15,899 research outputs found
The z=5.8 Quasar SDSSp J1044-0125: A Peek at Quasar Evolution?
The newly discovered z=5.8 quasar SDSSp J104433.04-012502.2 was recently
detected in X-rays and found to be extremely X-ray weak. Here we present the
hardness ratio analysis of the XMM-Newton observation. We consider various
models to explain the detection in the soft X-ray band and non-detection in the
hard band, together with its X-ray weakness. We show that the source may have a
steep power-law slope, with an absorber partially covering the continuum. This
may be X-ray evidence to support the argument of Mathur (2000) that narrow line
Seyfert 1 galaxies, which show steep power-law slopes, might be the low
redshift, low luminosity analogues of the high redshift quasars. Heavily
shrouded and steep X-ray spectrum quasars may indeed represent the early stages
of quasar evolution (Mathur 2000, Fabian 1999) and SDSSp J104433.04-012502.2 is
possibly giving us a first glimpse of the physical evolution of quasar
properties.Comment: To appear in A
The z>4 Quasar Population Observed by Chandra and XMM-Newton
The current status of our Chandra and XMM-Newton project on high-redshift
(z>4) quasars is briefly reviewed. We report the main results obtained in the
last few years for the detected quasars, along with a few (~10%) intriguing
cases where no detection has been obtained with Chandra snapshot observations.Comment: 4 pages, to appear in the proceedings of 'Multiwavelength AGN
surveys' (Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujic
A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945
We report on a powerful and variable off-nuclear X-ray source in the nearby
spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten
in 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months or
less. It is seen by ASCA about one month after the second PSPC pointing, and is
seen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one year
after the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity is
about 8E38 erg/s, making it brighter than any known persistent X-ray binary in
the Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s.
The observed variability argues against a superbubble interpretation, and the
off-nuclear position argues against a low-luminosity active galactic nucleus.
The source is therefore probably either an ultra-powerful X-ray binary or an
ultra-powerful supernova remnant. Optical monitoring has not identified any
supernovae in NGC 4945 during the time of the X-ray observations, and any
supernova would have had to have been either very highly absorbed or
intrinsically optically faint.Comment: 5 pages, uuencoded compressed tar file, MNRAS in pres
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