Swift imaging shows no evidence for active AGN near Crab

ATel #2868; Craig O. Heinke (U. of Alberta)
on 24 Sep 2010; 18:40 UT
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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The detection of enhanced gamma-ray (>100 MeV) flux from the direction of the Crab Nebula by AGILE (ATEL #2855) and Fermi (ATEL #2861) raises the question of whether the gamma-rays are from the Crab itself, or from a previously unknown gamma-ray source, such as an AGN, in close proximity to the Crab (the Fermi 1-sigma error radius is 3.6'). Although no changes have been noted in the overall X-ray flux from the Crab's direction (ATEL #2856, ATEL #2858), it is possible that a fainter X-ray source could have appeared, motivating imaging observations. Swift/XRT observed the Crab in photon-counting mode on Sept. 24, 2010, at UT 14:00 for a total livetime of 75 seconds (along with window-timing observations; see ATEL #2866). The Crab itself is saturated, and scattered light dominates the image, but limits may be placed on the X-ray flux from sources more than 1' from the center of the Crab (taken as RA=83.63, Dec=+22.02). We place conservative limits on the number of photons observed (0.2-10 keV; other choices gave similar results) from possible sources within the 9" Swift HEW radius at different distances from the Crab's center, and convert these to 1-10 keV X-ray fluxes assuming a power-law of photon index 2 absorbed by N_H=3.9e21 cm^-2 (from the Colden utility, Dickey & Lockman 1990, ARAA, 28, 215). Between 1' and 1.8' from the Crab, we find Fx<1.2e-10 ergs/cm^2/s, or <3.5 mCrab. Between 1.8' and 3', Fx<2.5e-11 ergs/cm^2/s, or <0.7 mCrab. Between 3' and 10', Fx<1.2e-11 ergs/cm^2/s, or <0.4 mCrab. The lack of evidence for a new X-ray source near the Crab suggests that the Crab itself is responsible for the enhanced gamma-ray flux, although more exotic scenarios cannot be ruled out. I thank the Swift team for prompt scheduling of this observation.