ESA Science & Technology - Publication Archive

Gamma-ray line emission from radioactive decay of ⁶⁰Fe provides constraints on nucleosynthesis in massive stars and supernovae. The spectrometer SPI on board INTEGRAL has accumulated nearly three years of data on gamma-ray emission from the Galactic plane. We have analyzed these data with suitable instrumental-background models and sky distributions to produce high-resolution spectra of Galactic emission. We detect the gamma-ray lines from ⁶⁰Fe decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9 sigma significance for the combination of the two lines. The average flux per line is (4.4 +- 0.9) x 10^-5 ph cm^-2 s^-1 rad^-1 for the inner Galaxy region. Deriving the Galactic ²⁶Al gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of ⁶⁰Fe/²⁶Al gamma-rays as 0.148 +- 0.06. The current theoretical predictions are still consistent with our result.

Contents

• Foreword
• 5th Announcement of Opportunity (AO-5)
• Key Programmes in AO-5
• Science Highlights
• Science Operations - Highlights
• The Fastest INTEGRAL TOO
• ISOC Science Data Archive Version 2.7
• INTEGRAL Visualisation Tool & Explorer
• INTEGRAL Publication Status
• Changes in ISOC

The work of ESA's INTEGRAL high-energy observatory usually follows a long-term plan that is established every year by selecting only the very best of the numerous observing proposals from the scientific community. However, nature does not always follow the same plan, so INTEGRAL and the people who keep it running have to react to unforeseen and sudden 'Targets of Opportunity'. The case here is neutron star IGR J00291+5934, an incredibly dense object with a mass similar to that of our Sun's compressed into a rapidly spinning sphere only a few kilometres across. Not only that, but it is busy swallowing its stellar companion.

The gamma-ray observatory INTEGRAL was launched in October 2002 and produces since then a wealth of discoveries and important new results. I will present a selection of scientific highlights obtained during the first 2.5 years of the mission.

Contents

• Foreword
• INTEGRAL AO-4 Results
• INTEGRAL AO-5 Key Programme
• INTEGRAL Publication Status
• Science Highlights
• Science Operations - Highlights
• The ISOC Science Data Archive
• The 6th INTEGRAL Workshop
• Changes at ISOC

Context. In coded mask techniques, reconstructed sky images are pseudo-images: they are maps of the correlation between the image recorded on a detector and an array derived from the coded mask pattern.

Aims. The INTEGRAL/IBIS telescope provides images where the flux of each detected source is given by the height of the local peak in the correlation map. As such, it cannot provide an estimate of the flux of an extended source. What is needed is intensity sky images giving the flux per solide angle as typically done at other wavelengths.

Methods. In this paper, we present the response of the INTEGRAL IBIS/ISGRI coded mask instrument to extended sources. We develop a general method based on analytical calculations in order to measure the intensity and the associated error of any celestial source and validated with Monte-Carlo simulations.

Results. We find that the sensitivity degrades almost linearly with the source extent. Analytical formulae are given as well as an easy-to-use recipe for the INTEGRAL user. We check this method on IBIS/ISGRI data but these results are general and applicable to any coded mask telescope.

We report the detection of both the 67.9 and 78.4 keV ⁴⁴Sc gamma-ray lines in Cassiopeia A with the INTEGRAL IBIS/ISGRI instrument. Besides the robustness provided by spectroimaging observations, the main improvements compared to previous measurements are a clear separation of the two ⁴⁴Sc lines together with an improved significance of the detection of the hard X-ray continuum up to 100 keV. These allow us to refine the determination of the ⁴⁴Ti yield and to constrain the nature of the nonthermal continuum emission. By combining COMPTEL, BeppoSAX PDS and ISGRI measurements, we find a line flux of (2.5+/-0.3)×10^-5 cm^-2s^-1 leading to a synthesized ⁴⁴Ti mass of 1.6^+0.6_-0.3×10^-4 M_solar. This high value suggests that Cas A is peculiar in comparison to other young supernova remnants, from which so far no line emission from ⁴⁴Ti decay has been unambiguously detected.

Using the IBIS Compton mode, the INTEGRAL satellite is able to detect and localize bright and hard GRBs, which happen outside of the nominal INTEGRAL field of view. We have developed a method of analyzing such INTEGRAL data to obtain the burst location and spectra. We present the results for the case of GRB 030406. The burst is localized with the Compton events, and the location is consistent with the previous Interplanetary Network position. A spectral analysis is possible by detailed modeling of the detector response for such a far off-axis source with an offset of 36.9°. The average spectrum of the burst is extremely hard: the photon index above 400 keV is -1.7, with no evidence of a break up to 1.1 MeV at a 90% confidence level.

