Abstract. The ESA observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory) is dedicated to the fine spectroscopy (2.5 keV FWHM @ 1 MeV) and fine imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray (3−35 keV) and optical (Vband, 550 nm) energy ranges. INTEGRAL carries two main gamma-ray instruments, the spectrometer SPI (Vedrenne et al. (until June 2003) reaches ∼1800 ks in the Galactic plane. The prospects are excellent for the scientific community to observe the high energy sky using state-of-the-art gamma-ray imaging and spectroscopy. This paper presents a high-level overview of INTEGRAL.

Triggered by the earlier surprising detection of pulsed hard X-ray emission from 1E 1841-045, we investigated the time-averaged high-energy spectral characteristics of the established Anomalous X-ray Pulsars, 1RXS J1708 -4009, 4U 0142 + 61, 1E 2259 + 586 and 1E 1048.1 -5937, all with persistent X-ray emission. We report on the discovery of hard spectral tails for energies above 10 keV in the total and pulsed spectra of AXPs 1RXS J1708 -4009, 4U 0142 + 61 and 1E 2259 + 586 using RXTE PCA (2-60 keV) / HEXTE (15-250 keV) data and INTEGRAL IBIS ISGRI (20-300 keV) data. 1E 1048.1 -5937 appeared to be too weak to be detected with the presently available exposure. Furthermore, improved spectral information for 1E 1841-045 is presented. The pulsed and total spectra measured 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 measured for four AXPs approximately in the range -1.0 -1.0. The indices measured for these pulsed spectra below 10 keV were in the range 2.0 -4.3, indicating the very drastic spectral changes in a narrow energy interval around 10 keV, where clear minima are found in luminosity. The best fit power-law models to the total spectra between ∼ 10 and 150 keV are significantly softer, with indices measured for three AXPs, 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: 4U 0142 + 61 ∼ 10%, 1E 1841-045 ∼25%, and 1RXS J1708 -4009 consistent with 100%. 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. This shows that also these new hard-X-ray components cannot be powered by rotational energy loss. We reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data to search for signatures from our set of AXPs. No detections can be claimed, but the obtained upper-limits to soft gamma-ray emission in this MeV range indicates for 1RXS J1708 -4009, 4U 0142 + 61 and 1E 1841-045 that strong breaks or bends must occur somewhere between ∼ 150 keV and 750 keV. We discuss predictions from first attempts to model our hard X-ray / soft gamma-ray spectra in the context of the magnetar model. Our spectral results cannot yet discriminate between the different proposed scenarios. Particularly, more constraining information is required on the detailed shape of the spectra between ∼ 150 keV (our highest data points from INTEGRAL) and 750 keV, from where we report the COMPTEL upper limits.

We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV -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 -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 -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.

Abstract. 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 significantly improved COMPTEL spectra for the Crab nebula and pulsar emissions, and for the first time to accurately determine at low-energy γ-rays the pulse profile 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 γ-rays from CGRO BATSE and at high-energy γ-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 profile 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 first pulse for increasing energies up to ∼1 MeV. Over the COMPTEL energy range above 1 MeV an abrupt morphology change happens: the first pulse becomes again dominant over the second pulse and the bridge emission loses significance such that the pulse profile 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 pulsephase 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 profile 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.

