ESA Science & Technology - Publication Archive

The prompt optical emission that arrives with the gamma-rays from a cosmic gamma-ray burst (GRB) is a signature of the engine powering the burst, the properties of the ultra-relativistic ejecta of the explosion, and the ejecta's interactions with the surroundings. Until now, only GRB 990123 had been detected at optical wavelengths during the burst phase. Its prompt optical emission was variable and uncorrelated with the prompt gamma-ray emission, suggesting that the optical emission was generated by a reverse shock arising from the ejecta's collision with surrounding material. Here we report prompt optical emission from GRB 041219a. It is variable and correlated with the prompt gamma-rays, indicating a common origin for the optical light and the gamma-rays. Within the context of the standard fireball model of GRBs, we attribute this new optical component to internal shocks driven into the burst ejecta by variations of the inner engine. The correlated optical emission is a direct probe of the jet isolated from the medium. The timing of the uncorrelated optical emission is strongly dependent on the nature of the medium.

We report the discovery of persistent hard X-ray emission extending up to 150 keV from the soft gamma-ray repeater SGR 1806-20 using data obtained with the INTEGRAL satellite in 2003-2004. Previous observations of hard X-rays from objects of this class were limited to short duration bursts and rare transient episodes of strongly enhanced luminosity ("flares''). The emission observed with the IBIS instrument above 20 keV has a power law spectrum with photon index in the range 1.5-1.9 and a flux of 3 milliCrabs, corresponding to a 20-100 keV luminosity of ~10³⁶ erg s^-1 (for a distance of 15 kpc). The spectral hardness and the luminosity correlate with the level of source activity as measured from the number of emitted bursts.

It is believed that core-collapse supernovae (CCSN), occurring at a rate ~once per century, have seeded the interstellar medium with long-lived radioisotopes such as ⁶⁰Fe (half-life 1.5 Myr), which can be detected by the gamma-rays emitted when they beta-decay. Here we report the detection of the ⁶⁰Fe decay lines at 1173 keV and 1333 keV with fluxes 3.7 ± 1.1 × 10^-5 photons cm^-2 s^-1 per line, in spectra taken by the SPI spectrometer on board INTEGRAL during its first year. The same analysis applied to the 1809 keV line of ²⁶Al yielded a line flux ratio ⁶⁰Fe/²⁶Al = 0.11 ± 0.03. This supports the hypothesis that there is an extra source of ²⁶Al in addition to CCSN.

GRB 040403 is one of the faintest gamma-ray bursts for which a rapid and accurate localization has been obtained. Here we report on the gamma-ray properties of this burst, based on observations with the IBIS instrument aboard INTEGRAL, and the results of searches for its optical afterglow. The steep spectrum (power law photon index = 1.9 in the 20-200 keV range) implies that GRB 040403 is most likely an X-ray rich burst. Our optical limit of R > 24.2 at 16.5 h after the burst, indicates a rather faint afterglow, similar to those seen in other relatively soft and faint bursts.

Very high energy gamma-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy gamma-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of "dark" nucleonic cosmic ray sources.

The center of our Galaxy is a known strong source of electron-positron 511 keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched in 2002 October, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high-resolution, coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution, and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic plane. No positive annihilation flux was detected outside of the central region (|l|>40°) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511 keV flux.

Contents

• 3rd Announcement of Opportunity
• Mission Status
• INTEGRAL science workshop
• Science Operations - Highlights
• ISOC at ESAC - latest news
• List of approved open time AO-3 observations

Contents

• 3rd Announcement of Opportunity (AO-3)
• Science Highlights
• Mission Status
• INTEGRAL Public Data Archive

We report the association of the recently discovered hard X-ray source IGR J17475-2822 with the giant molecular cloud Sgr B2 in the Galactic Center region. The broad band (3-200 keV) spectrum of the source constructed from data of different observatories strongly supports the idea that the X-ray emission of Sgr B2 is Compton scattered and reprocessed radiation emitted in the past by the Sgr A* source. We conclude that 300-400 years ago Sgr A* was a low luminosity (L~ 1.5×10³⁹ erg s^-1 at 2-200 keV) AGN with a characteristic hard X-ray spectrum (photon index Gamma ~ 1.8). We estimate the mass and iron abundance of the Sgr B2 scattering gas at 2×10⁶ M_Sun(r/10 pc)² and 1.9 solar, respectively (where r is the radius of the cloud).

ISOC is completing the preparations for the forthcoming Announcement of Opportunity (AO-3) which will be released on 13 September 2004 calling for new INTEGRAL observing proposals for the period February 2005 until August 2006.

