NuSTAR observations of a heavily X-ray obscured AGN in the dwarf galaxy J144013+024744

Abstract

We present a multi-wavelength analysis of the dwarf Seyfert-2 galaxy J$144013+024744$, a candidate obscured active galactic nucleus (AGN) thought to be powered by an intermediate-mass black hole (IMBH, $M_{\bullet }\approx 10^{4-6}{M}_{\odot }$) of mass $M_{\bullet }\sim 10^{5.2}{M}_{\odot }$. To study its X-ray properties, we targeted J$144013+024744$ with NuSTAR for $\approx 100$ ks. The X-ray spectrum was fitted with absorbed power law, Pexmon and a physical model (RXTorus). A Bayesian X-ray analysis was performed to estimate the posteriors. The phenomenological and the physical models suggest the AGN to be heavily obscured by a column density of $N_{\mathrm{H}}=(3.4-7.0)\times 10^{23}$ cm${}^{-2}$. In particular, the RXTorus model with a sub-solar metallicity suggests the obscuring column to be almost Compton-thick. We compared the $2-10$ keV intrinsic X-ray luminosity with the inferred X-ray luminosities based on empirical scaling relations for unobscured AGNs using $L_{[\mathrm{OIV}](25.89\mu \mathrm{m})}$, $L_{[\mathrm{OIII}](5007\mathrm{angstrom})}$, and $L_{6\mu \mathrm{m}}$ and found that the high-excitation $[\mathrm{OIV}]$ line provides a better estimate of the intrinsic $2-10$ keV X-ray luminosity ($L_{2-10}^{\mathrm{int}}\sim 10^{41.41}{\mathrm{ergs}}^{-1}$). Our results suggest that J$144013+024744$ is the first type-2 dwarf galaxy that shows X-ray spectroscopic evidence for obscuration. The column density that we estimated is among the highest measured to date for IMBH-powered AGNs, implying that a typical AGN torus geometry might extend to the low-mass end. This work has implications for constraining the black hole occupation fraction in dwarf galaxies using X-ray observations.

Accepted for Publication in ApJ

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