Multi-wavelength Observations of PG 1553+113 with HESS

TL;DR

This paper reports on multi-wavelength observations of PG 1553+113, including VHE gamma-ray detection with HESS and infrared spectroscopy with VLT, revealing a steep gamma-ray spectrum and an unknown redshift due to lack of spectral lines.

Contribution

First combined VHE gamma-ray and infrared observations of PG 1553+113, providing spectral data and constraints on its redshift despite no spectral lines detected.

Findings

Strong VHE gamma-ray signal detected with HESS

Steep gamma-ray spectrum with photon index ~4.5

No variability observed in gamma-ray flux

Abstract

Very high energy (VHE; >100 GeV) gamma-ray observations of PG 1553+113 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. A strong signal, ~10 standard deviations, is detected by HESS during the 2 years of observations (24.8 hours live time). The time-averaged energy spectrum, measured between 225 GeV to ~1.3 TeV, is characterized by a very steep power law (photon index of Gamma = (4.5 +- 0.3 {stat} +- 0.1 {syst}). The integral flux above 300 GeV is ~3.4% of the Crab Nebula flux and shows no evidence for any variations, on any time scale. 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. The redshift of PG 1553+113 is still unknown, as no absorption or emission lines were found.