Break in the VHE spectrum of PG 1553+113: new upper limit on its redshift?

Abstract

PG 1553+113 is a known BL Lac object, newly detected in the GeV-TeV energy range by H.E.S.S and MAGIC. The redshift of this source is unknown and a lower limit of $z>0.09$ was recently estimated. The very high energy (VHE) spectrum of PG 1553+113 is attenuated due to the absorption by the low energy photon field of the extragalactic background light (EBL). Here we correct the combined H.E.S.S and MAGIC spectrum of PG 1553+113 for this absorption assuming a minimum density of the evolving EBL. We use an argument that the intrinsic photon index cannot be harder than $\Gamma =1.5$ and derive an upper limit on the redshift of $z<0.69$. Moreover, we find that a redshift above $z=0.42$ implies a possible break of the intrinsic spectrum at about 200 GeV. Assuming that such a break is absent, we derive a much stronger upper limit of $z<0.42$. Alternatively, this break might be attributed to an additional emission component in the jet of PG 1553+113. This would be the first evidence for a second component is detected in the VHE spectrum of a blazar.

arXiv commentrevised version submitted to ApJ

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