NuSTAR Observations of a Heavily X-Ray-obscured AGN in the Dwarf Galaxy J144013+024744

Abstract

We present a multiwavelength analysis of the dwarf Seyfert 2 galaxy J144013+024744, a candidate obscured active galactic nucleus (AGN) thought to be powered by an intermediate-mass black hole (IMBH, M <SUB>•</SUB> ≈ 10<SUP>4</SUP>-10<SUP>6</SUP> M <SUB>⊙</SUB>) of mass M <SUB>•</SUB> ~ 10<SUP>5.2</SUP> M <SUB>⊙</SUB>. To study its X-ray properties, we targeted J144013+024744 with NuSTAR for ≈100 ks. The X-ray spectrum was fitted with an 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 <SUB>H</SUB> = (3.4-7.0) × 10<SUP>23</SUP> cm<SUP>-2</SUP>. In particular, the RXTorus model with a subsolar 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 <SUB>[OIV] 25.89 μm</SUB>, L <SUB>[OIII] λ5007</SUB>, and L <SUB>6μm</SUB> and found that the high-excitation [OIV] line provides a better estimate of the intrinsic 2-10 keV X-ray luminosity ( ${L}_{2\mbox{--}10}^{\mathrm{int}}\sim {10}^{41.41}$ erg s<SUP>-1</SUP>). Our results suggest that J144013+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 BH occupation fraction in dwarf galaxies using X-ray observations.