Swift BAT, Fermi LAT, and the Blazar Sequence

Details Swift BAT, Fermi LAT, and the Blazar Sequence Swift BAT, Fermi LAT, and the Blazar Sequence

2009-12-14

arxiv.org/abs/0912.2721v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

Accepted by ApJ

Scientific paper

Using public \fermi LAT and \swift BAT observations, we constructed the first sample of blazars selected at both hard X-rays and gamma-rays. Studying its spectral properties, we find a luminosity dependence of the spectral slopes at both energies. Specifically, luminous blazars, generally classified as FSRQs, have {\it hard} continua in the medium-hard X-ray range but {\it soft} continua in the LAT gamma-ray range (photon indices $\Gamma_X$ \ltsima 2 and $\Gamma_G$ \gtsima 2), while lower luminosity blazars, classified as BL Lacs, have opposite behavior, i.e., {\it soft} X-ray and {\it hard} gamma-ray continua ($\Gamma_X$ \gtsima 2.4 and $\Gamma_G < 2$). The trends are confirmed by detailed Monte Carlo simulations explicitly taking into account the observational biases of both instruments. Our results support the so-called ``blazar sequence'' which was originally based on radio samples of blazars and radio luminosities. We also argue that the X-ray-to-gamma-ray continua of blazars may provide independent insights into the physical conditions around the jet, complementing/superseding the ambiguities of the traditional classification based on optical properties.

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