Unidentified gamma-ray sources and gamma-ray bursts
[Unidentified gamma ray sources][Gamma-ray bursts][Satellites]

Third EGRET catalog distribution of gamma-ray sources. Clearly, most of them, including the unidentified ones, are in the galactic plane.

The existence of a galactic population of gamma-ray sources is known since the days of the COS B experiment. At that time (early 80's) about 50% of the unidentified detections (i.e. detections without known counterparts at other frequencies) were in regions containing young objects, like supernova remnants (SNRs) and OB massive stars. The gamma-ray emission could stem from pion decays resulting from hadronic interactions of high-energy protons (or nuclei) and ambient matter. These protons would be locally injected by young stars in the SNR shocks where they would be diffusively accelerated up to high energies by Fermi mechanism. Also, particle acceleration at the terminal shock of strong stellar winds alone could be responsible for the gamma-ray sources without the mediation of the SNR shock waves advocated before.

Since 1991, with the advent of the Energetic Gamma Ray Experiment Telescope (EGRET) onboard the Compton satellite -we show a picture of it below-, the observational data on galactic gamma-ray sources have been dramatically improved. Two of the previously unidentified COS-B sources, Geminga and PSR 1706-44, are now known to be pulsars. The detection of pulsed high-energy emission from other sources and the identification of Geminga as a radio quiet object have prompted several authors to explore the possibility that all unidentified low latitude sources in the EGRET catalog are pulsars (with the exception of a small extragalactic component which is seen through the disc of the Galaxy).

However, recent spectral analysis clearly show that several sources are quite at odds with the pulsar explanation. Time variability in the gamma-ray flux of many sources also argues against a unique population behind the unidentified galactic gamma-ray detections. Possible association of gamma-sources with SNRs was analized, finding significant statistical support for the idea that some remnants could be gamma-ray emitters. We have been involved in several of these recent discoveries, read about them in our Supernovae page.

Another highlight of our research in this area has been the study of the level of two-dimensional positional coincidences between unidentified EGRET sources at low galactic latitudes in the last catalog and different populations of galactic objects. It provided proofs that there is overwhelming statistical evidence for the association of gamma-ray sources with SNRs and OB star forming regions (these latter considered as pulsar tracers), marginally significant evidence for the association with early-type stars endowed with very strong winds, like Wolf-Rayet stars and Of stars.

Besides, about ten sources, for which we have there is not any positional coincidence with known objects, are the most strange. They are highly variable, with steep spectral index, and can not be explained by any usual model. We think this could be indicative a new discovery: Kerr-Newman isolated black holes.

Gamma ray bursts are flashes of high energy radiation that can be brighter, during their brief existence, than any other gamma ray source in the sky. The bursts present an amazing variety of temporal profiles, spectra, and time-scales that have puzzled astrophysicists for almost three decades. In recent years, our observational insight of this phenomenon has been dramatically increased by the huge amount of data collected by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO), a satellite launched by NASA in 1991. BATSE observations have confirmed that no large clustering or anisotropies are present in the sky distribution of GRBs. The isotropic distribution of GRBs strongly suggests an extragalactic origin which has been recently confirmed by the direct measurement of high-redshifted absorption lines of the optical counterpart of the GRB 970508. If the sources are so far, the energy necessary to produce the observed events by an intrinsic mechanism is astonishing: about 10^{51} erg of gamma rays must be released in less than 1 second. The most popular model to date to produce such an event is the merger of two compact stars (two neutron stars or a neutron star and a black hole) in a distant galaxy. As a result of the merging, a relativistic expanding fireball is formed. It is believed that the interaction of the blast with the surrounding medium produces lower energy (X-ray, optical, may be radio) counterparts of the original GRB. However, the wide variety of burst profiles, a possible statistical evidence for GRB repetition and some spectral properties remain unexplained by an unique, consistent model. There is such a large variety of individual events that every model proposed has to face a large number of counter-examples. These facts are suggesting, perhaps, that the origin of such a complex phenomenon might have more than one explanation.

Distribution of the gamma-ray positions and fluences of the updated catalog of BATSE gamma-ray bursts. Clearly, opposite to the steady sources distribution, they are isotropically distributed.

The temporal distribution of the bursts is one of the most striking signatures of the GRB phenomenon. There are at least four classes of distributions, from single-peaked bursts, including the fast rise and exponential decaying FREDs, their inverse or anti-FREDs to chaotic structures. There are well separated episodes of emission, and bursts with extremely complex profiles. Most of the bursts are time asymmetric but there are some symmetric as well.

Left: Typical temporal profiles of several gamma ray bursts. Right: When a gamma-ray burst is observed, all the gamma-ray sky is dominated by it.

Burst time-scales go through the 30ms scale to hundreds of seconds. The measurement of these time-scales is a rather complicated task, since it may depend on the intensity of both the background and the source. At high energies (>100 MeV), some extremely long bursts have been detected. A unique and common characteristic of GRBs is that most of their power is received in energies higher than 50KeV. Their spectrum approximately follows a power law with negative exponents within (-2.7,-1.7). It is interesting to note that there is no correlation between the spectral index and the morphology of the temporal profile or the location in the sky.

A special issue for us has been the possible repetition of the bursts. The most recent and complete repetition study on the BATSE catalogue has been carried out by Tegmark et al. They analyzed the angular power spectrum of 1122 GRBs finding that no more than 5% can be labeled as repeaters at the 99% confidence level. By now, evidence for repetition is very suggestive but not compelling. This point might be clarified with forthcoming technologies, especially when detectors with improved spatial resolution become available and studies on individual GRB repetition can be made unambiguously.

Based on the study of temporal asymmetry of 631 gamma ray bursts from the BATSE 3B catalog, we identified the population of bursts whose rising times are longer than their decays, thus showing atypical profiles. We analyzed their sky distribution, morphology, time-space clustering and other average properties and compared them with those associated with the bulk of the bursts. We show how most of the peculiar bursts analyzed are consistent with recent fireball models, but a fraction of bursts (4%) of the total sample) appear to be inconsistent. Go here if you like to know the details.

It is clear that to make gamma-ray astronomy in the MeV range we must go outside the Earth atmosphere. Below you will find some links (click in the pictures) to some of the satellites which produced, or are going to produce, the most relevant discoveries in the field.

From top and left: Compton Satellite, GLAST, Swift, Constellation-X and Integral.

Discovery of a new radio galaxy within the error box of the unidentified gamma-ray source 3EG J1735-1500 (J.A. Combi, G.E. Romero, J.M. Paredes, D.F. Torres, M. Ribó 2003, ApJ 588, 731)

In an effort to identify the gamma-ray source 3EG J1735-1500, we have discovered a new radio galaxy within its location error box. The galaxy is a double-sided jet source forming a large angle with the line of sight (see the Figure below). Optical observations reveal a V ~18 magnitude galaxy at the position of the radio core. Although the association with the EGRET source is not confirmed at the present stage, because there is a competing, alternative gamma-ray candidate within the location error contours which is also studied here, the case deserves further attention. The new radio galaxy can be used to test the recently proposed possibility of gamma-ray emitting radio galaxies beyond the already known case of Centaurus A.

High-resolution radio image of the galaxy J1737-15 at 1.4 GHz overlapped to the optical image obtained with the Calar Alto 2.2-m telescope using a Johnson's I filter. Radio contours are shown in steps of 0.6 mJy/beam, starting from 1 mJy/beam. 
Lower panel: An enlargement of the central region of the radio galaxy showing the possible host galaxy. Radio contours are in steps of 0.3 mJy/beam, starting from 1.3 mJy/beam.

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