamma-ray observations, and whether they could be constrained by
upcoming observations with the Cherenkov Telescope Array (CTA). We generated
synthetic gamma-ray maps using a CR transport model with spherical injection,
different diffusion regimes (in and out of the CMZ), polar advection, and
mono-energetic particles of 1 PeV, and including different CR populations
injected from the Arches, Quintuplet, and nuclear clusters of young massive
stars, plus supernova Sgr A East. We adopted two different 3D gas distributions
consistent with the observed gas column density, either with or without an
inner cavity. In order to reproduce the existing observations detected by the
High Energy Stereoscopic System (HESS), a ring-like gas distribution, with its
mass set by the standard Galactic CO-to-H conversion factor, and CR
acceleration from all relevant sources are required. For a conversion factor
one order of magnitude lower, injection rates that are ten times higher are
needed. We show that CTA will be able to differentiate between models with
different CR dynamics, proton sources, and CMZ morphology, owing to its
unprecedented sensitivity and angular resolution. More realistic CR dynamics
suggest that the CMZ has a large inner cavity and that the GC PeVatron is a
composite CR population accelerated by the Arches, Quintuplet, and nuclear star
clusters, and Sgr A East.Comment: Accepted for publication in A&A. 15 pages, 11 figure
Magellan/MMIRS near-infrared multi-object spectroscopy of nebular emission from star forming galaxies at 2<z<3
To investigate the ingredients, which allow star-forming galaxies to present
Lyalpha line in emission, we studied the kinematics and gas phase metallicity
(Z) of the interstellar medium. We used multi-object NIR spectroscopy with
Magellan/MMIRS to study nebular emission from z=2-3 star-forming galaxies
discovered in 3 MUSYC fields. We detected emission lines from four active
galactic nuclei and 13 high-z star-forming galaxies, including Halpha lines
down to a flux of 4.E-17 erg/sec/cm^2. This yielded 7 new redshifts. The most
common emission line detected is [OIII]5007, which is sensitive to Z. We were
able to measure Z for 2 galaxies and to set upper(lower) limits for another
2(2). The Z values are consistent with 0.3<Z/Zsun<1.2. Comparing the Lyalpha
central wavelength with the systemic redshift, we find
Delta_v(Lyalpha-[OIII])=70-270 km/sec. High-redshift star-forming galaxies,
Lyalpha emitting (LAE) galaxies, and Halpha emitters appear to be located in
the low mass, high star-formation rate (SFR) region of the SFR versus stellar
mass diagram, confirming that they are experiencing burst episodes of star
formation, which are building up their stellar mass. Their Zs are consistent
with the relation found for z<2.2 galaxies in the Z versus stellar mass plane.
The measured Delta_v(Lyalpha-[OIII]) values imply that outflows of material,
driven by star formation, could be present in the z=2-3 LAEs of our sample.
Comparing with the literature, we note that galaxies with lower Z than ours are
also characterized by similar Delta_v(Lyalpha-[OIII]) velocity offsets. Strong
[OIII] is detected in many Lyalpha emitters. Therefore, we propose the
Lyalpha/[OIII] flux ratio as a tool for the study of high-z galaxies; while
influenced by Z, ionization, and Lyalpha radiative transfer in the ISM, it may
be possible to calibrate this ratio to primarily trace one of these effects.Comment: 22 pages, 13 figures, 6 table
Near-Infrared Counterparts to Chandra X-ray Sources toward the Galactic Center. I. Statistics and a Catalog of Candidates
We present a catalog of 5184 candidate infrared counterparts to X-ray sources
detected towards the Galactic center. The X-ray sample contains 9017 point
sources detected in this region by the Chandra X-ray Observatory, including
data from a recent deep survey of the central 2 x 0.8 deg of the Galactic
plane. A total of 6760 of these sources have hard X-ray colors, and the
majority of them lie near the Galactic center, while most of the remaining 2257
soft X-ray sources lie in the foreground. We cross-correlated the X-ray source
positions with the 2MASS and SIRIUS near-infrared catalogs, which collectively
contain stars with a 10-sigma limiting flux of K_s<=15.6 mag. In order to
distinguish absorbed infrared sources near the Galactic center from those in
the foreground, we defined red and blue sources as those which have H-K_s>=0.9
and <=0.9 mag, respectively. We find that 5.8(1.5)% of the hard X-ray sources
have real infrared counterparts, of which 228(99) are red and 166(27) are blue.
The red counterparts are probably comprised of WR/O stars, HMXBs, and
symbiotics near the Galactic center. We also find that 39.4(1.0)% of the soft
X-ray sources have blue infrared counterparts; most of these are probably
coronally active dwarfs in the foreground. There is a noteworthy collection of
~20 red counterparts to hard X-ray sources near the Sagittarius-B H II region,
which are probably massive binaries that have formed within the last several
Myr. For each of the infrared matches to X-ray sources in our catalog we
derived the probability that the association is real, based on the results of
the cross-correlation analysis. The catalog will serve spectroscopic surveys to
identify infrared counterparts to X-ray sources near the Galactic center.Comment: Submitted to ApJ January 16, 2009; accepted July 21, 2009; 30 pages,
6 figure
Chandra Observations of the Three Most Metal-Deficient Blue Compact Dwarf Galaxies known in the Local Universe, SBS 0335-052, SBS 0335-052W, and I Zw 18
We present an X-ray study of the three most metal-deficient blue compact
dwarf (BCD) galaxies known in the local Universe, based on deep Chandra
observations of SBS 0335-052 (0.025 solar abundance), SBS 0335-052W (0.02 solar
abundance) and I Zw 18 (0.02 solar abundance). All three are detected, with
more than 90% of their X-ray emission arising from point-like sources. The
0.5-10.0 keV luminosities of these point sources are in the range
(1.3-8.5)x1e39 erg/s. We interpret them to be single or a collection of
high-mass X-ray binaries, the luminosities of which may have been enhanced by
the low metallicity of the gas. There are hints of faint extended diffuse X-ray
emission in both SBS 0335-052 and I Zw 18, probably associated with the
superbubbles visible in both BCDs. The spectrum of I Zw 18 shows a OVIII
hydrogen-like emission line. The best spectral fit gives an O overabundance of
the gas in the X-ray point source by a factor of ~7 with respect to the Sun, or
a factor of ~350 with respect to the O abundance determined for the HII region.Comment: emulateapj.cls used, 7 pages, 7 figures + 1 table, accepted for
publication in Ap
Massive Stellar X-ray Sources in the Galactic Center
We present results of a spectroscopic survey of bright near-infrared
counterparts to X-ray point sources from a deep Chandra survey of the Galactic nuclear bulge. K-band spectroscopy has revealed 13 new Wolf-Rayet and
O-supergiant counterparts to Chandra sources in the Galactic center (GC). Although they are systematically softer in X-rays than the general GC source population of accretion powered cataclysmic variables (CVs), their X-ray colors indicate a hard component consistent with emission from plasmas with E > 2 keV. Such hard X-ray emission is not ubiquitous among single Wolf-Rayet and O stars,
but is common among Wolf-Rayet+OB binaries with colliding supersonic winds.
Although we regard colliding-wind binary hypothesis as the most likely scenario,
it remains possible that several of these objects are wind-accreting neutron stars
or black holes in supergiant high-mass X-ray binaries, or extraordinary single
stars emitting hard X-rays
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