8,771 research outputs found
X-ray imaging above 3 keV
Energy dependent X-ray maps of clusters of galaxies are discussed. A design for a stacked, etched grid collimator is discussed
What can gamma rays tell us about binary x ray sources and SNR's
Black holes, neutron stars in binary x ray source systems, and supernova remnants (SNRs) are briefly discussed
X-ray Shapes of Distant Clusters: the Connection to Blue Galaxy Fractions
Based on ROSAT PSPC pointed observations, we have determined the aggregate
X-ray shapes of 10 distant (z = 0.17-0.54) rich clusters: A2397, A222, A520,
A1689, A223B, A1758, A2218, A2111, A2125, and CL0016+16. Four of the clusters
have global X-ray ellipticities greater than 0.2, as measured on a scale of
diameter 3 h_{50}^{-1} Mpc. These strongly elongated clusters tend to show
substantial amounts of substructure, indicating that they are dynamically young
systems. Most interestingly, the global X-ray ellipticities of the clusters
correlate well with their blue galaxy fractions; the correlation coefficient is
0.75 with a 90% confidence range of 0.44-0.92. This correlation suggests that
blue cluster galaxies originate in the process of cluster formation, and that
the blue galaxy proportion of a cluster decreases as the intracluster medium
relaxes onto equipotential surfaces.Comment: Submitted to ApJL, 9 pages including 2 figure
A Tidal Disruption Flare in Abell 1689 from an Archival X-ray Survey of Galaxy Clusters
Theory suggests that a star making a close passage by a supermassive black
hole at the center of a galaxy can under most circumstances be expected to emit
a giant flare of radiation as it is disrupted and a portion of the resulting
stream of shock-heated stellar debris falls back onto the black hole itself. We
examine the first results of an ongoing archival survey of galaxy clusters
using Chandra and XMM-selected data, and report a likely tidal disruption flare
from SDSS J131122.15-012345.6 in Abell 1689. The flare is observed to vary by a
factor of >30 over at least 2 years, to have maximum L_X(0.3-3.0 keV)> 5 x
10^{42} erg s^{-1} and to emit as a blackbody with kT~0.12 keV. From the galaxy
population as determined by existing studies of the cluster, we estimate a
tidal disruption rate of 1.2 x 10^{-4} galaxy^{-1} year^{-1} if we assume a
contribution to the observable rate from galaxies whose range of luminosities
corresponds to a central black hole mass (M_bh) between 10^6 and 10^8 M_sun.Comment: 24 pages, including 6 figures and 2 tables Accepted for publication
in the Astrophysical Journa
Search for cold gas in clusters with and without cooling flows
The dominant galaxy in each of approx. 40 clusters was studied using co-added Infrared Astronomy Satellite (IRAS) survey data, and 11 of these galaxies were observed for CO (J=1 to 0) emission with the 12 m telescope at Kitt Peak. Half of the galaxies in the sample are in clusters reported to have cooling flows while the other half are not. Six of the galaxies appear to have been detected by IRAS at fairly low flux levels, in addition to one previously known strong detection; all seven have reported cooling flows. No detectable CO emission (to 2 to 3 mK) was found in any of the 11 galaxies observed. Assuming accretion rates of approx. 100 Solar Mass yr(-1), the star formation rates and efficiencies in these galaxies must be quite high in order to render the CO undetectable. At the same time, the infrared luminosities of these galaxies is unremarkable, suggesting that the correlation between star formation efficiency and infrared luminosity found for spirals may not hold for cooling flows
X-ray Observations of Distant Optically Selected Cluster
We have measured fluxes or flux limits for 31 of the 79 cluster candidates in
the Palomar Distant Cluster Survey (PDCS) using archival ROSAT/PSPC pointed
observations. Our X-ray survey reaches a flux limit of erg s cm (0.4 - 2.0 keV), which corresponds to
luminosities of erg s ( = 50 km
s Mpc, = ), if we assume the PDCS estimated
redshifts. Of the 31 cluster candidates, we detect six at a signal-to-noise
greater than three. We estimate that (90% confidence
limits) of these six detections are a result of X-ray emission from objects
unrelated to the PDCS cluster candidates. The net surface density of X-ray
emitting cluster candidates in our survey, clusters
deg, agrees with that of other, X-ray selected, surveys. It is possible,
given the large error on our contamination rate, that we have not detected
X-ray emission from any of our observed PDCS cluster candidates. We find no
statistically significant difference between the X-ray luminosities of PDCS
cluster candidates and those of Abell clusters of similar optical richness.
This suggests that the PDCS contains objects at high redshift similar to the
low redshift clusters in the Abell catalogs. We show that the PDCS cluster
candidates are not bright X-ray sources, the average luminosity of the six
detected candidates is only erg s (0.4-2.0
keV). This finding is in agreement with previous X-ray studies of high
redshift, optically selected, rich clusters of galaxies.Comment: 19 pages, LaTeX with AAS Preprint Macros (v. 4), 3 embedded
postscript figures, 3 Seperate Tables using aj_pt4.sty, Accepted by the
Astronomical Journal for November 199
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
Optical properties of Mn4+ ions in GaN:Mn codoped with Mg acceptors
The optical properties of Mn-Mg codoped epitaxial GaN were studied. Addition
of Mg acceptors quenches the weak manganese-related photoluminescence (PL) band
at 1.3 eV in GaN:Mn and a series of sharp PL peaks are observed at 1 eV in
codoped epilayers. The change in PL spectra indicates that Mg addition
stabilizes the Mn4+ charge state by decreasing the Fermi level. The 1 eV PL
peaks are tentatively attributed to intra center transitions involving Mn4+
ions. Spin allowed 3d-shell 4T2-4T1 transitions and their phonon replicas are
involved. The relative intensities of the sharp peaks are strongly dependent on
the excitation wavelength, indicating the optically active Mn4+ centers
involved in the separate peaks are different. The temperature dependence of the
PL spectrum suggests the presence of at least three distinct Mn4+ complex
centers.Comment: 14 pages, 3 figures, 1 table, accepted by Appl. Phys. Let
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