PKS 2005-489 at VHE: Four Years of Monitoring with HESS and Simultaneous Multi-wavelength Observations

TL;DR

This study presents four years of VHE gamma-ray observations of PKS 2005-489 with HESS, combined with simultaneous X-ray data, revealing flux variability, spectral changes, and the emergence of a new emission component.

Contribution

First comprehensive multi-year VHE and X-ray monitoring of PKS 2005-489, identifying spectral evolution and a new emission component in the source.

Findings

VHE flux varies weakly over days to years.

X-ray flux varies by over an order of magnitude.

Spectral changes indicate a new emission component.

Abstract

VHE observations of PKS 2005-489 were made with HESS from 2004 through 2007, together with three simultaneous multi-wavelength campaigns performed with XMM-Newton and RXTE in 2004 and 2005. A strong VHE signal, ~17sigma total, is detected during the four years of HESS observations (90.3 hrs live time). The integral flux above the average analysis threshold of 400 GeV is ~3% of the Crab and varies weakly on time-scales from days to years. At X-ray energies the flux is observed to vary by more than an order of magnitude between 2004 and 2005. Strong changes in the X-ray spectrum (DeltaGamma~0.7) are also observed, which appear to be mirrored in the VHE band. The SED of PKS 2005-489, constructed for the first time with contemporaneous data on both humps, shows significant evolution. The large flux variations in the X-ray band, coupled with weak or no variations in the VHE band and a similar spectral behavior, suggest the emergence of a new, separate, harder emission component in September 2005.