W. Hermsen

Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1σ) between 1° and 2° within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm 2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons

The Soft Gamma-Ray Spectral Variability of Cygnus X-1

We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV γ-ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder γ-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft γ-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

The crab pulsar in the 0.75-30 MeV range as seen by cgro comptel

We present the time-averaged characteristics of the Crab pulsar in the 0.75{30 MeV energy window using data from the imaging Compton Telescope COMPTEL aboard the Compton Gamma-Ray Observatory (CGRO) collected over its 9 year mission. Exploiting the exceptionally long COMPTEL exposure on the Crab allowed us to derive signicantly improved COMPTEL spectra for the Crab nebula and pulsar emissions, and for the rst time to accurately determine at low-energy -rays the pulse prole as a function of energy. These timing data, showing the well-known main pulse and second pulse at a phase separation of 0:4 with strong bridge emission, are studied together with data obtained at soft/hard X-ray energies from the ROSAT HRI, BeppoSAX LECS, MECS and PDS, at soft -rays from CGRO BATSE and at high-energy -rays from CGRO EGRET in order to obtain a coherent high-energy picture of the Crab pulsar from 0.1 keV up to 10 GeV. The morphology of the pulse prole of the Crab pulsar is continuously changing as a function of energy: the intensities of both the second pulse and the bridge emission increase relative to that of the rst pulse for increasing energies up to 1 MeV. Over the COMPTEL energy range above 1 MeV an abrupt morphology change happens: the rst pulse becomes again dominant over the second pulse and the bridge emission loses signicance such that the pulse prole above 30 MeV is similar to the one observed at optical wavelengths. A pulse-phase-resolved spectral analysis performed in 7 narrow phase slices consistently applied over the 0.1 keV{10 GeV energy interval shows that the pulsed emission can empirically be described with 3 distinct spectral components: i) a power-law emission component (1 keV{5 GeV; photon index 2:022 0:014), present in the phase intervals of the two pulses; ii) a curved spectral component required to describe soft (<100 keV) excess emission present in the same pulse- phase intervals; iii) a broad curved spectral component reflecting the bridge emission from 0.1 keV to 10 MeV. This broad spectral component extends in phase over the full pulse prole in an approximately triangular shape, peaking under the second pulse. Recent model calculations for a three-dimensional pulsar magnetosphere with outer magnetospheric gap acceleration by Cheng et al. (2000) appear at present most successful in explaining the above complex high-energy characteristics of the Crab pulsar.

Gamma Radiation from PSR B1055–52

The telescopes on the Compton Gamma Ray Observatory (CGRO) have observed PSR B1055-52 a number of times between 1991 and 1998. From these data a more detailed picture of the gamma radiation from this source has been developed, showing several characteristics that distinguish this pulsar: the light curve is complex; there is no detectable unpulsed emission; the energy spectrum is flat, with no evidence of a sharp high-energy cutoff up to greater than 4 GeV. Comparisons of the gamma-ray data with observations at longer wavelengths show that no two of the known gamma-ray pulsars have quite the same characteristics; this diversity makes interpretation in terms of theoretical models difficult.

New high energy |[gamma]|-ray sources observed by COS B

LOCALISED γ-ray sources contribute to the overall galactic emission; some of these sources have been identified with known astronomical objects1,2, while several unidentified γ-ray sources have also been reported3,4. We describe here a search for γ-ray sources using data from the ESA γ-ray satellite COS B which revealed 10 new unidentified sources. These sources seem to be galactic with typical γ-ray luminosities above 100 MeV in excess of 1035 erg s−1.

A High-Sensitivity Measurement of the MeV Gamma-Ray Spectrum of Cygnus X-1

The Compton Gamma Ray Observatory (CGRO) has observed the Cygnus region on several occasions since its launch in 1991. The data collected by the COMPTEL experiment on CGRO represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. A spectrum accumulated by COMPTEL over 10 weeks of observation time shows significant evidence for emission extending out to several MeV. We have combined these data with contemporaneous data from both BATSE and OSSE to produce a broadband γ-ray spectrum, corresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to ~5 MeV. Although there is no evidence for any broad-line-like emissions in the MeV region, these data further confirm the presence of a hard tail at energies above several hundred keV. In particular, the spectrum at MeV energies can be described as a power law with a photon spectral index of α = -3.2, with no evidence for a cutoff at high energies. For the 200 keV-5 MeV spectrum, we provide a quantitative description of the underlying electron spectrum, in the context of a hybrid thermal/nonthermal model for the emission. The electron spectrum can be described by a thermal Maxwellian with a temperature of kTe = 86 keV and a nonthermal power-law component with a spectral index of pe = 4.5. The spectral data presented here should provide a useful basis for further theoretical modeling.

