11,381 research outputs found
V751 Cyg and V Sge as transient supersoft X-ray sources
I review the observational evidence for luminous, soft X-ray emission during
optical low-states in the two cataclysmic variables V751 Cyg and V Sge, and
discuss the possible link to the canonical supersoft X-ray sources.Comment: 6 pages with newarcrc.sty; inv. talk given at the 60th birthday Symp.
in honour of Brian Warner, Oxford, April 1999, to appear in New Astronomy
Reviews (Elsevier), eds. P. Charles, A. King, D. O'Donoghue; also available
from http://www.aip.de/~jcg/publis.htm
Gamma-ray bursts with ROSAT
I review the use of ROSAT over the last years for the investigation of well
localized gamma-ray burst (GRB) error boxes. In particular, I cover (i) the
systematic study of several dozens of IPN locations using the ROSAT
All-Sky-Survey data, (ii) results of deep ROSAT pointings of selected small GRB
error boxes, (iii) the attempts for and results of quick follow-up observations
after GRB events including the three GRBs localized with BeppoSAX, (iv) the
correlation of GRB locations with serendipitous ROSAT pointings and (v) the
search for X-ray flashes in the database of pointed ROSAT observations.Comment: Invited talk at the workshop on "All-sky X-ray observations in the
next decade", held at RIKEN, Tokyo, March 3-5, 1997; 6 pages incl. 3
ps-figures, plus workshop style file
Density correlators in a self-similar cascade
Multivariate density moments (correlators) of arbitrary order are obtained
for the multiplicative self-similar cascade. This result is based on the
calculation by Greiner, Eggers and Lipa (reference [1]) where the correlators
of the logarithms of the particle densities have been obtained. The density
correlators, more suitable for comparison with multiparticle data, appear to
have even simpler form than those obtained in [1].Comment: 9 pages, 3 figures, uses epsfig.st
Thermalization through Hagedorn states - the importance of multiparticle collisions
Quick chemical equilibration times of hadrons within a hadron gas are
explained dynamically using Hagedorn states, which drive particles into
equilibrium close to the critical temperature. Within this scheme master
equations are employed for the chemical equilibration of various hadronic
particles like (strange) baryon and antibaryons. A comparison of the Hagedorn
model to recent lattice results is made and it is found that for both Tc =176
MeV and Tc=196 MeV, the hadrons can reach chemical equilibrium almost
immediately, well before the chemical freeze-out temperatures found in thermal
fits for a hadron gas without Hagedorn states.Comment: 8 pages, 3 figures, talk presented at the International Conference on
Strangeness in Quark Matter, Buzios, Rio de Janeiro, Brazil, Sept. 27 - Oct.
2, 200
Optical counterparts of ROSAT X-ray sources in two selected fields at low vs. high Galactic latitudes
The optical identification of large number of X-ray sources such as those
from the ROSAT All-Sky Survey is challenging with conventional spectroscopic
follow-up observations. We investigate two ROSAT All-Sky Survey fields of size
10 * 10 degrees each, one at galactic latitude b = 83 deg (Com), the other at b
= -5 deg (Sge), in order to optically identify the majority of sources. We used
optical variability, among other more standard methods, as a means of
identifying a large number of ROSAT All-Sky Survey sources. All objects fainter
than about 12 mag and brighter than about 17 mag, in or near the error circle
of the ROSAT positions, were tested for optical variability on hundreds of
archival plates of the Sonneberg field patrol.
The present paper contains probable optical identifications of altogether 256
of the 370 ROSAT sources analysed. In particular, we found 126 AGN (some of
them may be misclassified CVs), 17 likely clusters of galaxies, 16 eruptive
double stars (mostly CVs), 43 chromospherically active stars, 65 stars brighter
than about 13 mag, 7 UV Cet stars, 3 semiregular resp. slow irregular variable
stars of late spectral type, 2 DA white dwarfs, 1 Am star, 1 supernova remnant
and 1 planetary nebula.
X-ray emission is, expectedly, tightly correlated with optical variability,
and thus our new method for optically identifying X-ray sources is demonstrated
to be feasible.Comment: 92 pages, 521 figures, A&A (accepted
- …