Astrid Orr

JEM-X: The X-ray monitor aboard INTEGRAL ?

The JEM-X monitor provides X-ray spectra and imaging with arcminute angular resolution in the 3 to 35 keV band. The good angular resolution and the low energy response of JEM-X plays an important role in the identification of gamma ray sources and in the analysis and scientific interpretation of the combined X-ray and gamma ray data. JEM-X is a coded aperture instrument consisting of two identical, coaligned telescopes. Each of the detectors has a sensitive area of 500 cm 2 , and views the sky through its own coded aperture mask. The two coded masks are inverted with respect to each other and provides an angular resolution of 3 0 across an eective field of view of about 10 diameter.

A swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory

BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times fainter than its historical average value, and remained steady along the whole observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense iron line (EW ~600 eV) are best explained assuming that the active nucleus has switched off, leaving only a residual reflection component visible.

XMM-Newton observations of the INTEGRAL X-ray transient IGR J17544-2619

On September 17, 2003 INTEGRAL discovered a bright transient source 3 ◦ from the Galactic Center, IGR J17544-2619. The field containing the transient was observed by XMM-Newton on March 17 and September 11 and 17, 2003. A bright source, at a position consistent with the INTEGRAL location, was detected by the European Photon Imaging Camera (EPIC) during both September observations with mean 0.5−10 keV unabsorbed luminosities of 1.1 × 10 35 and 5.7 × 10 35 erg s −1 for an (assumed) distance of 8 kpc. The source was not detected in March 2003, with a 0.5−10 keV lu- minosity of <3.8 × 10 32 erg s −1 . The September 11 and 17 EPIC spectra can be represented by a power-law model with photon indices of 2.25 ± 0.15 and 1.42 ± 0.09, respectively. Thus, the 0.5−10 keV spectrum hardens with increasing intensity. The low-energy absorption during both September observations is comparable to the interstellar value. The X-ray lightcurves for both September observations show energy dependent flaring which may be modeled by changes in either low-energy absorption or power-law index.

Evidence for a neutron star in the non-pulsating massive X-ray binary 4U2206+54

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54 . For the first time, high energy data (≥ 30 keV) are analyzed for this source. The data are well described by comptonization models (CompTT andBMC) in which seed photons with temperatures between 1.1 keV and 1.5 keV are comptonized by a hot plasma at 50 keV thereby producing a hard tail which extends up to, at least, 100 keV. We offer a new method of identification of neutron star systems using a temperature - luminosity relation. If a given X-ray source is characterized by a low bolometric luminosity and a relatively high color blackbody temperature (> 1 keV) it has necessarily to be a neutron star rather than a bla ck hole. From these arguments it is shown that the area of the soft photon source must be small (r≈ 1 km) and that the the accretion disk, if present, must be truncated very far from the compact object. Here we report on the possible existence of a cyclotron line around 30 keV. The presence of a neutron star in the system is strongly favored by the available data.

Ambiguities in Fits to the Complex X-Ray Spectra of Starburst Galaxies

Spectral fits to X-ray data from both NGC 253 and M82 provide ambiguous results. The so-called best-fit results depend on the instrument with which the data were obtained and, obviously, on the choice of spectral model composition. We show that different spectral models can be fit equally well to BeppoSAX data of both galaxies. Metallicities are unreliable in general, with a strong dependence on the choice of model. Preference to one particular spectral model can be given only by combining spectroscopic and imaging X-ray data from all available satellites (ROSAT, ASCA, and BeppoSAX). Based on spectra of NGC 253, we demonstrate that a model consisting of two or more thermal plasma components plus a hard power-law continuum and Fe Kα line emission can explain all observations. These model components represent the integral spectrum of thermal gas and compact sources in starburst galaxies that are most likely supernova remnants and X-ray binaries. The same model can fit the X-ray data of M82, but there the evidence, from ROSAT imaging, of the existence of compact sources which might represent high-mass X-ray binaries is weaker. This implies that its hard X-ray emission, which is extended in ROSAT images, might—if truly diffuse—be dominated by a very hot (several keV energy) thermal gas component.

BeppoSAX survey of Be/X-ray binary candidates

We present a BeppoSAX survey of ve Be/X-ray binary candidates. We report on the identication of two of them, HD 110432 and HD 141926, as low luminosity Be/X-ray binaries. For HD 110432 we report on the detection of a pulsation period of14 ks. Because the luminosity of these sources is low and their spectra do not require non-thermal emission models, these systems are good Be+White Dwarf candidates. If the pulsation period for HD 110432 is conrmed, this system would be the most rm Be+WD candidate found up to date. The other three objects HD 65663, HD 249179 and BD+53 2262 did not show detectable X-ray emission. We argue that, while the properties of BD+53 2262 are still consistent with a quiescent Be+Neutron Star scenario, the lack of detection for the other two objects implies that they are most probably not X-ray binaries.

