Hard X-Ray broadband spectroscopy of Mrk 876: characterizing its spectrum

Ever since the launch of the NuSTAR mission, the hard X-ray range is being covered to an unprecedented sensitivity. This range encodes the reflection features arising from active galactic nuclei (AGNs). Especially, the reflection of the primary radiation off the accretion disc carries the features of the manifestation of General Relativity described by the Kerr metric due to rotating supermassive black holes (SMBHs). We show the results of the broadband analyses of Mrk 876. The spectra exhibit the signature of a Compton hump at energies above 10 keV and a broadened and skewed excess at energies ~6 keV. We establish this spectral excess to be statistically significant at 99.71 per cent (~3σ) that is the post-trail probability through Monte Carlo simulations. Based on the spectral fit results and the significance of spectral features, the relativistic reflection model is favoured over the distant reflection scenario. The excess at ~6 keV has a complex shape that we try to recover along with the Compton hump through a self-consistent X-ray reflection model. This allows inferring an upper limit to the black hole spin of a ≤ 0.85, while the inclination angle of the accretion disc results in i = 32.84$^{\circ }{}^{+12.22}_{-8.99}$, which is in agreement within the errors with a previous independent measurement (i = 15.4$^{\circ }{}^{+12.1}_{-6.8}$). While most spin measurements are biased towards high spin values, the black hole mass of Mrk 876 (2.4$\times 10^{8}\, \mbox{M}_\odot \le$ M_SMBH $\le ~1.3 \times 10^{9}\, \mbox{M}_\odot$) lies in a range where moderately spinning SMBHs are expected. Moreover, the analyses of 12 Chandra observations reveal for the first time X-ray variability of Mrk 876 with an amplitude of 40 per cent.