Simultaneous multi-wavelength campaign on PKS 2005-489 in a high state

TL;DR

This paper reports a comprehensive multi-wavelength observational campaign of PKS 2005-489 during a high flux state, revealing complex variability patterns and challenging simple emission models.

Contribution

It provides detailed simultaneous observations across multiple bands and critically evaluates the applicability of a one-zone SSC model to the source's variability.

Findings

X-ray flux was ~50 times higher in 2009 compared to 2004

The VHE spectrum connects smoothly with the GeV spectrum

One-zone SSC model cannot fully explain the variability patterns

Abstract

The high-frequency peaked BL Lac object PKS 2005-489 was the target of a multi-wavelength campaign with simultaneous observations in the TeV gamma-ray (H.E.S.S.), GeV gamma-ray (Fermi/LAT), X-ray (RXTE, Swift), UV (Swift) and optical (ATOM, Swift) bands. This campaign was carried out during a high flux state in the synchrotron regime. The flux in the optical and X-ray bands reached the level of the historical maxima. The hard GeV spectrum observed with Fermi/LAT connects well to the very high energy (VHE, E>100GeV) spectrum measured with H.E.S.S. with a peak energy between ~5 and 500 GeV. Compared to observations with contemporaneous coverage in the VHE and X-ray bands in 2004, the X-ray flux was ~50 times higher during the 2009 campaign while the TeV gamma-ray flux shows marginal variation over the years. The spectral energy distribution during this multi-wavelength campaign was fit by a one zone synchrotron self-Compton model with a well determined cutoff in X-rays. The parameters of a one zone SSC model are inconsistent with variability time scales. The variability behaviour over years with the large changes in synchrotron emission and small changes in the inverse Compton emission does not warrant an interpretation within a one-zone SSC model despite an apparently satisfying fit to the broadband data in 2009.