JEM-X inflight performance

We summarize the inflight performance of JEM-X, the X-ray monitor on the INTEGRAL mission during the initial ten months of operations. The JEM-X instruments have now been tuned to stable operational conditions. The performance is found to be close to the pre-launch expectations. The ground calibrations and the inflight calibration data permit to determine the instruments characteristics to fully support the scientific data analysis.

The INTEGRAL ground segment and its science operations centre

The INTEGRAL ground segment is divided into operational and scientific components. The operational component consists of the Mission Operations Centre, the ground stations and communications lines while the scientific component com- prises of a Science Operations Centre and Science Data Centre. The overall architecture of the ground segment is described paying particular attention to the tasks and functionalities of the INTEGRAL Science Operations Centre. 2. The INTEGRAL operational ground segment The MOC is responsible for the operation of the spacecraft and instruments, for ensuring the spacecraft safety and health, for the maintenance of the satellites on-board software, for the provision of flight dynamics support including determination and control of the satellites orbit and attitude and for provision of the telemetry and auxiliary data to the ISDC. The MOC is the sole interface to the spacecraft and all commands to be sent to the spacecraft or instruments are generated at the MOC. INTEGRAL operations are executed using an automated timeline. The timeline is generated on a revolution by revo- lution basis, i.e. one timeline covers the period from perigee passage to the next perigee passage, which is approximately three days. Although there are already activities much be- fore (the planning of ground station resources commences a

Non-thermal emission from AGN coronae

Accretion disk coronae are believed to account for X-ray emission in Active Galactic Nuclei (AGNs). In this paper the observed emission is assumed to be due to a population of relativistic, non-thermal electrons (e.g. produced in a flare) injected at the top of an accretion disk magnetic loop. While electrons stream along magnetic field lines, their energy distribution evolves in time essentially because of inverse Compton and synchrotron losses. The corresponding time-dependent emission due, in the X-ray energy range, to the inverse Compton mechanism, has been computed. Since the typical decay time of a flare is shorter than the integration time for data acquisition in the X-ray domain, the resulting spectrum is derived as the temporal mean of the real, time-dependent emission, as originated from a series of consecutive and identical flares. The model outcome is compared to both the broad band BeppoSAX X-ray data of the bright Seyfert 1 NGC 5548 and to a few general X-ray spectral properties of Seyfert 1s as a class. The good agreement between model and observations suggests that the presently proposed non-thermal, non-stationary model could be a plausible explanation of AGN X-ray emission as an alternative to thermal coronae models.

The INTEGRAL Galactic bulge monitoring program: the first 1.5 years

Aims.The Galactic bulge region is a rich host of variable high-energy point sources. Since 2005, February 17 we are monitoring the source activity in the Galactic bulge region regularly and frequently, i.e., about every three days, with the instruments onboard INTEGRAL. Thanks to the large field of view, the imaging capabilities and the sensitivity at hard X-rays, we are able to present for the first time a detailed homogeneous (hard) X-ray view of a sample of 76 sources in the Galactic bulge region. Methods: We describe the successful monitoring program and show the first results from the start of the monitoring up to 2006, April 21, i.e., for a period of about one and a half year, during three visibility seasons. We focus on the short (hour), medium (month) and long-term (year) variability in the hard X-ray bands, i.e., 20-60 keV and 60-150 keV. When available, we discuss the simultaneous observations in the soft X-ray, 3-10 keV and 10-25 keV, bands. Results: Almost all the sources in the Galactic bulge region we detect in the 20-60 keV and 60-150 keV bands are variable. During the last two and a half weeks of the third visibility season most of the known persistent (hard) X-ray sources in the Galactic Center region were not detected. Of our sample of sources, per visibility season we detect 32/33 sources in the 20-60 keV band and 8/9 sources in the 60-150 keV band above a signal to noise of 7. On average, we find per visibility season one active bright (âa†100 mCrab, 20-60 keV) black-hole candidate X-ray transient and three active weaker (âa‰25 mCrab, 20-60 keV) neutron star X-ray transients. Most of the time a clear anti-correlation can be seen between the soft and hard X-ray emission in some of the X-ray bursters. Hard X-ray flares or outbursts in X-ray bursters, which have a duration of the order of weeks are accompanied by soft X-ray drops. On the other hand, hard X-ray drops can be accompanied by soft X-ray flares/outbursts. During the course of our program we found a number of new sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGR J17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here on some of the high-energy properties of these sources. Conclusions: The high-energy light curves of all the sources in the field of view, and the high-energy images of the region, are made available through the WWW, as soon as possible after the observations have been performed, at http://isdc.unige.ch/Science/BULGE/. Appendices are only available in electronic form at http://www.aanda.org