W. Collmar

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

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

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.

Discovery of Luminous Pulsed Hard X-Ray Emission from Anomalous X-Ray Pulsars 1RXS J1708?4009, 4U 0142+61, and 1E 2259+586 by INTEGRAL and RXTE

We investigated the time-averaged high-energy spectral characteristics of the persistent anomalous X-ray pulsars (AXPs) 1RXS J1708-4009, 4U 0142+61, 1E 2259+586, and 1E 1048.1-5937, using RXTE PCA (2-60 keV), RXTE HEXTE (15-250 keV), and INTEGRAL IBIS ISGRI (20-300 keV) data. We discovered hard spectral tails for energies above 10 keV in the total and pulsed spectra of AXPs 1RXS J1708-4009, 4U 0142+4009, and 1E 2259+586, but 1E 1048.1-5937 appeared to be too weak to be detected. Improved hard X-ray spectral information for AXP 1E 1841-045 is also presented. The pulsed and total spectra above 10 keV have power-law shapes, and there is so far no significant evidence for spectral breaks or bends up to ~150 keV. The pulsed spectra above 10 keV are exceptionally hard with indices for four AXPs in the range -1.0 to 1.0. Below 10 keV these indices were in the range 2.0-4.3, indicating the very drastic spectral changes in a narrow energy interval around 10 keV. The best-fit power-law models to the total spectra between ~10 and 150 keV are significantly softer, with indices measured for 1E 1841-045, 1RXS J1708-4009, and 4U 0142+61 in the range 1.0-1.4. For the latter AXPs the pulsed fractions are consistent with 100% around 100 keV but are different at 10 keV: ~10% for 4U 0142+61, ~25% for 1E 1841-045, and consistent with 100% for 1RXS J1708-4009. The luminosities of these total and pulsed spectral tails (10-150 keV) largely exceed the total available spin-down powers by factors ranging from ~100 to ~600. We also reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data: no AXP detections can be claimed, and the obtained upper limits indicate for 1RXS J1708-4009, 4U 0142+61, and 1E 1841-045 that strong breaks or bends must occur somewhere between ~150 and 750 keV. We discuss predictions from first attempts to model our hard X-ray and soft gamma-ray spectra in the context of the magnetar model.

Multiepoch multiwavelength spectra and models for blazar 3C 279

Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.

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.

INTEGRAL survey of the Cassiopeia region in hard X rays

We report on the results of a deep 1.6 Ms INTEGRAL observation of the Cassiopeia region performed from December 2003 to February 2004. Eleven sources were detected with the imager IBIS-ISGRI at energies above 20 keV, includ- ing three new hard X-ray sources. Most remarkable is the discovery of hard X-ray emission from the anomalous X-ray pulsar 4U 0142+61, which shows emission up to∼150 keV with a very hard power-law spectrum with photon index = 0.73± 0.17. We derived flux upper limits for energies between 0.75 MeV and 30 MeV using archival data from the Compton telescope COMPTEL. In order to reconcile the very hard spectrum of 4U 0142+61 measured by INTEGRAL with the COMPTEL upper limits, the spectrum has to bend or break between∼75 keV and∼750 keV. 1E 2259+586, another anomalous X-ray pulsar in this region, was not detected. INTEGRAL and COMPTEL upper limits are provided. The new INTEGRAL sources are IGR J00370+6122 and IGR J00234+6144. IGR J00370+6122 is a new supergiant X-ray binary with an orbital period of 15.665± 0.006 days, derived from RXTE All-Sky Monitor data. Archival BeppoSAX Wide-Field Camera data yielded four more detections. IGR J00234+6144 still requires a proper identification. Other sources f or which INTEGRAL results are pre- sented are high-mass X-ray binaries 2S 0114+650, Cas, RX J0146.9+6121 and 4U 2206+54, intermediate polar V709 Cas and 1ES 0033+595, an AGN of the BL-Lac type. For each of these sources the hard X-ray spectra are fitted with di fferent models and compared with earlier published results.

Multiwavelength Observations of 3C 273 in 1993-1995

We present the results of the multiwavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993, this quasar showed an increase of its flux for energies ≥100 MeV from about 2.1 × 10-7 photons cm-2 s-1 to approximately 5.6 × 10-7 photons cm-2 s-1 during a radio outburst at 14.5, 22, and 37 GHz. However, no one-to-one correlation of the γ-ray radiation with any frequency could be found. The photon spectral index of the high-energy spectrum changed from Γγ = (3.20 ± 0.54) to Γγ = (2.20 ± 0.22) in the sense that the spectrum flattened when the γ-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization, and the proton-initiated cascade model) for the explanation of the high γ-ray emission of active galactic nuclei were performed to the multiwavelength spectrum of 3C 273. All three models are able to represent the basic features of the multiwavelength spectrum. Although there are some differences, the data are still not decisive enough to discriminate between the models.

Day-Scale Variability of 3C?279 and Searches for Correlations in Gamma-Ray, X-Ray, and Optical Bands

Light curves of 3C 279 are presented in optical (R band), X-rays (RXTE/PCA), and γ rays (CGRO/EGRET) for 1999 January-February and 2000 January-March. During both of those epochs the γ-ray levels were high and all three observed bands demonstrated substantial variation, on timescales as short as 1 day. Correlation analyses provided no consistent pattern, although a rather significant optical/γ-ray correlation was seen in 1999, with a γ-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the γ-ray and X-ray light curves during the large γ ray flare in 1996 February and the two γ-bright weeks leading up to it; the correlation at that time was strong, with a γ-ray/X-ray offset of no more than 1 day.

INTEGRAL and RXTE Observations of Centaurus A

INTEGRAL and RXTE performed three simultaneous observations of the nearby radio galaxy Centaurus A in 2003 March, 2004 January, and 2004 February with the goals of investigating the geometry and emission processes via the spectral/temporal variability of the X-ray/low-energy gamma-ray flux, and intercalibration of the INTEGRAL instruments with respect to those on RXTE. Cen A was detected by both sets of instruments from 3 to 240 keV. When combined with earlier archival RXTE results, we find the power-law continuum flux and the line-of-sight column depth varied independently by 60% between 2000 January and 2003 March. Including the three archival RXTE observations, the iron-line flux was essentially unchanging, and from this we conclude that the iron-line-emitting material is distant from the site of the continuum emission, and that the origin of the iron-line flux is still an open question. Taking X-ray spectral measurements from satellite missions since 1970 into account, we discover a variability in the column depth between 1.0 × 1023 and 1.5 × 1023 cm-2 separated by approximately 20 yr, and suggest that variations in the edge of a warped accretion disk viewed nearly edge-on might be the cause. The INTEGRAL OSA 4.2 calibration of JEM-X, ISGRI, and SPI yields power-law indices consistent with the RXTE PCA and HEXTE values, but the indices derived from ISGRI alone are about 0.2 greater. Significant systematics are the limiting factor for INTEGRAL spectral parameter determination.