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INTEGRAL observations of Mrk 421

ATel #1507; D. Petry (MPE/ISDC), , W. Ishibashi (ISDC), V. Beckmann (ISDC) and A. v.Kienlin (MPE)
on 3 May 2008; 12:17 UT
Credential Certification: Dirk Petry (petry@mpe.mpg.de)

Subjects: X-ray, Gamma Ray, AGN

Following the observation of intense flaring at TeV energies by the VERITAS Cherenkov telescopes (later announced in Swordy et al. 2008, Atel #1506), a TOO observation of the blazar Mrk 421 by INTEGRAL was triggered on 2 May 2008. Observations started the same day at 21:29 h UTC during INTEGRAL revolution 678. The observations are ongoing and scheduled to continue until 7 May 2008.
From the first 31 ks of data, we extract the following information about Mrk 421:
At 3-20 keV, the JEM-X1 data (25 ks of effective exposure time) are well represented by a single power law model with photon index 3.0+-0.1 (reduced chi-squared is 1.06 for 122 degrees of freedom). The model flux in this energy range is 1.9e-10 erg/cm**2/sec, i.e. about 7 mCrab.
At 20-40 keV, the IBIS/ISGRI data (31 ks) yield a 6 sigma detection, with a flux of 5±1 mCrab.
At 40-80 keV, the source is not detected so far by IBIS/ISGRI. The 3-sigma upper limit for the 40-80 keV flux is 5 mCrab.
Both IBIS/ISGRI measurements are well compatible with the extrapolation of the JEM-X1 spectrum.
The flux appears to be variable and reaches up to 25 mCrab in individual 30 minute observations.