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Fermi LAT detection of GeV flare in high redshift blazar 4C 38.41

ATEL # 2456; S. Buson, D. Bastieri (University/INFN Padova) on behalf of the Fermi Large Area Telescope Collaboration
on 24 Feb 2010; 20:34 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Denis Bastieri (

Subjects: Optical, Gamma Ray, >GeV, AGN, Transients

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, detected on February 23, 2010 increasing gamma-ray activity from the source positionally consistent with the EGRET flat spectrum radio quasar 4C 38.41 also known as B2 1633+38, OS 356, 3EG J1635+3813, 1FGL J1635.0+3808 (RA: 16h 35m 15.493s, Dec. +38d 08m 04.50s, J2000.0, ref. 1995, AJ, 110, 880, redshift 1.814, ref. 2004, SDSS, release 3).

Preliminary analysis indicates that the source on 23 Feb 2010 was in a high state with a gamma-ray flux (E>100MeV) of (1.5 +/- 0.4) x 10^-6 ph cm^-2 s^-1 (statistical uncertainty only). This corresponds to approximately an order of magnitude increase with respect to the average flux of 1FGL J1635.0+3808 which is the LAT 1-year point source catalog object associated with 4C 38.41. This is the second time that Fermi is announcing a similar GeV flare in this blazar (July 2010, ATEL#2136 ).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source and the rather high redshift of the source, we strongly encourage multiwavelength observations. For this source the Fermi LAT contact persons are S. Ciprini ( and S. Buson (

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 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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