HESS Observations and VLT Spectroscopy of PG 1553+113

TL;DR

This study detected very high energy gamma-ray emission from PG 1553+113 with HESS, measured its spectrum, and attempted to determine its redshift using VLT spectroscopy, but found no spectral lines to confirm its distance.

Contribution

First combined VHE gamma-ray observations with HESS and near-infrared spectroscopy with VLT to study PG 1553+113's properties and redshift.

Findings

Detected VHE gamma-ray emission with high significance.

Measured a very soft gamma-ray spectrum with photon index ~4.5.

Failed to determine the redshift due to lack of spectral lines.

Abstract

AIMS: The properties of the very high energy (VHE; E>100 GeV) gamma-ray emission from the high-frequency peaked BL Lac PG 1553+113 are investigated. An attempt is made to measure the currently unknown redshift of this object. METHODS: VHE Observations of PG 1553+113 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. H+K (1.45-2.45 micron) spectroscopy of PG 1553+113 was performed in March 2006 with SINFONI, an integral field spectrometer of the ESO Very Large Telescope (VLT) in Chile. RESULTS: A VHE signal, ~10 standard deviations, is detected by HESS during the 2 years of observations (24.8 hours live time). The integral flux above 300 GeV is (4.6 +- 0.6{stat} +- 0.9{syst}) x 10^{-12} cm^{-2} s^{-1}, corresponding to ~3.4% of the flux from the Crab Nebula above the same threshold. The time-averaged energy spectrum is measured from 225 GeV to ~1.3 TeV, and is characterized by a very soft power law (photon index of Gamma = 4.5 +- 0.3{stat} +- 0.1{syst}). No evidence for any flux or spectral variations is found on any sampled time scale within the VHE data. The redshift of PG 1553+113 could not be determined. Indeed, even though the measured SINFONI spectrum is the most sensitive ever reported for this object at near infrared wavelengths, and the sensitivity is comparable to the best spectroscopy at other wavelengths, no absorption or emission lines were found in the H+K spectrum presented here.