Resonant absorption troughs in the gamma-ray spectra of QSO

Abstract

Abstract. We report the very first evidence of the possible detection of γ-ray resonant absorption along the line of sight towards γ-ray bright quasars (QSOs), like 3C 279, 3C 273, PKS 0528+0134, and BL Lacertae. These detections resulted from the analysis of COMPTEL and EGRET data that were collected either during monitoring campaigns of the Virgo and galactic anticenter regions by the Compton Gamma Ray Observatory (CGRO) or during ToO observations of QSOs flares. We discuss three resonant absorption mechanisms that affect the γ-ray spectrum of point-like sources when crossing the surrounding warm and cold absorbers, as well as the potential of this γ-ray photon absorption method to constrain the abundance of the absorber. We detected two absorbers along the line of sight towards γ-ray bright QSOs, one at the QSO rest frame redshift and another at approximately zero redshift. We tentatively identify the latter with an absorber in the Galactic halo, while the former is undoubtedly caused by photon absorption in the host galaxy of the QSO. We discuss the advantages and drawbacks of this method in studies of absorbers in different astrophysical environments and compare this new method to absorber studies at X-ray or other wavelengths. We applied this γ-ray absorption method to identify a few of the EGRET unidentified (EUID) sources as QSOs, and determine their redshifts.

Gamma-ray absorption method (GRAM) application to the identification of EGRET unidentified sources

“…To a first order, one can assume that we are dealing with absorbers at two redshifts, one in the QSO host galaxy and the second absorber in the Milky Way. This assumption is supported by our fitting results for SEDs of the γ-ray bright QSOs (see Iyudin et al 2005).…”

“…Line colours, diamonds, and boxes have the same meaning as in Fig. 4. given in Table 1 of Iyudin et al (2005), and it shows generally very significant improvement of the fits with the absorption line included in the fit function. Table A.1 contains values of the absorption trough position E 1 for the ∆-resonance related trough of the QSO covering the redshift range from z ∼ 0.07 for BL Lac up to z = 2.07 for PKS0528+134.…”

“…A side result of such a comparison will be either prove, or disprove of the necessity of using flux correction coefficients for the two lowest energy bins of EGRET SEDs. See the discussion of this point in Iyudin et al (2005).…”

“…The γ-quanta absorption method (GRAM), introduced by Iyudin et al (2005), can be used to measure the absorbing column in the cases of QSOs with known redshift. At the same time, this approach can be used to evaluate the redshift of objects with unknown identification (EUID).…”

“…GRAM is based on the absorption trough analysis (Iyudin et al 2005) in the spectral energy distribution (SED). Based on the ∆-resonance trough measurements of 3C 279 SED, we derived the "γ-measured" averaged redshift of 3C 279 as z GRAM = 0.60 ± 0.02 (see redshifts for 3C 279 in Table A.1).…”