Absolute timing with IBIS, SPI and JEM-X aboard INTEGRAL: Crab main-pulse arrival times in radio, X-rays and high-energy γ-rays

We have verified the absolute timing capabilities of the high-energy instruments aboard INTEGRAL, i.e. the imager IBIS, the spectrometer SPI and the X-ray monitor JEM-X. Calibration observations of the Crab, contemporaneous with the Rossi X-ray Timing Explorer (RXTE), have been used to measure the absolute phase of the main pulse of the Crab pulse profile using the same Jodrell Bank radio ephemeris. The three INTEGRAL instruments and RXTE give within the statistical and systematic uncertainties consistent results: The X-ray main pulse is leading the radio pulse by 280 ′ 40 μs. Also the shapes of the X-ray pulse profiles as measured by the different instruments are fully consistent with each other. In addition, we present the first measurement of the absolute phase of the main pulse at γ-ray energies above 30 MeV using data from the EGRET instrument aboard the Compton Gamma-Ray Observatory: The γ-ray main pulse is leading the radio one by 241 ′ 29 μs, consistent with the value for the X-ray main pulse. Comparing absolute arrival times at multiple frequencies gives important constraints to models explaining the production of non-thermal emission in magnetospheres of rotation powered neutron stars.

High-energy characteristics of the schizophrenic pulsar PSR J1846-0258 in Kes 75. Multi-year RXTE and INTEGRAL observations crossing the magnetar-like outburst

PSR J1846-0258 is a radio-quiet rotation-powered pulsar at the center of Supernova remnant Kes 75. It is the youngest pulsar (~723 year) of all known pulsars and slows down very predictably since its discovery in 2000. Till June 7, 2006 very stable behavior has been displayed both in the temporal and spectral domains with pulsed emission detectable by INTEGRAL IBIS ISGRI and RXTE HEXTE up to ~150 keV. Then, a dramatic brightening was detected of the pulsar during June 7-12, 2006 Chandra observations of Kes 75. This radiative event, lasting for ~55 days, was accompanied by a huge timing glitch, reported on for the first in present work. Moreover, several short magnetar-like bursts were discovered. In this work not only the time-averaged pre-outburst X-ray/soft gamma-ray characteristics are discussed in detail, but also the spectral evolution during the outburst and its relaxation phase are addressed using RXTE PCA and HEXTE and INTEGRAL IBIS ISGRI data.

The Revised COMPTEL Orion Results

COMPTEL observations of the Orion/Monoceros region have shown distinct evidence for excessive 3-7 MeV emission that was attributed to nuclear de-excitation lines from accelerated 12C and 16O nuclei. Unfortunately, we must conclude now that this appears to be a spurious result. This conclusion follows from a better understanding of the instrumental background, from a better exposure of the region, and from an improved analysis method. We show here how the impact of each of these gradually reduces the signal to a less than 3 σ result. The prime underlying cause seems to be 24Na activation in and around the upper COMPTEL detectors. Combining all available data, we now set a 2 σ flux upper limit on the 3-7 MeV emission of Orion of 3 × 10-5 γ cm-2 s-1, to be compared with the previously derived flux of ~10-4 γ cm-2 s-1.

The phase of the radio and X-ray pulses of PSR B1937+21

We present timing and spectral results of PSR B1937+21, the fastest known millisecond pulsar (P � 1.56 ms), observed with RXTE. The pulse profile, detected up to ∼20 keV, shows a double peak with the main component much stronger than the other. The peak phase separation is 0.526 ± 0.002 and the pulsed spectrum over the energy range 2-25 keV is well described by a power law with a photon index equal to 1.14 ± 0.07. We find that the X-ray pulses are closely aligned in phase with the giant pulses observed in the radio band. This results suggest that giant radio pulses and X-ray pulses originate in the same region of the magnetosphere due to a high and fluctuating electron density that occasionally emits coherently in the radio band. The X-ray events, however, do not show any clustering in time indicating that no X-ray flares are produced.