3,090 research outputs found
Development and Operation of the Astrophysics Data System
The Astrophysics Data System (ADS) provides access to astronomical bibliographic information, including references, abstracts, and full journal articles, as well as links to other on-line information sources like on-line electronic journals and on-line data. This section will first provide a brief history of the ADS, a general introduction of the current system, and a more detailed description of some of the parts of the ADS
Obscuration by Gas and Dust in Luminous Quasars
We explore the connection between absorption by neutral gas and extinction by
dust in mid-infrared (IR) selected luminous quasars. We use a sample of 33
quasars at redshifts 0.7 < z < 3 in the 9 deg^2 Bo\"otes multiwavelength survey
field that are selected using Spitzer Space Telescope Infrared Array Camera
colors and are well-detected as luminous X-ray sources (with >150 counts) in
Chandra observations. We divide the quasars into dust-obscured and unobscured
samples based on their optical to mid-IR color, and measure the neutral
hydrogen column density N_H through fitting of the X-ray spectra. We find that
all subsets of quasars have consistent power law photon indices equal to 1.9
that are uncorrelated with N_H. We classify the quasars as gas-absorbed or
gas-unabsorbed if N_H > 10^22 cm^-2 or N_H < 10^22 cm^-2, respectively. Of 24
dust-unobscured quasars in the sample, only one shows clear evidence for
significant intrinsic N_H, while 22 have column densities consistent with N_H <
10^22 cm^-2. In contrast, of the nine dust-obscured quasars, six show evidence
for intrinsic gas absorption, and three are consistent with N_H < 10^22 cm^-2.
We conclude that dust extinction in IR-selected quasars is strongly correlated
with significant gas absorption as determined through X-ray spectral fitting.
These results suggest that obscuring gas and dust in quasars are generally
co-spatial, and confirm the reliability of simple mid-IR and optical
photometric techniques for separating quasars based on obscuration.Comment: 5 pages, 3 figure
Is the Compact Source at the Center of Cas A Pulsed?
A 50 ksec observation of the Supernova Remnant Cas A was taken using the
Chandra X-Ray Observatory High Resolution Camera (HRC) to search for periodic
signals from the compact source located near the center. Using the HRC-S in
imaging mode, problems with correctly assigning times to events were overcome,
allowing the period search to be extended to higher frequencies than possible
with previous observations. In an extensive analysis of the HRC data, several
possible candidate signals are found using various algorithms, including
advanced techniques developed by Ransom to search for low significance periodic
signals. Of the candidate periods, none is at a high enough confidence level to
be particularly favored over the rest. When combined with other information,
however (e.g., spectra, total energetics, and the historical age of the
remnant), a 12 ms candidate period seems to be more physically plausible than
the others, and we use it for illustrative purposes in discussing the possible
properties of a putative neutron star in the remnant. We emphasize that this is
not necessarily the true period, and that a follow-up observation, scheduled
for the fall of 2001, is required.
A 50 ksec Advanced CCD Imaging Spectrometer (ACIS) observation was taken, and
analysis of these data for the central object shows that the spectrum is
consistent with several forms, and that the emitted X-ray luminosity in the 0.1
-10 keV band is 10^{33}-10^{35}erg cm^{-2}sec^{-1} depending on the spectral
model and the interstellar absorption along the line of sight to the source.Comment: 14 pages, 3 figures Submitted to ApJ 2001 June 2
Thermal gravity, black holes and cosmological entropy
Taking seriously the interpretation of black hole entropy as the logarithm of
the number of microstates, we argue that thermal gravitons may undergo a phase
transition to a kind of black hole condensate. The phase transition proceeds
via nucleation of black holes at a rate governed by a saddlepoint configuration
whose free energy is of order the inverse temperature in Planck units. Whether
the universe remains in a low entropy state as opposed to the high entropy
black hole condensate depends sensitively on its thermal history. Our results
may clarify an old observation of Penrose regarding the very low entropy state
of the universe.Comment: 5 pages, 2 figures, RevTex. v4: to appear in Phys. Rev.
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