Discovery Of Hard X-ray Emission Research Articles

Hard X‐Rays and Fluorescent Iron Emission from the Embedded Infrared Cluster in NGC 2071

We present first results of XMM-Newton X-ray observations of the infrared cluster lying near the NGC 2071 reflection nebula in the Orion B region. This cluster is of interest because it is one of the closest regions known to harbor embedded high-mass stars. We report the discovery of hard X-ray emission from the dense central NGC 2071-IR subgroup, which contains at least three high-mass young stellar objects (NGC 2071 IRS 1, IRS 2, and IRS 3). A prominent X-ray source is detected within 1'' of the infrared source IRS 1, which is thought to drive a powerful bipolar molecular outflow. The X-ray spectrum of this source is quite unusual compared to the optically thin plasma spectra normally observed in young stellar objects (YSOs). The spectrum is characterized by a hard broadband continuum plus an exceptionally broad emission line at ≈6.4 keV from neutral or near-neutral iron. The fluorescent Fe line likely originates in cold material near the embedded star (i.e., a disk or envelope) that is irradiated by the hard, heavily absorbed X-ray source.

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, including 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 ± 0.17. 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 for which INTEGRAL results are presented are high-mass X-ray binaries 2S 0114+650, γ 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 different models and compared with earlier published results.

The discovery of hard X-ray emission in the persistent flux of the Rapid Burster

We report the first detection with INTEGRAL of persistent hard X-ray emission (20 to 100 keV) from the Rapid Burster (MXB 1730-335), and describe its full spectrum from 3 to 100 keV. The source was detected on February/March 2003 during one of its recurrent outbursts. The source was clearly detected with a high signal to noise ratio during the single pointings and is well distinguished from the neighboring source GX 354-0. The 3 - 100 keV X-ray spectrum of the persistent emission is well described by a two-component model consisting of a blackbody plus a power-law with photon index ~ 2.4. The estimated luminosity was ~ 8.5x10^{36} erg/s in the 3 - 20 keV energy band and \~ 1.3x10^{36} erg/s in the 20 - 100 keV energy range, for a distance of 8 kpc.

Low-radiative-efficiency accretion in the nuclei of elliptical galaxies

The discovery of hard X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo, Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad-band spectral energy distributions for these galaxies, which accrete from their hot gaseous haloes at rates comparable to their Bondi rates, can be explained by low-radiative-efficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron components in the radio band (excluding the case of M87) and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low radiative efficiencies. We briefly discuss an alternative model for the observed X-ray emission, in which it is due to non-thermal synchrotron-self-Compton processes in the accretion flows, or outflows. This appears to require implausibly weak magnetic fields. Emission from a collimated jet viewed off-axis should be distinguishable from the bremsstrahlung model by variability and thermal line emission studies. We argue that the difference in radiative efficiency between the nuclei of spiral and elliptical galaxies may arise from the different manner in which interstellar gas is fed into the nuclei. In ellipticals, matter fed from the hot (slowly cooling) interstellar medium (ISM) is likely to be highly magnetized and have low specific angular momentum, which favours low-radiative-efficiency accretion solutions and, possibly, the formation of the observed jets.

Discovery of hard X-ray emission from supernova 1987A

We report the discovery of hard X-rays from the region of the supernova SN1987A in the Large Magellanic Cloud. The observations were made from the Mir-Kvant observatory 'Rontgen'. Hard X-rays were first observed on 10 August 1987 and SN1987A then became the main target of the observatory. Up to 15 September 1987, a total of 115 pointings on 21 days were made. The flux showed little variation during the observation period. The measured spectrum extends from 20 keV to 300 keV and is extremely hard, having a photon power law index of ˜1.4. At low energies the spectrum becomes even flatter and there is indication of a cutoff between 10 and 25keV. The luminosity over the energy range 20–300 keV is ˜2x1038 erg s–1 (assuming a distance of 55 kpc). The error box for the hard source has a (2σ) radius of 10 arc min and contains SN1987A. The results have important implications for current models of SN1987A.