Break in the Very High Energy 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 Γ = 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 ~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 detected in the VHE spectrum of a blazar.