NuSTAR and Swift observations of SMSS J114447.770-430859.3

ATel #15510; E. Kammoun (IRAP, Toulouse), J. M. Miller (Univ. of Michigan), A. C. Fabian (Univ. of Cambridge)
on 11 Jul 2022; 09:10 UT
Credential Certification: Elias Kammoun (ekammoun@irap.omp.eu )

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SMSS J114447.77-430859.3 (J1144) has been identified as the most luminous known non-jetted quasar in the last 9 Gyr of cosmic history (Onken et al. 2022), thanks to observations with the SkyMapper Southern Survey (SMSS). For a spectroscopic redshift of z = 0.83, optical data give a mass estimate of log (M_BH/M_sun) = 9.4 +/- 0.5 and a bolometric luminosity of L_bol = 4.7 +/- 1.0 E+47 erg/s. This suggests an Eddington ratio of 1.4 (mildly super-Eddington). J1144 was not detected with the ROSAT All-Sky Survey, and was not targeted by any of the modern X-ray observatories. A recent observation (ATel#15450, ATel#15452) with the Neil Gehrels Swift Observatory (Swift), allowed us to detect the source with an observed flux of F_obs = 1.5 +/- 0.5 E-12 erg/s/cm^2. We have recently observed the source with NuSTAR for a total of 60 ks starting 2022-06-28 20:35:13 UTC. The source is detected up to 30 keV in the observed frame (~55 keV, rest frame), with a net count rate of 0.023 count/s. No variability has been detected during the observation. We extracted the source spectrum using a 50 arc second (radius) region, and a separate background region of double this size. The spectra were re-binned to require a minimum signal-to-noise (S/N) ratio of 4. We fitted simultaneously the spectra obtained from each of the NuSTAR FPM detectors and the Swift XRT spectrum using an absorbed cutoff power law. In the XSPEC parlance the model can be written as follows: TBabs[1] * zTBabs[2] * zcutoffpl[3]. The first absorption component accounts for the Galactic absorption in our line of sight. The second component accounts for neutral absorption at the rest frame of the source. This results in a statistically acceptable fit of C-stat/dof = 126/125. The best fit equivalent hydrogen column density is 1.04_{-0.8}^{+1.1} E+22 cm^-2. The best-fit photon index, high-energy cutoff, and normalization are 1.6+/-0.3, 25_{-8}^{+24} keV, and 1.2_{-0.3}^{+0.6}E-3 photons/keV/cm^2/s, respectively. This is equivalent to an un-absorbed 2-10 keV luminosity of 4.9+/-0.6 E+45 erg/s. All uncertainties are reported at the 90% confidence range. We tested a model where we added a reflection component. However, this component was not required by the data. The NuSTAR observation confirmed the flux level of the source during the Swift observation that took place ~1 week earlier. A new Swift observation took place on 2022-07-04 05:36:00 (UTC), in which the source increased in flux by 50% which can be attributed to an increase in the intrinsic luminosity, reaching an F_obs = 2.36 +/- 0.76E-12 erg/s/cm^2. This is equivalent to an unabsorbed luminosity L = 8.5 +/- 2.1 erg/s in the 2-10 keV range. The soft X-ray spectra obtained by Swift show hints of absorption features, albeit with a low significance. Accounting for these features may alter the best-fit values of photon index and high-energy cutoff. However, a higher S/N spectrum in the soft X-rays is required in order to confirm the existence of such features and better constrain the spectral shape. We thank Fiona Harrison and the NuSTAR planning team for executing this observation. We also thank Swift for executing the ToO observations. References: Onken, C., et al., 2022, PASA, submitted, arxiv:2206.04204