Hard X-ray luminosity function of tidal disruption events: First results from the MAXI extragalactic survey

Abstract

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 unbiased sample observed by the Monitor of All-sky X-ray Image (MAXI). On the basis of the light curves characterized by a power-law decay with an index of −5/3, a systematic search using the MAXI data in the first 37 months detected four TDEs, all of which have been found in the literature. To formulate the TDE XLF, we consider the mass function of SMBHs, that of disrupted stars, the specific TDE rate as a function of SMBH mass, and the fraction of TDEs with relativistic jets. We perform an unbinned maximum likelihood fit to the MAXI TDE list and check the consistency with the observed TDE rate in the ROSAT all sky survey. The results suggest that the intrinsic fraction of the jet-accompanying events is 0.0007-34%. We confirm that at z < ∼ 1.5 the contamination by TDEs to the hard X-ray luminosity functions of active galactic nuclei is not significant and hence that their contribution to the growth of SMBHs is negligible at the redshifts.

A Comparison of the X-Ray Emission from Tidal Disruption Events with those of Active Galactic Nuclei

“…However, it would be beneficial if we could constrain observationally the LF of TDEs, as this would provide us with a way to experimentally constrain the contribution of TDEs to the growth of BHs in the local universe, while providing a useful tool to predict the number of possible events one could detect in future X-ray surveys using instruments such as eROSITA 11 . Recently, Kawamuro et al (2016) made the first attempt to characterise observationally the hard (4-10 keV) X-ray LF of TDEs using a flux limited sample of TDE events detected using 37 months of MAXI data. From their study, they find TDEs do not contribute significantly to the hard X-ray LF of AGN for z < 1.5, and thus their contribution to the BH growth at these redshifts is negligible.…”

“…From their study, they find TDEs do not contribute significantly to the hard X-ray LF of AGN for z < 1.5, and thus their contribution to the BH growth at these redshifts is negligible. Unfortunately, due to the limited number of TDEs that emit significantly in the hard X-ray energy band, Kawamuro et al (2016) likely miss a large number of TDE candidates that could be used to derive the X-ray …”

Evidence for a TDE origin of the radio transient Cygnus A-2

“…Alternatively, it has previously been shown in the literature that some thermal TDE's also have a non-thermal emission component in the 2-10 keV band, as in the TDE XMMSL1 J0740-85 (Kawamuro et al 2016;Saxton et al 2017). This power-law component is thought to be from the hot corona which Comptonizes the thermal optically thick disk emission.…”

Discovery of the New X-Ray Transient MAXI J1807+132: A Candidate of a Neutron Star Low-mass X-Ray Binary

“…The source position estimated with the MAXI/GSC turned out to be consistent with that of 2MAXIt J1807+132, which is listed in the MAXI/GSC transient source catalog (Kawamuro et al 2016), based on an X-ray flaring event detected in 2011 May.…”

“…Kawamuro et al (2016) concluded that 2MAXIt J1807+132 is unlikely a TDE, because it has exhibited multiple enhancements (though with lower significances, at ∼2σ levels) from 2009 to 2013. Identifying the present source MAXI J1807+132 with 2MAXIt J1807+132, the TDE interpretation becomes even less likely, because the interval of the two strongest flaring events is much shorter than those predicted for TDEs (typically ∼10 4 to ∼10 5 years; e.g., Kawamuro et al 2016). To search for other brightening episodes of MAXI J1807+132 (=2MAXIt J1807+132), we analyzed the entire MAXI/GSC data of this sky region using the ondemand process system 3 .…”