High excitation emission line nebula associated with an ultra-luminous X-ray source at z = 0.027 in the AKARI North Ecliptic Pole Deep Field

¹ Instituto de Astronomía sede Ensenada, Universidad Nacional Autónoma de México, 22860 Ensenada, Mexico
e-mail: jadiazt@astrosen.unam.mx
² Physics Section, Faculty of Science and Engineering, Iwate University, 020-8551 Morioka, Japan
³ Leibniz Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Department of Astronomy, Kyoto University, 606-8502 Kyoto, Japan
⁵ Max-Planck-Institut für extraterrestrische Physik, 8046 Garching, Germany
⁶ National Tsinghua University, 30013 Hsingchu, Taiwan
⁷ Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, 10617 Taipei, Taiwan
⁸ PO Box 439027, San Ysidro, CA 92143-9027, USA

Received: 13 February 2017
Accepted: 2 June 2017

Abstract

Aims. We report our finding of a high excitation emission line nebula associated with an ultra-luminous X-ray source (ULX) at z= 0.027, which we found in our Chandra observation of the AKARI North Ecliptic Pole Deep Field.

Methods. We present a Chandra X-ray and Gran Telescopio Canarias (GTC) optical spectral analysis of the ULX blob. We investigated the nature of the emission line nebula using line ratio diagnostic diagrams and estimated the physical properties of the nebula with spectral energy distribution (SED) fitting.

Results. The optical spectrum of this ULX blob shows emission line ratios that are located on the borderlines between star formation and Seyfert regimes in [OIII]/Hβ-[OI]/Hα, [OIII]/Hβ-[SII]/Hα, and [OIII]/Hβ-[OIII]/[OII] diagnostic diagrams. These are in contrast with those of a nearby blob observed with the same slit, which occupies the HII regimes.

Conclusions. This result suggests that the energy input from the accretion power of the ULX, in addition to the star formation activity in the blob, significantly contribute to the ionization of the emission line nebula associated with the ULX, suggesting the existence of an accretion disk in the ULX emitting UV radiation or exerting shock waves.