Get access

Optical counterpart of HLX-1 during the 2010 outburst

Authors

  • Roberto Soria,

    Corresponding author
    1. International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
      E-mail: roberto.soria@icrar.org (RS); pahakala@utu.fi (PJH); ghau@eso.org (GKTH); jgladsto@ualberta.ca (JCG); akong@phys.nthu.edu.tw (AKHK)
    Search for more papers by this author
  • Pasi J. Hakala,

    Corresponding author
    1. Finnish Centre for Astronomy with ESO (FINCA), Väisäläntie 20, University of Turku, FIN-21500 Piikkiö, Finland
      E-mail: roberto.soria@icrar.org (RS); pahakala@utu.fi (PJH); ghau@eso.org (GKTH); jgladsto@ualberta.ca (JCG); akong@phys.nthu.edu.tw (AKHK)
    Search for more papers by this author
  • George K. T. Hau,

    Corresponding author
    1. European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
      E-mail: roberto.soria@icrar.org (RS); pahakala@utu.fi (PJH); ghau@eso.org (GKTH); jgladsto@ualberta.ca (JCG); akong@phys.nthu.edu.tw (AKHK)
    Search for more papers by this author
  • Jeanette C. Gladstone,

    Corresponding author
    1. Department of Physics, University of Alberta, Edmonton, Alberta T6G 2C7, Canada
      E-mail: roberto.soria@icrar.org (RS); pahakala@utu.fi (PJH); ghau@eso.org (GKTH); jgladsto@ualberta.ca (JCG); akong@phys.nthu.edu.tw (AKHK)
    Search for more papers by this author
  • Albert K. H. Kong

    Corresponding author
    1. Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
      E-mail: roberto.soria@icrar.org (RS); pahakala@utu.fi (PJH); ghau@eso.org (GKTH); jgladsto@ualberta.ca (JCG); akong@phys.nthu.edu.tw (AKHK)
    Search for more papers by this author

E-mail: roberto.soria@icrar.org (RS); pahakala@utu.fi (PJH); ghau@eso.org (GKTH); jgladsto@ualberta.ca (JCG); akong@phys.nthu.edu.tw (AKHK)

ABSTRACT

We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO 243−49. We used a set of Very Large Telescope imaging observations from 2010 November, integrated by Swift X-ray data from the same epoch. We measured standard Vega brightnesses U= 23.89 ± 0.18 mag, B= 25.19 ± 0.30 mag, V= 24.79 ± 0.34 mag and R= 24.71 ± 0.40 mag. Therefore, the source was ≈1 mag fainter in each band than in a set of Hubble Space Telescope images taken a couple of months earlier, when the X-ray flux was a factor of 2 higher. We conclude that during the 2010 September observations, the optical counterpart was dominated by emission from an irradiated disc (which responds to the varying X-ray luminosity), rather than by a star cluster around the black hole (which would not change). We modelled the Comptonized, irradiated X-ray spectrum of the disc, and found that the optical luminosity and colours in the 2010 November data are still consistent with emission from the irradiated disc, with a characteristic outer radius rout≈ 2800rin∼ 1013 cm and a reprocessing fraction ≈2 × 10−3. The optical colours are also consistent with a stellar population with age ≲6 Myr (at solar metallicity) and mass ≈104 M; this is only an upper limit to the mass, if there is also a significant contribution from an irradiated disc. We strongly rule out the presence of a young superstar cluster, which would be too bright. An old globular cluster might be associated with HLX-1, as long as its mass ≲2 × 106 M for an age of 10 Gyr, but it cannot significantly contribute to the observed very blue and variable optical/ultraviolet emission.

Get access to the full text of this article

Ancillary