Michael Maisack

OSSE Observations of Blazars

Abstract : Results are reported on observations obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO) of 17 active galactic nuclei known to exhibit blazar properties at other wavelengths. These observations span the period from 1991 June through 1994 May. Of the 33 high-confidence EGRET detections of blazars during CGRO observing phases 1 and 2 (1991 May 16 - 1993 Sept. 7), OSSE has observed 8 and detected 5, namely 3C 273, 3C 279, PKS 0528+134, CTA 102, and 3C 454.3. Additionally, OSSE has detected the BL LAC sources H 1517+65.6 and PKS 2155-304, which were not detected with EGRET. Variablility in the energy band 50 -150 keV is observed for all of the detected AGN. The OSSE blazar sources are all well described by simple power law models with photon number indices,,, varying from 1.0 to 2.1 among sources. When combined with available, although not necessarily contrmporaneous, COMPTEL and EGRET observations, 4 out of 5 detected blazars show clear evidence for spectral breaks between the hard X-ray and medium energy gamma-ray bands. The exception is the combined OSSE/EGRET data for 3C 279 during 1991 October, where a simple power-law with, approximately equal 1.9 works equally well. Gamma-ray evidence for beaming in CTA 102, PKS 0528+134, and 3C 454.3 is presented.

The soft gamma-ray spectrum of A0535+26: Detection of an absorption feature at 110 keV by OSSE

Abstract : We present soft gamma-ray observations by the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (GRO) of the transient X-ray binary pulsar A0535+26. The observations were made 1994 February 8{17, immediately prior to the peak of a giant outburst. The phase- averaged spectrum is complex and cannot be described by a single-component model. We find that structure in the spectrum above 100 keV can best be modeled by an absorption feature near 110 keV, which we interpret as the signature of cyclotron resonant scattering. Because of OSSEs 45-keV threshold, we are unable to make a definitive statement on the presence of a 55-keV absorption line; however we can conclude that if this line does exist, it must have a smaller optical depth than the line at 110 keV. A first harmonic (= fundamental) cyclotron resonance at 110 keV corresponds to a magnetic field strength at the surface of the neutron star of ~1 x 10(exp 13) G (~5 x 10(exp 12) G if the first harmonic is at 55 keV).

Observations of a large flare in GX 1+4 with the Compton Gamma Ray Observatory

The pulsating X-ray binary GX 1+4 (4U 1728-24) was observed by Oriented Scintillation Spectrometer Experiment (OSSE) onboard the Compton Gamma Ray Observatory (CGRO) from 9 to 21 September 1993 as a target of oppurtunity after Burst and Transient Source Experiment (BATSE) had detected the onset of a large flare by the greatly increased pulsed flux at the period of approximately 2 min. The total flux in the 40-100 keV range as observed by the OSSE reached its maximum of 83 mCrab on 14/15 September, after which it fell sharply to about 31 mCrab within 2 days. The spectrum is well described by thermal type spectra. The characteristic temperature of the average OSSE spectrum for a thermal Bremsstrahlung model is kT = (35.5 +/- 0.5) keV. A single power law can be ruled out. There is evidence for a hardening of the spectrum with decreasing intensity at the end of the flare. The barycentric pulse period was (120.567 +/- 0.005) s on 5 September. The average spin-down rate as taken from the standard BATSE analysis was dP/dt = 0.0105 s/day, and constant over the time of the flare. A further target of oppurtunity (TOO) observation with the ROSAT Position Sensitive Proportional Counter (PSPC) on 18 September led to the first detection of the source with a reflecting X-ray telescope and to a signifcantly improved position: RA(2000) = 17h 32m 2.1s and DEC(2000) = -24 deg 44 min 44 sec. This position 3.5 sec from V2116 Oph, with a 90% error radius of 8 sec is the most accurate so far obtained with an X-ray instrument, thus confirming the identification with the suspected stellar counterpart.

