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Crab flux no longer elevated in Fermi-LAT band

ATEL # 2879; E. Hays (NASA/GSFC), R. Buehler (SLAC/KIPAC), F. D'Ammando (INAF-IASF Palermo), J. E. Grove (NRL), P. S. Ray (NRL) on behalf of the Fermi Large Area Telescope Collaboration
on 28 Sep 2010; 16:31 UT
Password Certification: Elizabeth Hays (

Subjects: >GeV, Pulsars
Referred to by ATEL #: 2893

A gamma-ray flux increase from the direction of the Crab Nebula was reported previously by the AGILE and LAT collaborations (ATEL #2855, #2861). We report that the Fermi LAT has been monitoring this source and has observed a flux enhancement during the interval 18-22 September. Measurements taken starting on 23 September are consistent with the typical flux of the Crab. The average total flux from this location (which includes the nebula and pulsed emission) above 100 MeV measured between 2010-09-23 00:00:00 and 2010-09-26 18:43:01 UTC was (280 +/- 20) x 10^-8 ph/cm2/sec. The systematic error on the flux is generally estimated to be ~10% at 100 MeV. A preliminary phase-resolved analysis of the flaring source shows that the excess flux due to the flare is the same in the on and off pulse selections for the pulsar and thus it does not appear that the enhanced emission is pulsed. This suggests that origin of the flare is related to the nebula rather than the pulsar emission.

All reported results are preliminary and only the statistical errors are given. For this source the Fermi LAT contact person is Rolf Buehler (

After completing a dedicated pointed observation of the Crab Nebula from 2010-09-23 15:50 UT to 2010-09-27 19:49 UT (during which the exposure per unit time was increased by a factor of ~4), Fermi continues regular gamma-ray monitoring of this source in all-sky scanning mode. Further multiwavelength observations are well supported by the continued coverage available with the sky survey.

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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