منابع مشابه
X–ray Emission from the First Quasars
It is currently unknown whether the Universe was reionized by quasars or stars at z ∼ > 5. We point out that quasars can be best distinguished from stellar systems by their X-ray emission. Based on a simple hierarchical CDM model, we predict the number counts and X-ray fluxes of quasars at high redshifts. The model is consistent with available data on the luminosity function of high-redshift qu...
متن کاملA new population of soft X-ray weak quasars
Quasars selected in optical surveys by means of their blue or UV excess are known to be strong emitters in the X-rays, except for Broad Absorption Line (BAL) objects. In this paper we study the X-ray emission of quasars selected through their emission line spectrum rather than their blue excess. X-ray data are obtained by cross-correlating two optical samples (the Hamburg Survey Catalogue and t...
متن کاملEvolution of the X-ray Emission of Radio-Quiet Quasars
We report new Chandra observations of seven optically faint, z ∼ 4 radio-quiet quasars. We have combined these new observations with previous Chandra observations of radio-quiet quasars to create a sample of 174 sources. These sources have 0.1 < z < 4.7, and 10 ergs s < νLν(2500Å) < 10 48 ergs s. The X-ray detection fraction is 90%. We find that the X-ray loudness of radio-quiet quasars decreas...
متن کاملBeppoSAX OBSERVATIONS OF SYNCHROTRON X–RAY EMISSION FROM RADIO QUASARS
We present new BeppoSAX LECS, MECS, and PDS observations of four flat–spectrum radio quasars (FSRQ) having effective spectral indices αro and αox typical of high-energy peaked BL Lacs. Our sources have X–ray–to–radio flux ratios on average ∼ 70 times larger than “classical” FSRQ and lie at the extreme end of the FSRQ X–ray–to–radio flux ratio distribution. The collected data cover the energy ra...
متن کاملRed Parkes Quasars: Evidence for Soft X-ray Absorption
The Parkes Half-Jansky Flat-Spectrum Sample contains a large number of sources with unusually red optical-toÈnear-infrared (NIR) continua. If this is to be interpreted as extinction by dust in the line of sight, then associated material might also give rise to absorption in the soft X-ray regime. This hypothesis is tested using broadband (0.1È2.4 keV) data from the ROSAT All-Sky Survey. SigniÐc...
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ژورنال
عنوان ژورنال: Symposium - International Astronomical Union
سال: 1994
ISSN: 0074-1809
DOI: 10.1017/s0074180900175667