31 research outputs found
A Systematic Search for Supersoft X-Ray Sources in the ROSAT All-Sky Survey
We have conducted a systematic search for supersoft X-ray sources using the
ROSAT all-sky survey data. With the optical identification of the selected
sources being almost complete, we discuss the statistics of the various source
classes and their observability. Besides supersoft close binary sources this
search also can be used to estimate the number of isolated neutron stars in the
Galaxy, such as those described by Stocke et al. 1995 and Walter et al. 1996.Comment: 4 pages postscript, Proc. of Workshop on Supersoft X-Ray Sources, to
appear in Lecture Notes in Physics vol. 472 (1996
Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation
We examine the recent report by Prochter et al. (2006) that gamma-ray burst
(GRB) sight lines have a much higher incidence of strong MgII absorption than
quasar sight lines. We propose that the discrepancy is due to the different
beam sizes of GRBs and quasars, and that the intervening MgII systems are
clumpy with the dense part of each cloudlet of a similar size as the quasars,
i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational
predictions of our proposed model. Most notably, in some cases the intervening
MgII absorbers in GRB spectra should be seen varying, and quasars with smaller
sizes should show an increased rate of strong MgII absorbers. In fact, our
prediction of variable MgII lines in the GRB spectra has been now confirmed by
Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be
strongly variable in the multi-epoch spectra of z=4.05 GRB060206.Comment: 12 pages, 2 figures; substantially revised model calculation;
accepted for publication in Astrophysics & Space Science as a Lette
FUV and X-ray absorption in the Warm-Hot Intergalactic Medium
The Warm-Hot Intergalactic Medium (WHIM) arises from shock-heated gas
collapsing in large-scale filaments and probably harbours a substantial
fraction of the baryons in the local Universe. Absorption-line measurements in
the ultraviolet (UV) and in the X-ray band currently represent the best method
to study the WHIM at low redshifts. We here describe the physical properties of
the WHIM and the concepts behind WHIM absorption line measurements of H I and
high ions such as O VI, O VII, and O VIII in the far-ultraviolet and X-ray
band. We review results of recent WHIM absorption line studies carried out with
UV and X-ray satellites such as FUSE, HST, Chandra, and XMM-Newton and discuss
their implications for our knowledge of the WHIM.Comment: 26 pages, 9 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 3; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Baryons: What, When and Where?
We review the current state of empirical knowledge of the total budget of
baryonic matter in the Universe as observed since the epoch of reionization.
Our summary examines on three milestone redshifts since the reionization of H
in the IGM, z = 3, 1, and 0, with emphasis on the endpoints. We review the
observational techniques used to discover and characterize the phases of
baryons. In the spirit of the meeting, the level is aimed at a diverse and
non-expert audience and additional attention is given to describe how space
missions expected to launch within the next decade will impact this scientific
field.Comment: Proceedings Review for "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", ed. X. Tielens, 38 pages, 10 color figures. Revised
to address comments from the communit