We report the first high-energy survey catalog obtained with the IBIS gamma-ray imager on board INTEGRAL. The analysis has been performed on the first-year Core Program ISGRI data comprising both Galactic Plane Scan and Galactic Centre Deep Exposure pointings for a total exposure time exceeding 5 Ms. This initial survey has revealed the presence of ~120 sources detected with the unprecedented sensitivity of ~1 mcrab in the energy range 20-100 keV. Each source is located to an accuracy between 1' and 3', depending on its brightness. The outstanding IBIS capability to locate soft gamma-ray emitters has allowed us to identify most of the detected sources with already known Galactic X-ray binary systems, while 28 of the objects are of unknown nature.

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton, and RXTE.

INTEGRAL is performing very well and we have witnessed a number of key events in the recent months which will be addressed in more detail here. Consequently this issue of the ISOC Newsletter is a bit longer than usual.

The Milky Way is known to be an abundant source of gamma-ray photons, now determined to be mainly diffuse in nature and resulting from interstellar processes. In the soft gamma-ray domain, point sources are expected to dominate, but the lack of sensitive high-resolution observations did not allow for a clear estimate of the contribution from such sources. Even the best imaging experiment revealed only a few point sources, accounting for about 50% of the total Galactic flux. Theoretical studies were unable to explain the remaining intense diffuse emission. Investigating the origin of the soft gamma-rays is therefore necessary to determine the dominant particle acceleration processes and to gain insights into the physical and chemical equilibrium of the interstellar medium. Here we report observations in the soft gamma-ray domain that reveal numerous compact sources. We show that these sources account for the entirety of the Milky Way's emission in soft gamma-rays, leaving at most a minor role for diffuse processes.

A special ESA publication to coincide with the 1st annivsary of the launch of the INTEGRAL spacecraft. Contribution were made by various members of the entire mission and project teams to give a unique account of a year in the life of a spacecraft.

This Special Letters Issue features the INTEGRAL observatory. About one year after its successful launch, this series of 75 publications describe the mission, the various instruments and their performance, as well as first scientific results from the spacecraft, ranging from gamma-ray bursts to Galactic sources.

Several important INTEGRAL events took place during the last two months.

We celebrated the first launch anniversary, on 17 October, with the spacecraft in excellent shape. Unfortunately, recent high solar activity substantially interrupted the observing programme, for the first time.

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a truly international enterprise. While ESA is responsible for the overall mission, the satellite's development and the flight operations, the launcher is provided by the Russian Space Agency and the second ground station is provided by NASA. The scientific instruments and the Science Data Centre are provided by the mission's Principal Investigators, with funding from national organisations.

The spectroscopic study of atomic and molecular lines in the infrared, optical and ultraviolet regions of the electromagnetic spectrum has provided us with knowledge and understanding of the physics of normal stars. Spectrallines in the gamma-ray region are produced by new and different processes. These include radioactivity, nuclear excitation, electron- positron annihilation and radiation in the vicinity of magnetic fields. Gamma-ray lines are indicators of all these processes and have now been observed at astrophysical sites.They provide powerful diagnostics for the high-energy processes occurring in some of the most violent and exotic objects in the Universe. The key features of the Integral satellite will allow us to study in great detail these, and other sites of key interest in today's astrophysical research, with high sensitivity combined with very good energy resolution and very good imaging capabilities (in order to avoid source confusion) using two main instruments, an 'Imager' and a 'Spectrometer'.

We have carried out a multiwavelength study of the plane of our Galaxy in order to establish a star-forming-complex catalogue which is as complete as possible. Features observed include H alpha, H109 alpha, CO, the radio continuum and absorption lines. For each complex we have determined the position, the systemic velocity, the kinematic distance and, when possible, the stellar distance and the corresponding uncertainties. All of these parameters were determined as homogeneously as possible, in particular all the stellar distances have been (re)calculated with the same calibration and the kinematic distances with the same mean Galactic rotation curve. Through the complexes with stellar distance determination, a rotation curve has been fitted. It is in good agreement with the one of Brand & Blitz (1993). We also investigated the residual velocities relative to the circular rotation model. We find that departures exist over large areas of the arms, with different values from one arm to another. From our data and in good agreement with previous studies, the Galactic warp is observed. It does not seem correlated with the departures from circular rotation. Finally, as segment-like features are noted from the complexes' distribution, we tried to find if they are indicative of a larger underlying structure. Then, we attempted to interpret the complexes' distribution in terms of spiral structure by fitting models with two, three and four logarithmic spiral arms. The four-arm model seems more appropriate to represent the grand design of our Galaxy. In this model the Norma arm and the external arm appear as being the two extremities of a single arm called the Norma-Cygnus arm. The new data and fitted model confirm the four-segment model of Georgelin & Georgelin (1976), clarifying the arms' design and extension and doubling their known length.