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On The Ultra-Luminous X-Ray Source In ESO 243-49: HLX-1 (2XMM J011028.1−460421)


The X-ray source 2XMM J011028.1−460421 (a.k.a HLX-1) is the brightest known ultraluminous X-ray source in the sky (ULX; Feng & Soria 2011) and is suspected to be an intermediate-mass black hole (IMBH) candidate. With the recent discovery  by Wiersema et al. 2010 in the optical HLX-1 spectrum of an emission line consistent with Hα at the redshift of ESO 243-49 (z = 0.0223) irrevocably confirms its association with this galaxy at a distance of 95 Mpc. As it is located within the outskirts of the S0 galaxy ESO 243-49, approximately 0.8 kpc out of the plane and ~3.3 kpc away from the nucleus, HLX-1 can not be considered as the supermassive black hole (SMBH) that resides within the nucleus of its host galaxy. Yet, considering it has comparable X-ray luminosities to that of many modest SMBHs, such observations make it a rather interesting object in the night sky.

Fig. 1: Composite Far-UV/Near-UV/C/V/IH band image of ESO 243-49 taken by the Hubble Space Telescope, HLX-1 is centered within the white circle. (Credit: Farrell et al. 2009)

Most black holes found thus far are either comparable to stellar masses (e.g. 3 – 10) or belong to an extreme brand of SMBH with masses of billions of times the mass of our Sun. However, estimates of the mass of HLX-1, based on its X-ray spectrum, range from 500 to 100000 (Farrell et al. 2009; Davis et al. 2011; Servillat et al. 2011), placing HLX-1 into the IMBH classification. The mass of a black hole is often associated to the amount of energy is emitted from the accretion disc. Due to the massive gravitational potentials created by black holes, gases being accreted (falling) onto a black hole are heated. As X-rays are highly energetic (second only to γ-rays) the flux associated with an object will always be proportional to its mass. Hence, it is possible to measure the mass of an object such as a black hole through its X-ray emission.

2XMM J011028.1–460421, referred to hereafter as HLX-1, was discovered serendipitously by XMM-Newton on November 23rd, 2004 in the outskirts of the edge on spiral galaxy ESO 243-49, at a redshift of z = 0.0224 (Afonso et al. 2005) is classified as an ultra-luminous X-ray source, with a peak X-ray luminosity of  (0.2-10 keV) (Servillat et al. 2011).

With observed X-ray luminosities reaching above  , HLX-1 is super-Eddington if the black-hole’s mass is less than ~100000. Beaming effects (e.g. King 2008; Kording et al. 2002) have been proposed as viable mechanisms for producing the apparent super-Eddington luminosities seen from other ULXs. However, beaming is unlikely to explain HLX-1’s extreme luminosity due to the observed large-scale variability (which appears similar to that seen from Galactic stellar mass black hole binaries that are not viewed down the jet-axis) and the luminosity of the Hα line, which is an order of magnitude above that expected from reprocessing in the local absorbing material (Wiersema et al. 2010). Hence, we are dealing with no differing effects, where the variability likely confirms HLX-1 as a clear IMBH (Lasota et al. 2011).

What is also very interesting to note is that the X-ray luminosity of HLX-1 varies from ~10 – 400 times the Eddington limit of a 20 black hole, where mass estimates for HLX-1 from Eddington scaling, accretion disc continuum fitting, and jet flare luminosity all support a mass of 100000, placing it quite clearly within the IMBH classification: approximately one hundred thousand times as large as a stellar sized black hole but not yet massive enough to be considered a “super massive”.

A rather interesting point to make, which is something I have not always felt plausible, is that if ESO 243-49 contains a SMBH at its heart and if we were to fast forward a couple of billion years is that HLX-1 may merge with such a possible SMBH. Supporting evidence is supplied by many studies of HLX-1, using derived stellar ages to show that the age of HLX-1 inconsistent with globular cluster, instead implying HLX-1 could be nucleus of stripped dwarf galaxy accreted by ESO 243-49. In essence, the observation of a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very low-mass dwarf galaxy. The dwarf galaxy was then gravitationally bound to and swallowed by ESO 243-49. Hence, many millions or perhaps billions of years from now could see the merger of HLX-1 and the SMBH at the centre of ESO 243-49.

Journal References:

  • Davis, S.W. et al. (2010) The Cool Accretion Disk In ESO 243-49 HLX-1: Further Evidence Of An Intermediate-Mass Black HoleThe Astrophysical Journal, 734 (2): Article I.D. #111.
  • Lasota, J.P. et al. (2011) The Origin of Variability Of The Intermediate-Mass Black-Hole ULX System HLX-1 In ESO 243-49The Astrophysical Journal, 735 (2):  Article I.D. #89.
  • Servillat, M. et al. (2011) X-Ray Variability & Hardness Of ESO 243-49 HLX-1: Clear Evidence for Spectral State TransitionsThe Astrophysical Journal743 (1): Article I.D. #6.

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