SWIFT BURST ALERT TELESCOPE, FERMI LARGE AREA TELESCOPE, AND THE BLAZAR SEQUENCE

Using public Fermi Large Area Telescope (LAT) and Swift Burst Alert Telescope 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 flat spectrum radio quasars, have hard continua in the medium-hard X-ray range but soft continua in the LAT gamma-ray range (photon indices Gamma(X) less than or similar to 2 and Gamma(G) greater than or similar to 2), while lower luminosity blazars, classified as BL Lacs, have opposite behavior, i.e., soft X-ray and hard gamma-ray continua (Gamma(X) greater than or similar to 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.