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Fermi LAT detection of increasing gamma-ray activity of blazars PKS 0537-441 and PKS 0301-243

ATEL # 2591; A. Cannon (University College Dublin, NASA/GSFC) on behalf of the Fermi Large Area Telescope Collaboration
on 30 Apr 2010; 19:37 UT
Password Certification: Teddy Cheung (

Subjects: Gamma Ray, >GeV, AGN

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed over 27th and 28th April 2010, increasing gamma-ray activity in sources positionally consistent with PKS 0537-441 (1FGL J0538.8-4404) and PKS 0301-243 (1FGL J0503.5-2406).

PKS 0537-441 (z=0.894, Peterson et al. 1976, ApJ, 207, L5) is one of the most variable BL Lac objects. Preliminary analysis indicates that the source gradually brightened over the previous weeks. On 27th April 2010, the source peaked at a daily gamma-ray flux (E>100MeV) of (1.2+/-0.2)x10^-6 ph cm-2 s-1, reaching a 6 hr peak flux of (2.0+/-0.7)x10^-6 ph cm-2 s-1 (statistical uncertainty only). Similar activity was previously reported in ATEL #1759 (October 2008) and ATEL #2124 (July 2009).

PKS 0301-243 (RA:03h03m26.50s Dec:-24d07m11.3s, J2000, Healey et al., 2007 ApJS, 171, 61) is also a BL Lac with redshift z=0.26 (Falomo et al 1993 AJ, 106, 11). It is a steep spectrum radio source with spectral index ~ -0.5. Fermi LAT observations indicate a daily flux (E>100MeV) of (1.4+/-0.3)x10^-6 ph cm-2 s-1 on 28th April 2010, with a peak 6 hr flux of (2.0+/-0.6)x10^-6 ph cm-2 s-1, which is ~40x larger than the average flux for this source over the first 11 months of LAT operation (arXiv:1002.2280v1). This is the first Fermi ATEL on this source.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. In consideration of the ongoing activity of these sources we strongly encourage multiwavelength observations. The Fermi LAT contact persons for these sources are Gino Tosti ( for PKS 0537-441 and Dave J. Thompson ( for PKS 0301-243.

The Fermi LAT is a pair conversion telescope designed to cover the energy band 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

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