BeppoSAX PDS detection of the AGILE transient in Musca

ATel #1419; Orlandini M., Frontera F., Bassani L., Landi R., Sguera V. (INAF/IASF Bologna)
on 9 Mar 2008; 11:05 UT
Credential Certification: Mauro Orlandini (orlandini@bo.iasf.cnr.it)

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Following ATel#1394 reporting the detection by the AGILE satellite of a variable gamma-ray emission from a region in Musca centered at (l,b) = (312.2,-0.3) and radius 30', we have searched in our BeppoSAX PDS Archive for possible PDS pointings in that region.

We actually find that the AGILE error box was observed by PDS four times: in three occasions in one of the offset positions of its rocking collimators, and in an on-axis position in one occasion.

In the 15-100 keV energy band, a marginal evidence of excess counts (0.422 +/- 0.115 cts/s) is found in the offset pointing occurred on 23/02/1997 (90 ksec of elapsed time). No evidence of emission (0.145 +/- 0.070 cts/s) is found in the pointing occurred on 19/02/2000 (47 ksec).

Instead, in the offset pointing of January 6, 2001 (220 ksec) a significant signal (0.450 +/- 0.032 cts/s) was detected. The center of the PDS FOV was almost coincident with the AGILE position. The spectrum of the excess counts is fit with a power law of photon index 2.2 +/- 0.2, with a normalized chi2 of 1.44 for 14 dof, and a 15-100 keV flux of 5x10**-11 erg/cm2/s (2.5 mCrab).

The light curve (32 bins of 1.5 hrs each) does not show significant variability, even if a flaring episode lasting a few bins seems to be there. A run test on the data gives a probability of 90% that the variation observed is due to chance.

The same region was also observed by all BeppoSAX NFIs on 29/01/2000 (42 ksec). A marginal signal (0.247 +/- 0.069 cts/s) is found in the PDS data, while the MECS image shows the clear presence of two sources, one of which (the weakest) in the AGILE error box, with 2000.0 equatorial coordinates (RA,DEC) = 14 08 00.6, -61 58 19.9.

The 2-10 keV spectrum of this source is fit with an absorbed power law of photon index 0.86 +/- 0.28, nH = 2.2x10**22 H/cm2 (compatible with galactic absorption), and a 2-10 keV unabsorbed flux of 2.5x10**-12 erg/cm2/s.

If the serendipitous source found with the PDS on January 11, 2001 is identified with the AGILE source, the flux expected assuming the measured spectrum is well below the AGILE sensitivity and would imply the presence of a variable high energy component, the origin of which (Comptonization?) should be investigated.