Swift Observations of the Extremely Luminous Quasar SMSS J114447.77-430859.3

ATel #15450; J. M. Miller (Univ. of Michigan), A. C. Fabian (Univ. of Cambridge)
on 21 Jun 2022; 15:16 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

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Recent observations with the SkyMapper Southern Survey (SMSS) identified SMSS J114447.77-430859.3 as the optically brightest unbeamed quasar at z > 0.4, and the most luminous known quasar of the last nine billion years of cosmic history (Onken et al. 2022). For a redshift of z = 0.83, optical data give a bolometric luminosity of L_bol = 4.7 +/- 1.0 E+47 erg/s, and suggest an Eddington ratio of 1.4 (mildly super-Eddington).

SMSS J1144 was not detected in the ROSAT All-Sky Survey, and was not previously targeted with any modern imaging X-ray telescope. We therefore requested an observation with the Neil Gehrels Swift Observatory. A short observation was obtained on 21 June 2022, starting at 01:04:02 UTC.

An X-ray source is clearly detected that is coincident with SMSS J1144. Source events were extracted in a circular region with a radius of 24 arc seconds. Background events were extracted in a nearby region of the same size. A total of 39 source counts were recorded in 1068 seconds of exposure, compared to 3 background counts. Assuming an unabsorbed power-law spectrum with a power-law index of Gamma = 2.0, the corresponding count rate of 0.034 +/- 0.006 c/s gives a flux of F_X = 1.1 +/- 0.2 E-12 erg/cm^2/s (0.3-10.0 keV, observed frame). At the host redshift of z=0.83, this signals an luminosity of L_X = 3.9 +/- 0.7 E+45 erg/s.

An implied X-ray to bolometric luminosity ratio of L_X / L_bol ~ 0.008 is broadly consistent with other massive black holes accreting close to (or above) their Eddington limit. In this accretion regime, the hard X-ray corona in massive black holes appears to be largely quenched, similar to stellar-mass black holes in the "high/soft" state. Future observations with Chandra, XMM-Newton, and NuSTAR can determine the broadband X-ray spectrum of SMSS J1144.

The UVOT UVM2 filter has a central wavelength of 2246 A, and the smallest red leak of those aboard Swift. It therefore functions as the best measure of UV flux. A strong source is detected at the position of SMSS J1144. We measure a flux density of F_uvm2_nu = 1.63 +/- 0.03 E-14 erg/cm^2/s/A, a flux of F_uvm2 = 8.1 +/- 0.2 E-12 erg/cm^2/s, and a luminosity of L_uvm2= 2.9 +/- 0.1 E+46 erg/s.

We thank Brad Cenko, Jamie Kennea, Caryl Gronwall, and the Swift team for making this observation.

References

Onken, C., et al., 2022, PASA, submitted, arxiv:2206.04204