GRB 040223 was observed by INTEGRAL and XMM-Newton. GRB 040223 has a peak flux of (1.6+/-0.13) x10^-8 ergs cm^-2 s^-1, a fluence of (4.4+/-0.4)x10^-7 ergs cm^-2 and a steep photon power law index of -2.3+/-0.2, in the energy range 20-200 keV. The steep spectrum implies it is an x- ray rich GRB with emission up to 200 keV and E_peak < 20 keV. If E_peak is < 10 keV, it would qualify as an x-ray flash with high energy emission. The x-ray data has a spectral index beta_x = -1.7+/-0.2, a temporal decay of t^(-0.75+/-0.25) and a large column density of (1.8 x 10^22) cm^-2. The luminosity-lag relationship was used to obtain a redshift (z = 0.3+/-0.06). The isotropic energy radiated in gamma-rays and x-ray luminosity after 10 hours are factors of 1000 and 100 less than classical GRBs. GRB 040223 is consistent with the extrapolation of the Amati relation into the region that includes XRF 030723 and XRF 020903.

In this paper we report the second soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific data set is based on more than 10 Ms of high-quality observations performed during the first 2 years of Core Program and public IBIS/ISGRI observations, and covers ~50% of the whole sky. The main aim of the first survey was to scan systematically, for the first time at energies above 20 keV, the whole Galactic plane to achieve a limiting sensitivity of ~1 mcrab in the central radian. The target of the second year of the INTEGRAL mission lifetime was to expand as much as possible our knowledge of the soft gamma-ray sky, with the same limiting sensitivity, to at least 50% of the whole sky, mainly by including a substantial coverage of extragalactic fields. This catalog comprises more than 200 high-energy sources detected in the energy range 20-100 keV, including new transients not active during the first year of operation, faint persistent objects revealed with longer exposure time, and several Galactic and extragalactic sources in sky regions not observed in the first survey. The mean position error for all the sources detected with significance above 10 sigma is ~40", enough to identify most of them with a known X-ray counterpart and to unveil the nature of most of the strongly absorbed ones, even though they are very difficult to detect in X-rays.

Gamma-rays from radioactive ²⁶Al (half-life ,7.2 x 10⁵ years) provide a 'snapshot' view of continuing nucleosynthesis in the Galaxy. The Galaxy is relatively transparent to such gamma-rays, and emission has been found concentrated along its plane. This led to the conclusion1 that massive stars throughout the Galaxy dominate the production of ²⁶Al. On the other hand, meteoritic data show evidence for locally produced ²⁶Al, perhaps from spallation reactions in the protosolar disk. Furthermore, prominent gamma-ray emission from the Cygnus region suggests that a substantial fraction of Galactic ²⁶Al could originate in localized star-forming regions. Here we report high spectral resolution measurements of ²⁶Al emission at 1808.65 keV, which demonstrate that the ²⁶Al source regions corotate with the Galaxy, supporting its Galaxy-wide origin. We determine a present-day equilibrium mass of 2.8 (±0.8) solar masses of 26Al. We use this to determine that the frequency of core collapse (that is, type Ib/c and type II) supernovae is 1.9 (±1.1) events per century.

SGR 1806-20 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Götz et al. (2004a). Our sample includes a very bright outburst that occurred on October 5 2004, during which over one hundred bursts were emitted in 10 minutes, involving an energy release of 3x10⁴² erg. We present a detailed analysis of it and discuss our results in the framework of the magnetar model.

Contents

• Foreword
  - 4th Announcement of Opportunity (AO-4)
  - The INTEGRAL Users Group
• INTEGRAL Mission Status
• Science Highlights
• Galactic Bulge Monitoring Program
• Science Operations - Highlights
• The ISOC Science Data Archive
• The 6th INTEGRAL workshop
• Outreach
• ISOC now at ESAC
• Contact INTEGRAL science operations

We present the study of one year of INTEGRAL data on the neutron star low mass X-ray binary GX 5-1. Thanks to the excellent angular resolution and sensitivity of INTEGRAL, we are able to obtain a high quality spectrum of GX 5-1 from ~5 keV to ~100 keV, for the first time without contamination from the nearby black hole candidate GRS 1758-258 above 20 keV. During our observations, GX 5-1 was mostly found in the horizontal and normal branch of its hardness intensity diagram. A clear hard X-ray emission is observed above ~30 keV which exceeds the exponential cut-off spectrum expected from lower energies. This spectral flattening may have the same origin of the hard components observed in other Z sources as it shares the property of being characteristic to the horizontal branch. The hard excess is explained by introducing Compton up-scattering of soft photons from the neutron star surface due to a thin hot plasma expected in the boundary layer. The spectral changes of GX 5-1 downward along the 'Z' pattern in the hardness intensity diagram can be well described in terms of monotonical decrease of the neutron star surface temperature. This may be a consequence of the gradual expansion of the boundary layer as the mass accretion rate increases.

In proceeding of "Workshop Astronomy with Radioactivities V", Clemson SC, USA, Sep 5-9 2005, Editors R. Diehl, D. Hartmann, E. Zinner, in New Reviews in Astronomy, Elsevier

The gamma-ray observatory INTEGRAL was launched in October 2002 and produces since then a wealth of discoveries and important new results. I will present a selection of scientific highlights obtained during the first 2.5 years of the mission.

