Skip to Main content Skip to Navigation
Theses

RECHERCHE SYSTEMATIQUE DE CONTREPARTIES
OPTIQUES DE SURSAUTS GAMMA
DANS LES IMAGES DU "VERY WIDE SURVEY"
DU TELESCOPE CANADA-FRANCE-HAWAII

Abstract : Gamma-Ray Bursts (hereafter GRBs) are short and bright flashes of high-energy photons which happen at random time and position on the sky, with a frequency of about two per days. These events are the most energetic known in the Universe, and during their life-time, which goes from a few milliseconds to a couple of minutes, they become most brilliant than the whole sources of the sky gathered.The fast and accurate localisations of the prompt gamma-ray emission of GRB by on-board detectors have led to the discovery of their counterparts in other wavelengths, which have been called afterglows. Due to the fast decay of their luminosity, optical afterglows can only be seen from a few hours to a few days after the burst. Afterglows observations allowed the confirmation of the cosmological nature of GRBs by measures of spectroscopic reddening, the presence of host galaxies, and their association with a pecular type of supernovae.

There is some evidence that the prompt emission is collimated into a jet, whereas the late emission is more isotropic. One argument is provided by the achromatic steepening observed in many afterglow light-curves. At about the time of the steepening, the jet spreads out, and the afterglow becomes visible for off-axis observers. Therefore, we expect to observe many more afterglows than GRBs. Afterglows that are not associated with an observed GRB are called orphan afterglows. Since all GRBs recorded to date have been detected by their high-energy emission, the search for orphan GRB afterglows offers a complementary way to test the beaming hypothesis and to measure the beaming factor. Untriggered searches for GRB afterglows have already been attempted by a few teams, but none of them leads to the detection of optical afterglows, because of little sensitivity or poor sky coverage.

We have performed a search for optical afterglows in the images acquired for the Very Wide Survey, a observational program at the Canada-France-Hawaii Telescope, which covers a thousand of square degrees with a decent limiting magnitude. Each field is observed several times within the same observational period. This recurrence can be used to compare images of the same field in order to detect new, vanishing, and variable objects. This work is done by a dedicated Real Time Analysis System called "Optically Selected GRB Afterglows". This fully automatic pipeline consists at first in creating catalogs of the astrophysical sources of the image, and in calibrating parameters of these sources, and then in comparing catalogs of images of the same field and extracts a list of transient and variable objects. Objects are finally checked manually by a member of the collaboration who choose to reject the object as a false detection or to validate it as a truly variable object.

Since the beginning of the RTAS, we analyzed more than 490 square degrees down to magnitude r'=22.5, which represents about 20 millions of objects. Amongst these objects, 0.07% are found to be variable by the process, but only 10% of them are truly variable sources. Most of them are variable stars. We are left with one strong afterglow candidate that seems to follow the decreasing of the luminosity of a typical afterglow. We have also performed simulations which randomly generates afterglow light-curves in the sky, and computes the number of afterglows expected for a given observationl strategy. These simulations show that the Very Wide Survey combined with the RTAS is the best strategy for the search for optical afterglows, since we expect 10 times more afterglows than in previous searches. These simulations also allows us to give a limit to the total number of afterglows in the sky, and to compare this value to the theoretical models. If we assume that we have detected one afterglow, the number of afterglows in the sky at a given time is 100. This value is consistent with the model of Totani & Panaitescu, but incompatible with the predictions of Nakar et al. and Zou et al..
Complete list of metadata

Cited literature [67 references]  Display  Hide  Download

https://tel.archives-ouvertes.fr/tel-00135769
Contributor : Williams Exbrayat <>
Submitted on : Thursday, March 8, 2007 - 4:38:17 PM
Last modification on : Monday, April 5, 2021 - 2:26:05 PM
Long-term archiving on: : Friday, September 21, 2012 - 12:40:09 PM

Identifiers

  • HAL Id : tel-00135769, version 1

Collections

Citation

Frederic Malacrino. RECHERCHE SYSTEMATIQUE DE CONTREPARTIES
OPTIQUES DE SURSAUTS GAMMA
DANS LES IMAGES DU "VERY WIDE SURVEY"
DU TELESCOPE CANADA-FRANCE-HAWAII. Astrophysique [astro-ph]. Université Paul Sabatier - Toulouse III, 2006. Français. ⟨tel-00135769⟩

Share

Metrics

Record views

254

Files downloads

365