NuSTAR observations of Mrk 766: distinguishing reflection from absorption

D. J. K. Buisson, M. L. Parker, E. Kara, R. V. Vasudevan, A. M. Lohfink, C. Pinto, A. C. Fabian, D. R. Ballantyne, S. E. Boggs, F. E. Christensen W. W. Craig, D. Farrah, C. J. Hailey, F. A. Harrison, C. Ricci, D. Stern, D. J. Walton, W. W. Zhang

Published 2018-07-31Version 1

We present two new NuSTAR observations of the narrow line Seyfert 1 (NLS1) galaxy Mrk 766 and give constraints on the two scenarios previously proposed to explain its spectrum and that of other NLS1s: relativistic reflection and partial covering. The NuSTAR spectra show a strong hard (>15 keV) X-ray excess, while simultaneous soft X-ray coverage of one of the observations provided by XMM-Newton constrains the ionised absorption in the source. The pure reflection model requires a black hole of high spin ($a>0.92$) viewed at a moderate inclination ($i=46^{+1}_{-4}$ degrees). The pure partial covering model requires extreme parameters: the cut-off of the primary continuum is very low ($22^{+7}_{-5}$ keV) in one observation and the intrinsic X-ray emission must provide a large fraction (75%) of the bolometric luminosity. Allowing a hybrid model with both partial covering and reflection provides more reasonable absorption parameters and relaxes the constraints on reflection parameters. The fractional variability reduces around the iron K band and at high energies including the Compton hump, suggesting that the reflected emission is less variable than the continuum.