A Long Hard-X-Ray Look at the Dual Active Galactic Nuclei of M51 with NuSTAR

Abstract

We present a broadband X-ray spectral analysis of the M51 system, including the dual active galactic nuclei (AGN) and several off-nuclear point sources. Using a deep observation by NuSTAR, new highresolution coverage of M51b by Chandra, and the latest X-ray torus models, we measure the intrinsic X-ray luminosities of the AGN in these galaxies. The AGN of M51a is found to be Compton thick, and both AGN have very low accretion rates (λ Edd < 10 −4 ). The latter is surprising considering that the galaxies of M51 are in the process of merging, which is generally predicted to enhance nuclear activity. We find that the covering factor of the obscuring material in M51a is 0.26 ± 0.03, consistent with the local AGN obscured fraction at L X ∼ 10 40 erg s −1 . The substantial obscuring column does not support theories that the torus, presumed responsible for the obscuration, disappears at these low accretion luminosities. However, the obscuration may have resulted from the gas infall driven by the merger rather than the accretion process. We report on several extra-nuclear sources with L X > 10 39 erg s −1 and find that a spectral turnover is present below 10 keV in most such sources, in line with recent results on ultraluminous X-ray sources.

NuSTAR observations of four nearby X-ray faint AGNs: low luminosity or heavy obscuration?

“…The bestfitting photon index measured is = 2.12 +0.61 −0.23 , and the APEC thermal component indicated a plasma temperature of kT = 0.95 +0.08 −0.16 keV. In general, our results are in agreement with those found by Brightman et al (2018) and Rampadarath et al (2018), who fitted the same data using slightly different models. We show the broad-band X-ray spectra of NGC 5195 in Fig.…”

“…A variable ULX (ULX-7), identified in the northern spiral arm of NGC 5194, was recently identified to be powered by neutron star (Brightman et al 2020;Rodríguez Castillo et al 2020). We observed NGC 5195 as part of the M51 system with NuSTAR in 2017 (Brightman et al 2018). Based on Fig.…”

Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

“…The galaxies of M51, which consist of the main spiral galaxy M51a/NGC5194 and its smaller companion M51b/ NGC5195, are a host to a multitude of X-ray sources (e.g., Brightman et al 2018a). These include two active galactic nuclei (AGNs) powered by accreting supermassive black holes (SMBHs), one at the center of each of the galaxies; several ultraluminous X-ray sources (ULXs); and transient sources, such as the type IIb supernova SN2011dh.…”

Discovery of a 2.8 s Pulsar in a 2 Day Orbit High-mass X-Ray Binary Powering the Ultraluminous X-Ray Source ULX-7 in M51

“…Among these galaxies is Messier 51a (M51a), also known as NGC 5194 or the Whirlpool Galaxy, a face-on spiral interacting with the dwarf galaxy M51b (NGC 5195). It is located at a distance of ( )  8.58 0.10 Mpc (McQuinn et al 2016) and hosts a large number of X-ray sources, including nine ULXs (Terashima & Wilson 2004;Brightman et al 2018). Based on optical studies, M51a was classified as a Seyfert 2 galaxy (Stauffer 1982).…”