Further Observations of the Intermediate Mass Black Hole Candidate ESO 243-49 HLX-1

The brightest ultra-luminous X-ray source (ULX) currently known (HLX-1) was discovered in the second XMM-Newton Serendipitous Source Catalogue [2XMM; 1] in the outskirts of the edge-on spiral galaxy ESO 243-49 [2]. Its 0.2 -10 keV unabsorbed X-ray luminosity, assuming the galaxy distance (95 Mpc), exceeded 1.1 × 10 42 ergs s -1 . Follow-up observations have revealed large scale luminosity and spectral variability in X-rays, in a way similar to Galactic black hole binary systems [3]. The extreme luminosities of ULXs, if the emission is isotropic and below the Eddington limit, imply the presence of an accreting black hole with a mass of ∼10 2 -10 5 M ⊙ . However, the existence of such intermediate mass black holes (IMBHs) is in dispute. Both super-Eddington accretion and beaming of the X-ray emission could account for X-ray luminosities up to ∼ 10 40 ergs s -1 for stellar mass black holes (10 -50 M ⊙ ), but would require extreme tuning to explain an X-ray luminosity of 10 42 ergs s -1 . Hence, HLX-1 is an excellent candidate IMBH [2]. The existence of IMBHs has profound implications for massive star evolution, and the formation and evolution of star clusters and galaxies in general [e.g. 4]. In this paper we present the results of new and archival multi-wavelength observations of HLX-1 in an attempt to confirm an association with ESO 243-49 and determine in particular where it lies within the galaxy (e.g. in a star cluster, star forming region, globular cluster etc.).

We obtained a 1 ks observation of HLX-1 under the Director’s Discretionary Time (DDT) program with the HRC-I camera onboard Chandra on 2009 July 4 (ObsID: 10919). No source was detected within the XMM-Newton error circle of HLX-1, indicating the count rate must have dropped down to < 0.006 cts s -1 , compared to the expected 0.03 cts s -1 based on the most recent XMM-Newton flux and spectrum measurements [2]. Swift XRT monitoring observations of HLX-1 confirmed the drop in flux and found that one month later the flux increased significantly [3]. Following this re-brightening we obtained a second deeper DDT observation of 10 ks with the HRC-I on 2009 August 17 (ObsID: 11803). A total of 11 sources were detected, including a source consistent with the position of HLX-1 with a net count rate of 0.098 ± 0.003 cts s -1 . After applying astrometry correction by cross-matching detected sources against the 2MASS catalogue [see 5, for a full discussion], a final position of RA = 01h 10m 28.29s, Dec = -46 • 04’ 22.3" was obtained for HLX-1, with a 95% error of 0.3".

The Swift UVOT observed the field of ESO 243-49 on 2009 August 5, 6, 16, 18, 19 and 20 for a total exposure of 38 ks. Observations were performed in the uvw2 (∼160 -250 nm) filter only. At the location of the core of ESO 243-49 there is an extended object (Figure 1), with some hints of an elongated emission towards the position of HLX-1. No point source is observed above the flux level of the galaxy at the Chandra position of HLX-1, although given the spatial resolution of 2.9" FWHM in the uvw2 band 1 we cannot rule out a point source unresolved from the nuclear emission. We estimate a 3σ upper-limit of 20.3 mag at this position [see 5, for details].

The field of HLX-1 was observed by GALEX as part of the deep survey on 2004 September 27 for ∼13 ks in the near-UV (NUV, ∼180 -280 nm) and ∼8 ks in the far-UV (FUV, ∼150 -200 nm). A clear extension towards the location of HLX-1 from the nucleus of ESO 243-49 can be seen in both images (Figure 2), with the dominant emission occurring in the FUV. No point source was detected coincident with the position of HLX-1 in either band, although again an unresolved point source cannot be ruled out due to the spatial resolution [4 -6" FWHM; 6]. 3σ upper limits of 20.4 mag and 21.4 mag were determined in the NUV and FUV respectively [see 5, for details].

The field of ESO 243-49 was observed as part of the Phoenix Deep Survey (PDS) with the Australia Telescope Compact Array at 1.4 GHz [7]. The final mosaic and a series of source catalogues are in the public domain 2 and we have used these data products to search for radio emission from HLX-1. A source is clearly detected coincident with the nucleus of ESO 243-49, but no source is present at the position of HLX-1. The flux density at the location of HLX-1 is ∼5 µJy. This is below the 3σ local rms noise level of 45 µJy, which we thus adopt as the flux density upper limit for HLX-1. We fitted a Gaussian to the radio source at the nucleus of ESO 243-49, yielding an integrated flux density of 0.15 ± 0.03 mJy. The derived position is consistent with the PDS position of RA = 01h 10m 27.69s, Dec = -46 • 04’ 27.8" with uncertainties of 0.2" and 0.5" respectively. Given the accurate astrometry in both radio and X-ray it is extremely unlikely that the 0.15 mJy source detected in the nucleus of ESO 243-49 is associated with HLX-1. The spatial resolution of the PDS observations is high enough such that any significant

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