We present the results of the analysis of the first 9 months of data of the Swift BAT survey of AGNs in the 14Y195 keV band. Using archival X-ray data or follow-up Swift XRT observations, we have identified 129 (103 AGNs) of 130 objects detected at jbj > 15 and with significance >4.8 . One source remains unidentified. These same X-ray data have allowed measurement of the X-ray properties of the objects. We fit a power law to the log N Ylog S distribution, and find the slope to be 1:42 AE 0:14. Characterizing the differential luminosity function data as a broken power law, we find a break luminosity log L Ã (erg s À1 ) = 43.85 AE 0.26, a low-luminosity power law slope a ¼ 0:84 þ0:16 À0:22 , and a highluminosity power law slope b ¼ 2:55 þ0:43 À0:30 , similar to the values that have been reported based on INTEGRAL data. We obtain a mean photon index 1.98 in the 14Y195 keV band, with an rms spread of 0.27. Integration of our luminosity function gives a local volume density of AGNs above 10 41 erg s À1 of 2:4 ; 10 À3 Mpc À3 , which is about 10% of the total luminous local galaxy density above M Ã ¼ À19:75. We have obtained X-ray spectra from the literature and from Swift XRT follow-up observations. These show that the distribution of log n H is essentially flat from n H ¼ 10 20 to1024 cm À2 , with 50% of the objects having column densities of less than 10 22 cm À2 . BAT Seyfert galaxies have a median redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have log n H > 22.

We report on Rossi X-Ray T iming Explorer observations of four type I X-ray bursters, namely, 1E 1724 [3045, GS 1826[238, SLX 1735[269, and KS 1731 The Ðrst three were in a low state, with 1È200 keV X-ray luminosities in the range Eddington luminosity for a neutron D0.05È0.1L Edd (L Edd : star, ergs s~1), whereas KS 1731[260 was in a high state, with luminosity L Edd \ 2.5 ] 1038 D0.35L Edd . The low-state sources have very similar power spectra, displaying high-frequency noise up to D200 Hz. For KS 1731[260, its power spectrum is dominated by noise at frequenciesHz ; in addition a [20 quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1È200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature around 25È30 keV. For KS 1731[260, the spectrum is also dominated by thermal (kT e ) Comptonization, but with a much lower keV and no signiÐcant hard X-ray emission. With the kT e D 3 exception of GS 1826[238, they each have an underlying soft component, carrying at most D25% of the total 1È200 keV luminosity. For all sources, we have detected an iron Ka line at 6.4 keV (although it is weak and marginal in 1E 1724 [3045). A reÑection component is present in the spectra of GS 1826[238 and SLX 1735[269, and for both we Ðnd that the reÑecting medium subtends only a small solid angle ()/2n D 0.15, 0.28). The origin of the line and the reÑection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion Ñow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reÑected component and associated iron line increase. At the same time, the Comptonization region cools o † in response to an increased cooling Ñux from the accretion disk and from the reprocessed/reÑected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion Ñows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The Ðrst one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have larger than D50 keV. However, this criterion is weakened by the fact that there are NSs diskT e playing nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptoni...

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.