NuSTAR unveils a Compton-thick Type 2 quasar in Mrk 34

TL;DR

NuSTAR observations reveal that Mrk 34 is a Compton-thick Type 2 quasar with high intrinsic luminosity, challenging previous classifications based on lower-energy X-ray data and establishing it as a key example of such objects.

Contribution

This study provides the first direct measurement of the Compton-thick column density in Mrk 34 using NuSTAR, confirming its status as a luminous, obscured quasar and highlighting the importance of hard X-ray observations.

Findings

Mrk 34 is confirmed as a Compton-thick quasar with N_H > 1.5 x 10^{24} cm^{-2}.

The intrinsic 2-10 keV luminosity is approximately 10^{44} erg/s, much higher than previous estimates.

X-ray data suggest the soft component is due to photoionization, not star-formation-related thermal emission.

Abstract

We present Nustar 3-40 keV observations of the optically selected Type 2 quasar (QSO2) SDSS J1034+6001 or Mrk 34. The high-quality hard X-ray spectrum and archival XMM-Newton data can be fitted self-consistently with a reflection-dominated continuum and strong Fe Kalpha fluorescence line with equivalent-width >1 keV. Prior X-ray spectral fitting below 10 keV showed the source to be consistent with being obscured by Compton-thin column densities of gas along the line-of-sight, despite evidence for much higher columns from multiwavelength data. NuSTAR now enables a direct measurement of this column, and shows that Nh lies in the Compton-thick (CT) regime. The new data also show a high intrinsic 2-10 keV luminosity of L_{2-10}~10^{44} erg/s, in contrast to previous low-energy X-ray measurements for which L_{2-10}<~10^{43} erg/s (i.e. X-ray selection below 10 keV does not pick up this source as an intrinsically luminous obscured quasar). Both the obscuring column and the intrinsic power are about an order of magnitude (or more) larger than inferred from pre-NuSTAR X-ray spectral fitting. Mrk34 is thus a 'gold standard' CT QSO2 and is the nearest non-merging system in this class, in contrast to the other local CT quasar NGC6240 which is currently undergoing a major merger coupled with strong star-formation. For typical X-ray bolometric correction factors, the accretion luminosity of Mrk34 is high enough to potentially power the total infrared luminosity. X-ray spectral fitting also shows that thermal emission related to star-formation is unlikely to drive the observed bright soft component below ~3 keV, favoring photionization instead.