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[en] Uhuru, the first X-ray astronomy satellite, was launched in 1970. Its instruments performed the first all sky survey in X-rays. Approximately half of the 400 sources discovered by Uhuru and its successor Ariel-5 are galactic objects: binary systems containing neutron stars and possibly blackholes, X-ray bursters, supernova remnants and transients. The extragalactic sources are mainly clusters of galaxies as well as Seyfert galaxies and BL Lac Objects. The identifications and follow-up studies of these sources have produced the main advances in X-ray astronomy in the first half of the 1970's. The follow-up surveys with the A-1, A-2 and A-4 instruments on HEAO-1, provided an increase in spectral coverage and somewhat higher sensitivities. The A-1 experiment detected more than 800 sources with fluxes 0.5 mCrab in the 1-25 keV band. In view of the limited positional resolution of the collimated detectors used in these missions, identifications have been a problem. In this context the rotation modulation collimators flown on SAS-3 and HEAO-1 (A-3 experiment) have played a major role. In this paper the author concentrates on some scientific and technical aspects of the X-ray sky survey. ROSAT is a German satellite with substantial US and UK (XUV wide field camera) contributions. The ROSAT X-ray optics consist of a fourfold nested mirror system of Wolter type I with 83 cm aperture having an intrinsic resolution of a few arcseconds. The focal plane assembly comprises two position sensitive proportional counters (PSPC) and one high resolution imager (HRI), which are mounted on a carousel. This paper also shows a cross section of the X-ray telescope
[en] Exosat observations of Sco X-1 obtained using the Xe and/or Ar detectors for a total of about 80,000 s during four runs on August 24-27, 1985 are reported and analyzed. Two modes of quasi-periodic oscillations (QPOs) corresponding to the quiescent and active states of Sco X-1 and to two modes of spectral behavior are identified and characterized, confirming the findings of Priedhorsky (1985) and Middleditch and Priedhorsky (1986). In the quiescent state, the QPO frequency is about 6 Hz and is anticorrelated with intensity, and the spectral hardness ratio (14-21 vs 2-7 keV) varies steeply with intensity; in the active state, QPO frequency is correlated with intensity and varies from 10 to 20 Hz, and the spectral-hardness-ratio/intensity curve is flatter. Previous observations of bimodal behavior in other bands are summarized, and theoretical models proposed to explain them are discussed. 20 references
[en] The Ada training experiences of the Gamma Ray Observatory Ada development team are related, and recommendations are made concerning future Ada training for software developers. Training methods are evaluated, deficiencies in the training program are noted, and a recommended approach, including course outline, time allocation, and reference materials, is offered
[en] Using archival Hubble Space Telescope (HST) imaging data, we report the multiband photometric properties of 13 ultraluminous X-ray sources (ULXs) that have a unique compact optical counterpart. Both magnitude and color variation are detected at timescales of days to years. The optical color, variability, and X-ray to optical flux ratio indicate that the optical emission of most ULXs is dominated by X-ray reprocessing on the disk, similar to that of low-mass X-ray binaries. For most sources, the optical spectrum is a power law, Fν∝να with α in the range 1.0-2.0 and the optically emitting region has a size on the order of 1012 cm. Exceptions are NGC 2403 X-1 and M83 IXO 82, which show optical spectra consistent with direct emission from a standard thin disk, M101 ULX-1 and M81 ULS1, which have X-ray to optical flux ratios more similar to high-mass X-ray binaries, and IC 342 X-1, in which the optical light may be dominated by the companion star. Inconsistent extinction between the optical counterpart of NGC 5204 X-1 and the nearby optical nebulae suggests that they may be unrelated.
[en] Computer software necessary for the analysis of the time variability of x-ray sources was developed and tested. Seventy-one Uhuru observations of the black hole candidate Cygnus X-1 were also examined. Data show that the x-ray emission can be best represented as arising from randomly occurring x-ray flares with a mean rate of occurrence of 18 flares/sec and a mean cooling time of approximately 1/2 sec. Furthermore data show that photons in the 2 to 16 keV bandwidth arise from within the same flares. Finally, it was found that the mean flare rate and cooling time have been constant since the source underwent a spectral transition in March 1971. Long term variations of several parameters associated with the x-ray emission from this object were studied. (auth)
[en] Major outbursts have been observed from the well known high-energy-peaked blazar Markarian 501 since its discovery in 1996. Two episodes of very-high-energy gamma-ray flaring events during 2005 May–July and 2012 June are of special significance, as the source exhibited extreme HBL-like behavior. The successful standard photohadronic model does not adequately explain these extraneous behaviors. We propose a two-zone photohadronic scenario to overcome this problem. In this picture, the low-energy regime (zone-1) of the spectrum follows the standard photohadronic interpretation, while the high-energy regime (zone-2) of the spectrum is new, with a spectral index δ 2 ≥ 3.1, which is solely due to the extreme nature of the flaring event. We also estimate the bulk Lorentz factor corresponding to these extreme flaring events. By analyzing many flaring events before and after these extreme events we argue that the extreme HBL-like events are transient and may repeat in the future.
[en] Preliminary results are presented for the SAS-3 observation of an x-ray flare from Cygnus X-1 in 1975 September. The 1.5 to 6 keV intensity rose by a factor of four and exhibited variability on several time scales from seconds to hours. The 6 to 15 keV intensity showed less activity. The event is similar to that observed in May by ANS and Ariel-5, but lasted less than two weeks