Relativistic particle transport in hot accretion disks

Accretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in AGNs above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 10 12-10 13 K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed EGRET outbursts if the black hole mass is 10 10 M 0 • Moreover, the accretion timescale for the inner region is comparable to the observed transient timescale of -1 week. Hence EGRET outbursts may be driven by instabilities in hot, two-temperature disks around supermassive black holes. In this paper we discuss turbulent (stochastic) acceleration in hot disks as a possible source of Ge V particles and radiation. We constrain the model by assuming the turbulence is powered by a collective instability that drains energy from the hot protons. We also provide some ideas concerning new, high-energy Penrose processes that produce GeV emission be directly tapping the rotational energy of Kerr black holes. Subject headings: acceleration of particlesaccretion, accretion disksgalaxies: nucleiquasars: individual ( 3C 279)radiation mechanisms: nonthermal

Broad-band X-ray observations of Cir X-1

Abstract We present broad-band (2–88 keV) X-ray observations of the X-ray binary Cir X-1 with the TTM and HEXE instruments on board of the Mir space station. The observations were made in January/February 1989. The spectrum is best described by a model with 3 components: a blackbody at low energies, an iron line and a Comptonised hard continuum. The spectrum is variable during our observations; when the Comptonised component becomes harder, the spectrum becomes softer below 15 keV. The high-energy spectrum resembles that of X-ray binary pulsars.

Observations of Vela X–1 with RXTE

We present results from observations of Vela X–1 obtained in February 1996 with RXTE. We have fitted phase averaged spectra with several models and compared our results with prior results obtained with HEXE and Ginga. No acceptable fit is possible with the models normally applied to X-ray binaries, instead we found a best fit using a power law with a Fermi-Dirac cutoff modified by one or two cyclotron absorption lines at ∼25 keV and ∼58 keV. In phase resolved spectral analysis we find that the cyclotron absorption is detectable only in the pulse, and is deeper in the first stronger pulse.

HEXE observations 1991/1992

We report observations with the HEXE instrument on the Kvant module aboard the Mir space station which were performed in late 1991 and 1992. We present results from the field containing MX 0836‐42 and GRS 0834‐430, and from the sources GX 5‐1, GRS 1758‐258, 1E 1740.7‐294. The most interesting observation is the first of these, since the high‐energy spectra of these two sources are not well known, contrary to those of the other objects reported here. We find a thermal spectrum with temperature ≊11 keV and no indication of a periodic signal. Since a power‐law type spectrum and pulsations have been observed from GRS 0834‐430, we believe that the source we observe is MX 0836‐42.

The results of the MIR-KVANT in 1987–1989

Abstract The main results of the MIR-KVANT X-ray observatory in 1987–1989 are reported. These results include the discovery of the hard X-ray radiation from Supernova 1987A, long term imaging observations of LMC field, broad band observations of bright transient sources Vul X Nova, GS2023+338, timing observations of the X-ray pulsars, cartography of the Galactic Plane in 2–30 keV band with arcmin angular resolution.

Black hole X‐ray novae and pair models

X‐ray novae are among the best black hole candidates to date. Since galactic black holes are often regarded as ‘‘mini‐AGN’’, we test a simple pair and Compton reflection model, developed for AGN, with statistically more significant data of black hole X‐ray novae (BHXN) and Cyg X‐1 observed by the HEXE instrument. We find that we can fit the data with a model by Zdziarski (1993) and discuss the allowed parameter range.

X-ray binary pulsars: Pulse vs. orbit and the high-energy continuum

Abstract Pulse period and binary orbit period in X-ray pulsars have been found to be correlated, depending on the nature of the mass-donor. We have examined the high-energy continua of these sources above 20 keV. We find that Be-star binaries have the hardest spectra, while those of objects accreting by Roche lobe overflow have the softest spectra. The temperatures derived from bremsstrahlung or exponential fits to archival HEAO-A4 all-sky survey data, BATSE and HEXE data of Be and supergiant binaries correlate with the position of the sources on the pulse-period vs. orbit period diagram.