The IBIS/ISGRI imager on Integral detected for the first time a hard X-ray source, IGR J17456-2901, located within 1' of Sgr A* over the energy range 20-100 keV. Here we present the results of a detailed analysis of ~7x10⁶s of Integral observations of the Galactic Centre. With an effective exposure of 4.7x10⁶s we have obtained more stringent positional constraints on this high-energy (HE) source and constructed its spectrum in the range 20-400 keV. Furthermore, by combining the Isgri spectrum with the total X-ray spectrum corresponding to the same physical region around SgrA* from XMM data, and collected during part of the Integral observations, we constructed and present the first accurate wide band HE spectrum for the central arcmins of the Galaxy. Our complete analysis of the emission properties of IGR shows that it is faint but persistent with no variability above 3 sigma contrary to what was alluded to in our first paper. This result, in conjunction with the spectral characteristics of the X-ray emission from this region, suggests that the source is most likely not point-like but, rather, that it is a compact, yet diffuse, non-thermal emission region. The centroid of IGR is estimated to be R.A.=17^h45^m42.5^s, decl.=-28°59'28'', offset by 1' from the radio position of Sgr A* and with a positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10³⁵ erg/sec. Very recently, Hess detected of a source of ~TeV gamma-rays also located within 1' of Sgr A*. We present arguments in favor of an interpretation according to which the photons detected by Integral and Hess arise from the same compact region of diffuse emission near the central black hole and that the supernova remnant Sgr A East could play an important role as a contributor of very HE gamma-rays to the overall spectrum from this region.

The closest Wolf-Rayet star, WR 11 in the binary system 2 Velorum, is the only star for which the spectral signature of the ²⁶Al produced in its core is expected to be detectable with current gamma-ray instruments, through the 1.8 MeV decay of that radioactive nucleus.

We present here the current status of both model predictions, from calculations of massive star evolution including rotation of stellar interior, and from data on 2 Velorum obtained by the ESA's gamma-ray satellite INTEGRAL over the first year of its mission.

The transient X-ray accreting millisecond pulsar XTE J1807-294 was observed during its February/March 2003 outburst by INTEGRAL, partly simultaneously with the XMM-Newton and RXTE satellites. We present here the first study of the 0.5-200 keV broad-band spectra of the source. On February 28, the source spectrum was consistent with thermal Comptonization by electrons of temperature ~40 keV, considerably higher than the value (~10 keV) previously derived from the low energy XMM-Newton data alone. The source is detected by INTEGRAL up to 200 keV with a luminosity in the energy band (0.1-200) keV of 1.3 x 10³⁷ erg s^-1 (assuming a distance of 8 kpc). 22 days later the luminosity dropped to 3.6 x 10³⁶ erg s^-1. A re-analysis of XMM-Newton data yields the orbital Doppler variations of the pulse period and refines the previous ephemeris. For this source, with shortest orbital period of any known binary radio or X-ray millisecond pulsar, we constrain the companion mass M_C < 0.022~M_Sun, assuming minimum mass transfer driven by gravitational radiation. Only evolved dwarfs with a C/O composition are consistent with the Roche lobe and gravitational radiation constraints, while He dwarfs require an unlikely low inclination.

We present the analysis of serendipitous sources in a deep, 500 ks, hard X-ray observation of the Coma Cluster region with the IBIS instrument on board INTEGRAL. In addition to the Coma Cluster, the final 20-50 keV image contains 12 serendipitous sources with statistical significance >4 sigma. We use these data (after correcting for expected number of false detections) to extend the extragalactic source counts in the 20-50 keV energy band down to a limiting flux of 1.0x10^-11 ergs s^-1 cm^-2 (~=1 mcrab). This is a more than a factor of 10 improvement in sensitivity compared to the previous results in this energy band obtained with the HEAO 1 A-4 instrument. The derived source counts are consistent with the Euclidean relation N(>f)~f^-3/2. A large fraction of identified serendipitous sources are low-redshift, z<0.02, active galactic nuclei (AGNs), mostly of Seyfert 1 type. The surface density of hard X-ray sources is (1.4+/-0.5)x10^-2 deg^-2 above a flux threshold of 10^-11 ergs s^-1 cm^-2. These sources directly account for ~3% of the cosmic X-ray background in the 20-50 keV energy band. Given the low redshift depth of our sample, we expect that similar sources at higher redshifts account for a significant fraction of the hard X-ray background. Our field covers only 3% of the sky; a systematic analysis of other extragalactic INTEGRAL observations can produce much larger source samples and is, therefore, critically important.

The explosion that results in a cosmic gamma-ray burst (GRB) is thought to produce emission from two physical processes: the central engine gives rise to the high-energy emission of the burst through internal shocking, and the subsequent interaction of the flow with the external environment produces long-wavelength afterglows. Although observations of afterglows continue to refine our understanding of GRB progenitors and relativistic shocks, gamma-ray observations alone have not yielded a clear picture of the origin of the prompt emission nor details of the central engine. Only one concurrent visible-light transient has been found and it was associated with emission from an external shock. Here we report the discovery of infrared emission contemporaneous with a GRB, beginning 7.2 minutes after the onset of GRB 041219a. We acquired 21 images during the active phase of the burst, yielding early multi-colour observations. Our analysis of the initial infrared pulse suggests an origin consistent with internal shocks.