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NuSTAR observations of the ultraluminous X-ray source M33 X-8: a black hole in a very high state?
Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Natl Res Univ Higher Sch Econ, Myasnitskaya Ul 20, Moscow 101000, Russia..
Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Tuorla Observ, Dept Phys & Astron, FI-20014 Turku, Finland..
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Tuorla Observ, Dept Phys & Astron, FI-20014 Turku, Finland.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..ORCID iD: 0000-0002-0983-0049
2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 2, p. 2357-2364Article in journal (Refereed) Published
Abstract [en]

The closest known ultraluminous X-ray source (ULX), M33 X-8, has been recently observed with NuSTAR during its Extragalactic Legacy program, which includes a hard X-ray survey of the M33 galaxy. We present results of two long observations of M33 taken in 2017 March and July, with M33 X-8 in the field of view. The source demonstrates a nearly constant flux during the observations, and its 3-20 keV spectrum can be well described by two distinct components: a standard accretion disc with a temperature of similar to 1 keV at the inner radius and a power law with a photon index Gamma approximate to 3, which is significantly detected up to 20 keV. There is also an indication of a high-energy cut-off in the spectrum, corresponding to a temperature of the Comptonizing medium of greater than or similar to 10 keV. The broad-band spectral properties of M33 X-8 resemble black hole X-ray binaries in their very high states, suggesting that M33 X-8 is a black hole accreting at a nearly Eddington rate, in contrast to super-Eddington accretion believed to take place in more luminous ULXs.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS , 2018. Vol. 480, no 2, p. 2357-2364
Keywords [en]
X-rays: binaries, X-rays: general, X-rays: individual: (M33 X-8)
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-239819DOI: 10.1093/mnras/sty1995ISI: 000449614800072Scopus ID: 2-s2.0-85055116028OAI: oai:DiVA.org:kth-239819DiVA, id: diva2:1272271
Note

QC 20181217

Available from: 2018-12-18 Created: 2018-12-18 Last updated: 2018-12-18Bibliographically approved

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Poutanen, Juri

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