Person: Fialkov, Anastasia
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Fialkov
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Anastasia
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Fialkov, Anastasia
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Publication Constraining the CMB optical depth through the dispersion measure of cosmological radio transients(IOP Publishing, 2016) Fialkov, Anastasia; Loeb, AbrahamThe dispersion measure of extragalactic radio transients, such as of recently discovered Fast Radio Burst FRB150418, can be used to measure the column density of free electrons in the intergalactic medium. The same electrons also scatter the Cosmic Microwave Background (CMB) photons, affecting precision measurements of cosmological parameters. We explore the connection between the dispersion measure of radio transients existing during the Epoch of Reionization (EoR) and the total optical depth for the CMB, τCMB, showing that the existence of such transients would provide a new sensitive probe of τCMB. As an example, we consider the population of FRBs. Assuming they exist during the EoR, we show that: (i) such sources can probe the reionization history by measuring τCMB to sub-percent accuracy, and (ii) they can be detected with high significance by an instrument such as the Square Kilometer Array.Publication Precise Measurement of the Reionization Optical Depth from The Global 21-cm Signal Accounting for Cosmic Heating(American Astronomical Society, 2016) Fialkov, Anastasia; Loeb, AbrahamAs a result of our limited data on reionization, the total optical depth for electron scattering, τ, limits precision measurements of cosmological parameters from the Cosmic Microwave Background (CMB). It was recently shown that the predicted 21-cm signal of neutral hydrogen contains enough information to reconstruct τ with sub-percent accuracy, assuming that the neutral gas was much hotter than the CMB throughout the entire epoch of reionization. Here we relax this assumption and use the global 21-cm signal alone to extract τ for realistic X-ray heating scenarios. We test our model-independent approach using mock data for a wide range of ionization and heating histories and show that an accurate measurement of the reionization optical depth at a sub-percent level is possible in most of the considered scenarios even when heating is not saturated during the epoch of reionization, assuming that the foregrounds are mitigated. However, we find that in cases where heating sources had hard X-ray spectra and their luminosity was close to or lower than what is predicted based on low-redshift observations, the global 21-cm signal alone is not a good tracer of the reionization history.Publication Jetted tidal disruptions of stars as a flag of intermediate mass black holes at high redshifts(Oxford University Press (OUP), 2017-07-14) Fialkov, Anastasia; Loeb, AbrahamTidal disruption events (TDEs) of stars by single or binary supermassive black holes (SMBHs) brighten galactic nuclei and reveal a population of otherwise dormant black holes. Adopting event rates from the literature, we aim to establish general trends in the redshift evolution of the TDE number counts and their observable signals. We pay particular attention to two types of TDEs which are expected to be observable out to high redshifts, namely (i) jetted TDEs whose luminosity is boosted by relativistic beaming, and (ii) TDEs around binary black holes. We show that the brightest (jetted) TDEs are expected to be produced by massive black hole binaries if the occupancy of intermediate mass black holes (IMBHs) in low mass galaxies is high. The same binary population will also provide gravitational wave sources for eLISA. In addition, we find that the shape of the X-ray luminosity function of TDEs strongly depends on the occupancy of IMBHs and could be used to constrain scenarios of SMBH formation. Finally, we make predictions for the expected number of TDEs observed by future X-ray telescopes as a function of their sensitivity limits. We find that an instrument which is 50 times more sensitive than the Burst Alert Telescope (BAT) on board the Swift satellite is expected to trigger ~10 times more events than BAT with 50% of the events coming from z>2. Because of their long decay times, high-redshift TDEs can be mistaken for fixed point sources in deep field surveys such as the 4Ms survey with the Chandra X-ray Observatory and targeted observations of the same deep field with year-long intervals could reveal TDEs. If the occupation fraction of IMBHs is high, 6-20 TDEs are expected in each deep field observed by a telescope 50 times more sensitive than Chandra.