We report the discovery of nonthermal pulsed X-ray/soft gamma-ray emission up to $150 keV from the anomalous 11.8 s X-ray pulsar AXP 1E 1841À045 located near the center of supernova remnant Kes 73 using Rossi X-Ray Timing Explorer (RXTE ) Proportional Counter Array and High Energy X-Ray Timing Experiment (HEXTE) data. The morphology of the double-peaked pulse profile changes rapidly with energy from 2 keV up to $8 keV, above which the pulse shape remains more or less stable. The pulsed spectrum is very hard; its shape above 10 keV can be described well by a power law with a photon index of 0:94 AE 0:16. 1E 1841À045 is the first AXP for which such very hard pulsed emission has been detected, which points to an origin in the magnetosphere of a magnetar. We have also derived the total emission spectrum from the Kes 73/1E 1841À045 complex for the $1-270 keV energy range using RXTE HEXTE and XMM-Newton pn data. A comparison of the total emission from the complex with the pulsed+DC emission from 1E 1841À045 as derived from Chandra ACIS CC-mode data (Morii et al. 2003) leaves little room for emission from Kes 73 at energies near 7 keV or above. This suggests that the HEXTE spectrum above $15 keV, satisfactorily described by a power law with index 1:47 AE 0:05, is dominated by emission from 1E 1841À045. In that case the pulsed fraction for energies above 10 keV would increase from about 25% near 10 keV to 100% near 100 keV. The origin of the DC-component extending up to $100 keV is probably magnetospheric and could be a manifestation of pulsed emission that is ''on'' for all phases.

At high-energy γ-rays (> 100 MeV) the Large Area Telescope (LAT) on the Fermi satellite already detected more than 145 rotation-powered pulsars (RPPs), while the number of pulsars seen at soft γ-rays (20 keV -30 MeV) remained small. We present a catalogue of 18 non-recycled RPPs from which presently non-thermal pulsed emission has been securely detected at soft γ-rays above 20 keV, and characterize their pulse profiles and energy spectra. For 14 of them we report new results, (re)analysing mainly data from RXTE, INTEGRAL, XMM-Newton and Chandra. The soft γ-pulsars are all fast rotators and on average ∼ 9.3× younger and ∼ 43× more energetic than the Fermi LAT sample. The majority (11 members) exhibits broad, structured single pulse profiles, and only 6 have double (or even multiple, Vela) pulses. Fifteen soft γ-ray pulsar show hard power-law spectra in the hard X-ray band and reach maximum luminosities typically in the MeV range. For only 7 of the 18 soft γ-ray pulsars pulsed emission has also been detected by the LAT, but 12 have a pulsar wind nebula (PWN) detected at TeV energies. For six pulsars with PWNe, we present also the spectra of the total emissions at hard X-rays, and for IGR J18490-0000, associated with HESS J1849-000 and PSR J1849-0001, we used our Chandra data to resolve and characterize the contributions from the point-source and PWN. Finally, we also discuss a sample of 15 pulsars which are candidates for future detection of pulsed soft γ-rays, given their characteristics at other wavelengths.

Nature © Macmillan Publishers Ltd 1998 8 reported 7,8 ; the spectra of RX J0852:0 2 4622 are similar to these if the power-law-like spectrum of its northern limb section is applied. The X-ray spectral parameters which Willingale et al. 8 have obtained for SN1006 and our results for RX J0852:0 2 4622 are basically the same concerning the spectral index a as well as the temperatures T r,1 and T r,2 of the SNR interior. Furthermore, the X-ray morphology of the two objects is very similar for E .

Simultaneous observations of the accretion-powered millisecond pulsar IGR J00291+5934 by the International Gamma-Ray Astrophysics Laboratory and Rossi X-ray Timing Explorer during the 2004 December outburst are analysed. The average spectrum is well described by thermal Comptonization with an electron temperature of 50 keV and Thomson optical depth τ T ∼ 1 in a slab geometry. The spectral shape is almost constant during the outburst. For the first time we detect a spin-up,ν = 8.4 × 10 −13 Hz s −1 , of an accreting millisecond pulsar. The ISGRI data reveal the pulsation of X-rays at a period of 1.67 milliseconds up to ∼150 keV. The pulsed fraction is shown to increase from 6 per cent at 6 keV to 12-20 per cent at 100 keV. This is naturally explained by the action of the Doppler effect on the exponentially cutoff Comptonization spectrum from the hot spot. The nearly sinusoidal pulses show soft lags with a complex energy dependence, increasing up to 7 keV, then decreasing to 15 keV, and seemingly saturating at higher energies.