Data Centre for Astrophysics
Astronomy Department of the University of Geneva

GRB 090817: INTEGRAL/OMC's unlucky gamma-ray burst

On 2009-08-17 at 2:51 a.m. (00:51 GMT) INTEGRAL caught another Gamma-Ray Burst (GRB) in the field of view of IBIS/ISGRI. Although this happens about 10 times a year, this GRB was special, as it was not only within the field of view of the imager IBIS/ISGRI and of the spectrograph SPI, but also in the smaller field of view of the X-ray monitor JEM-X 1 and (almost) of the optical camera OMC.

Usually not all the data of OMC are downloaded from the satellite to the ground, but in case of a GRB in its field of view, a command is sent to the spacecraft to download the data around the burst position. This worked fine. But OMC was unlucky again (see INTEGRAL Picture of the Month of July 2005) - the burst was just outside its detector area. This is illustrated in the image to the left showing the IBIS/ISGRI error circle ontop of the OMC image.

Nevertheless GRB 090817 provides a lot of data to analyse. Both the IBIS/ISGRI and the JEM-X 1 light curves show two prominent peaks and a comparably long duration of about 250 seconds. The combined INTEGRAL spectrum covers the whole range of a gamma-ray burst, from 3 keV to several hundreds of keV, but, unluckily, not simultaneously the optical range. Other observations of this GRB were reported and can be accessed on the GCN site.