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تسجيل مستخدم جديدFirst tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z<0.6
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We present the first sample of TDEs discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected on the 0<l<180 hemisphere by eROSITA during its second scan of the sky (10 June-14 Dec. 2020) and confirmed as TDEs by our optical follow-up observations. The most distant event occurred at z=0.581. One TDE continued to brighten after its discovery for at least another 6 months. The X-ray spectra can be described by emission from a standard accretion disk with kT between 0.05 and 0.5 keV, consistent with near-critical accretion onto black holes of a few 10^3 to 10^8 Msun, although super-critical accretion is possibly taking place. In 2 TDEs, a spectral hardening is observed 6 months after the discovery, possibly indicating the formation of an accretion disk corona. 4 TDEs show an optical brightening concurring with or preceding the X-ray outburst. All 13 TDEs are optically faint, with Lg/Lx<0.1 in most cases, where Lg and Lx are the intrinsic g-band and 0.2-6 keV luminosities, respectively. This sample is thus drastically different from TDEs selected at optical wavelengths. We have constructed a TDE X-ray luminosity function in the 10^42.5-10^45 erg/s range. The TDE volume rate decreases with increasing X-ray luminosity approximately as a power law with alpha=-0.6+/-0.2. This is similar to a trend observed for optically selected TDEs. The total rate at z<0.6 is (1.1+/-0.5)10^-5 TDEs/galaxy/year, an order of magnitude lower than previously estimated from optical studies. This might indicate that X-ray bright events constitute a minority of all TDEs, which would provide support to models predicting a strong dependence on the viewing angle. Our current TDE detection threshold can be lowered by a factor of ~2, which should make it possible to find ~700 TDEs by the end of the SRG survey over the entire sky.
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We derive the first hard X-ray luminosity function (XLF) of stellar tidal disruption events (TDEs) by supermassive black holes (SMBHs), which gives an occurrence rate of TDEs per unit volume as a function of peak luminosity and redshift, utilizing an
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We report the discovery of X-ray emission from CFHQS J142952+544717, the most distant known radio-loud quasar at z=6.18, on Dec. 10--11, 2019 with the eROSITA telescope on board the SRG satellite during its ongoing all-sky survey. The object was iden
tified by cross-matching an intermediate SRG/eROSITA source catalog with the Pan-STARRS1 distant quasar sample at 5.6 < z < 6.7. The measured flux $sim 8 times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 0.3--2 keV energy band corresponds to an X-ray luminosity of $2.6^{+1.7}_{-1.0}times 10^{46}$ erg s$^{-1}$ in the 2--10 keV rest-frame energy band, which renders CFHQS J142952+544717 the most X-ray luminous quasar ever observed at z > 6. Combining our X-ray measurements with archival and new photometric measurements in other wavebands (radio to optical), we estimate the bolometric luminosity of this quasar at $sim (2$--$3) times 10^{47}$ erg s$^{-1}$. Assuming Eddington limited accretion and isotropic emission, we infer a lower limit on the mass of the supermassive black hole of $sim 2times 10^9 M_odot$. The most salient feature of CFHQS J142952+544717 is its X-ray brightness relative to the optical/UV emission. We argue that it may be linked to its radio-loudness (although the object is not a blazar according to its radio properties), specifically to a contribution of inverse Compton scattering of cosmic microwave background photons off relativistic electrons in the jets. If so, CFHQS J142952+544717 might be the tip of the iceberg of high-z quasars with enhanced X-ray emission, and SRG/eROSITA may find many more such objects during its 4 year all-sky survey.
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