A spectral study of gamma-ray emitting AGN

In this paper we present a statistical analysis of the gamma-ray spectra of flat-spectrum radio quasars (FSRQ) compared to those of BL Lacs. The average spectra and possible systematic deviations from power-law behaviour are investigated by summing up the intensity and the power-law fit statistic for both classes of objects. We also compare the time-averaged spectrum to that at the time of gamma-ray outbursts. The spectrum of the average AGN is softer than that of the extragalactic gamma-ray background. It may be that BL Lacs, which on average have a harder spectrum than FSRQs, make up the bulk of the extragalactic background. We also find apparent cut-offs at both low and high energies in the spectra of FSRQs at the time of gamma-ray outbursts. While the cut-off at high energies may have something to do with opacity, the cut-off at low energies may be taken as indication that the gamma-ray emission of FSRQs is not a one component spectrum.Comment: 8 pages, accepted for publication in A&

High-energy gamma-radiation from Geminga observed by EGRET

Available from TIB Hannover: RN 9303(265) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A heliospheric density model and type III radio bursts

A heliospheric density model is derived by evaluating the spherical solutions of magnetohydrostatic equations including the thermal pressure and the gravitational force of the Sun. The model resulting as a special solution of Parker's wind equation covers a range from the low corona up to 5 AU and, surprisingly, agrees very well with observations. Such a model is required for the interpretation of solar and interplanetary radio observations since the emission of the radio radiation is regarded to be generated near the local electron plasma frequency, which depends on the electron number density. Thus, the density model yields the radial distance of the radio source from the Sun and, consequently, the radial source velocity from the drift in dynamic radio spectra. The model is applied for estimating the velocity of electron beams generating solar and interplanetary type III radio bursts. (orig.)SIGLEAvailable from TIB Hannover: RR 7310(99-11) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Nonthermal radio signatures of coronal disturbances with and without coronal mass ejections

This paper demonstrates that the radio signature of nonthermal electrons can be a sensitive indicator of small scale energy release related to topological changes in coronal structures. This study presents data on two events from the Extreme-UV Imaging Telescope (EIT) and the Large Angle and Spectroscopic Coronagraph (LASCO) instruments, GOES soft X-ray data, and 40-800 MHz radio spectra of the Astrophysical Institute Potsdam (AIP). At first glance, the two events do not appear very different. Both are long duration soft X-ray events (LDEs). However, one event decays after a flare-related ejection of cold and hot matter into the lower corona causing only brightness changes in EIT and LASCO-C1 while the other event marks the onset of a coronal mass ejection (CME) that propagates with a leading-edge speed of #approx# 530 kms "1 between 2-30 R_sun. One characteristic difference between the two events is a 1.5 hours duration plateau between two energy release phases detected by the GOES soft X-ray flux curves. The formation of the CME appears during this soft X-ray flux stagnation. It is first detected in EIT and C1 and is announced by an unspectacular faint group of metric drift bursts starting in the spectral range 170-200 MHz (corresponding to a height interval in 1.2-1.3 R_sun for different density models). The burst group lasts for #propor to#3 min. During the later CME lift-off, the start frequency drift of a simultaneously appearing type IV continuum translates to a speed in the range 80-130 km s"-"1. The same speed is obtained from the change in height of leading CME features in LASCO-C1. The non-CME event shows a brief (five minutes) break during the rise of the soft X-ray flux curve, only. It is accompanied by a type IV continuum without start frequency drift and lacks metric type III burst activity. In the non-CME case a bright blob remains stable on the top of a loop arcade for more than 2 hours. The brightness decline of this blob corresponds to the soft X-ray LDE decay. (orig.)SIGLEAvailable from TIB Hannover: RR 7310(98-24) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

ROSAT observations of the 4U 1543-47 outburst in 1992. ROSAT and BATSE observations of GRS 1915+105

Contributions to the 2. Compton symposium, Washington, Sep. 1993SIGLEAvailable from TIB Hannover: RN 9303(275) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Catalogue of solar type II radio bursts observed from September 1990 to December 1993 and their statistical analysis

Solar type II radio bursts represent the radio signature of shock waves travelling through the solar corona. They are associated with flares, coronal mass ejections (CME's) and interplanetary shocks. Type II radio bursts appear as emission stripes slowly drifting from high to low frequencies in dynamic radio spectra. The spectral features of all solar type II radio bursts observed by the new radiospectrograph of the Astrophysikalisches Institut Potsdam in Tremsdorf during the time period from September 1, 1990 to December 31, 1993, i.e., during the first part of the ULYSSES spacecraft mission, are summarized and statistically investigated. (orig.)Available from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Near-simultaneous ROSAT and Ginga observations of the 1991 X-ray transient in Musca

During its All-Sky-Survey ROSAT has observed the Musca 1991 X-ray transient on January 24/25, two weeks after outburst, for about 170 s. The intensity was found to be nearly 6 Crab in the ROSAT band. A combined fit of ROSAT and Ginga data from 1991 January 25 with a multitemperature disk blackbody model plus a power law component results in a maximal temperature of the disk of about kT=0.96 keV at an absorbing column of N_H=2.2x10"2"1 cm"-"2. With the knowledge of the mass function f(M)=3.1 M_sun from optical observations we derive a minimum distance to the black hole binary of at least 4-5 kpc. Including the accretion disk inclination angle of i=26 #+-#25 determined from the shape of the positron annihilation line, and the quiescent optical brightness together with the most probable spectral type of the companion we drive a black hole mass of M=(6#+-#1.5) M_sun and a distance of about (11#+-#3) kpc. (orig.)Available from TIB Hannover: RN 9303(273) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The energy-dependent temporal variation of the ROSAT PSPC gain

The existence of systematic spectral-fit residuals in ROSAT PSPC spectra, and their dependence on time, is by now a well established fact. This paper describes how those residuals may be related to second order variations of the gain of the PSPC as a function of energy and time. As a result, the energy scale used for the interpretation of PSPC spectra can be incorrect producing significant systematic effects, in particular at energies where the effective area of the instrument changes rapidly. A monotonic gain decay of #approx# 1% per year is measured at 1 keV. A quantitative description of the PSPC gain-time variation as a function of energy is provided. The functional dependence of this variation is found to be well represented by a second order polynomial. (orig.)Available from TIB Hannover: RN 9303(355) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Ten past and ten future GAS/MAUS-payloads

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Monitoring of a shock wave propagation from the solar atmosphere to Earth

In the special case of the event on April 7, 1997 a shock wave was travelling from the Sun to Earth. This shock wave was accompanied by a solar and interplanetary type II radio burst remotely measured by the radiospectrometers of the Astrophysical Institute Potsdam and on the WIND spacecraft. The shock wave caused a sudden commencement at Earth and enhancements of the total electron content of the ionosphere on April 10, 1997. Moreover an aurora was observed by the Polar satellite on April 11, 1997. A heliospheric density model derived from a special solution of Parker's wind equation is a good tool to describe the propagation of this shock wave from the low corona up to several AU. The presented method gives the possibility for forecasting solar induced magnetic activities at Earth. (orig.)8 refs.Available from TIB Hannover: RR 7310(99-12) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman