Astronomy and Astrophysics – Astrophysics
Scientific paper
2000-06-19
Astronomy and Astrophysics
Astrophysics
2 pages, to appear in the proceedings of the Fourth Italian Conference on AGNs (MemSAIt)
Scientific paper
The X-ray sky is not as well known as is sometimes thought. We report on our search of minority populations (Kim & Elvis 1999). One of the most intriguing is that of `blank field sources', i.e. bright ROSAT sources (F(X)>1e-13 erg/cm2/s) with no optical counterpart on the Palomar Sky Survey (to O=21.5) within their 39"(99%) radius error circle. The nature of Blank Field sources is unknown: no known extragalactic population has such extreme X-ray to optical ratios (f(X)/f(V) >60). Moreover blank field source tend to have much flatter PSPC spectra compared to radio-quiet AGN. Both properties suggest obscuration. `Blank field sources' could be: Quasar-2s, low-mass AGNs, isolated neutron stars, high redshift clusters of galaxies, failed clusters, AGNs with no big blue bump. Identification with any of these populations would be an interesting discovery.
Cagnoni Ilaria
Celotti Annalisa
Elvis Martin
Kim Dong-Woo
Nicastro Fabrizio
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