Constraining blazar distances with combined Fermi and TeV data: an empirical approach

arXiv:1003.1674v2 [astro-ph.CO] 29 Mar 2010 Mon. Not. R. Astron. Soc. 000, 1–5 () Printed 17 January 2020 (MN LATEX style file v2.2) Constraining blazar distances with combined Fermi and TeV data: an empirical approach E. Prandini1⋆, G. Bonnoli2, L. Maraschi3, M. Mariotti1, F. Tavecchio2 1 Dipartimento di Fisica, Padova University and INFN Sez. di Padova, via Marzolo 8, I–35131 Padova, Italy 2 INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, I–23807 Merate, Italy 3 INAF – Osservatorio Astronomico di Brera, via Brera 28, I–20100 Milano, Italy 17 January 2020 ABSTRACT We discuss a method to constrain the distance of blazars with unknown redshift using com- bined observations in the GeV and TeV regimes. We assume that the VHE spectrum corrected for the absorption through the interaction with the Extragalactic Background Light can not be harder than the spectrum in the Fermi/LAT band. Starting from the observed VHE spectral data we derive the EBL-corrected spectra as a function of the redshift z and fit them with power laws to be compared with power law fits to the LAT data. We apply the method to all TeV blazars detected by LAT with known distance and derive an empirical law describing the relation between the upper limits and the true redshifts that can be used to estimate the dis- tance of unknown redshift blazars. Using different EBL models leads to systematic changes in the derived upper limits. Finally, we use this relation to infer the distance of the unknown redshift blazar PKS 1424+240. Key words: galaxies: distances and redshifts - gamma-rays: observations - radiation mecha- nisms: non–thermal 1 INTRODUCTION The extragalactic TeV sky catalogue (E > 100 GeV), counts nowadays 35 objects1. Many of these sources have recently been detected also at GeV energies by the Fermi satellite (Abdo et al. 2009), allowing for the first time a quasi-continuous coverage of the spectral shape of extragalactic VHE emitters over more than 4 decades of energy. Except for two starburst galaxies and two radiogalaxies, all the others are blazars, radio-loud active galac- tic nuclei with a relativistic jet closely oriented toward the Earth, as described in Urry & Padovani (1995). The apparent luminos- ity of the non-thermal radiation emitted by the jet is then largely enhanced by relativistic beaming and dominates the observed high energy emission. Typically, the spectral energy distribution (SEDs) emitted from these objects, extending from radio waves to gamma- ray frequencies, is composed of two broad humps. In the case of TeV detected blazars, the first component usually peaks in the UV- X-ray band, and the second peak is located at GeV-TeV energies. The first component is identified as electron synchrotron radiation, whilst the second component is widely attributed to inverse Comp- ton scattering of ambient photons by the same synchrotron emitting electrons. Relativistic electrons are accelerated within a region in bulk relativistic motion along the jet (e.g. Tavecchio et al. 1998). VHE photons emitted by cosmological sources are effectively ⋆E–mail: prandini@pd.infn.it 1 for an updated list see: http://www.mppmu.mpg.de/∼rwagner/sources/ absorbed, through the pair production process, γγ →e+−, by the interaction with the so-called Extragalactic Background Light (EBL) (Stecker, de Jager & Salamon 1992). EBL is composed of stellar light emitted and partially reprocessed by dust throughout the entire history of cosmic evolution. The expected EBL spectrum is composed by two bumps at near-infrared and far-infrared wave- lengths (Hauser & Dwek 2001). Direct measurement of the EBL has proved to be a difficult task, primarily due to the zodiacal light that forms a bright foreground which is difficult to suppress. Due to the lack of direct EBL knowledge, many models have been elab- orated in the last years (Stecker, Malkan & Scully 2006; Frances- chini, Rodighiero & Vaccari 2008; Gilmore et al. 2009; Kneiske & Dole 2010). Moreover, for some blazars the derivation of the in- trinsic spectrum is also difficult due to the uncertainty or lack of a redshift measurement. In particular a direct spectroscopic measure of the redshift is often difficult in BL Lac objects, which are char- acterized by extremely weak emission lines (equivalent width < 5 ˚A). In this paper we discuss a method to derive upper limits on the redshift of a source based on the comparison between the spec- tral index at GeV energies as measured by LAT (unaffected by the cosmological absorption up to redshifts far beyond those of inter- est here) and the deabsorbed TeV spectrum. Basically, for larger distances the deabsorbed spectrum becomes harder. A solid upper limit to the redshift can be inferred deriving the redshift at which c⃝RAS 2 Prandini et al. Source Name z[real] Fermi/LAT VHE z∗ z∗ z∗ z[rec] slope slope low EBL model mean EBL model high EBL model mean EBL model Mkn 421 0.030 1.78±0.03 2.3± 0.1(1) 0.101+0.021 −0.022 0.078+0.016 −0.018 0.054+0.012 −0.012

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