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

On 17 August 2009, INTEGRAL caught another long Gamma-Ray Burst (GRB) in the field of view of IBIS/ISGRI. This one was special as it was also in the smaller field of view of the X-ray monitor JEM-X 1 and (almost) of the optical camera OMC. Beckmann et al. 2009, GCN 9815

JEM-X 1 image of GRB 090817
GRB 090817 is well detected near the centre of the INTEGRAL field of view in this JEM-X 1 image.
Credits: ISDC/V. Beckmann

OMC missed image of GRB 090817
OMC image taken during GRB 090817, but unfortunately the gamma-ray burst was just outside (see IBIS/ISGRI error circle on the right) the instrument's field of view.
Credits: ISDC/V. Beckmann

IBIS/ISGRI lightcurve of GRB 090817
The light-curve of GRB 090817 measured by IBIS/ISGRI lasted for about 250 seconds and consists of two main flares separated by a quiet period.
Credits: ISDC/V. Beckmann

JEM-X 1 lightcurve of GRB 090817
The double-peaked lightcurve of GRB 090817 as measured in the softer X-ray band of the JEM-X 1 instrument.

Credits: ISDC/V. Beckmann

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.