68 research outputs found
Observational Signatures of Supermassive Black Hole Binaries
Despite solid theoretical and observational grounds for the pairing of
supermassive black holes (SMBHs) after galaxy mergers, definitive evidence for
the existence of close (sub-parsec) separation SMBH binaries (SMBHBs)
approaching merger is yet to be found. This chapter reviews techniques aimed at
discovering such SMBHBs in galactic nuclei. We motivate the search with a brief
overview of SMBHB formation and evolution, and the gaps in our present-day
theoretical understanding. We then present existing observational evidence for
SMBHBs and discuss ongoing efforts to provide definitive evidence for a
population at sub-parsec orbital separations, where many of the aforementioned
theoretical gaps lie. We conclude with future prospects for discovery with
electromagnetic (primarily time-domain) surveys, high-resolution imaging
experiments, and low-frequency gravitational-wave detectors.Comment: To appear in Chapter 5 in the book Black Holes in the Era of
Gravitational Wave Astronomy, ed. Arca Sedda, Bortolas, Spera, pub. Elsevier.
All authors equally contributed to the Chapter writin
Correlation between optical and UV variability of a large sample of quasars
The variability of quasars across multiple wavelengths is a useful probe of physical conditions in active galactic nuclei. In particular, variable accretion rates, instabilities, and reverberation effects in the accretion disc of a supermassive black hole are expected to produce correlated flux variations in ultraviolet (UV) and optical bands. Recent work has further argued that binary quasars should exhibit strongly correlated UV and optical periodicities. Strong UV–optical correlations have indeed been established in small samples of (N ≲ 30) quasars with well-sampled light curves, and have extended the ‘bluer-when-brighter’ trend previously found within the optical bands. Here, we further test the nature of quasar variability by examining the observed-frame UV–optical correlations among bright quasars extracted from the Half Million Quasars (HMQ) catalogue. We identified a large sample of 1315 quasars in HMQ with overlapping UV and optical light curves from the Galaxy Evolution Explorer and the Catalina Real-time Transient Survey, respectively. We find that strong correlations exist in this much larger sample, but we rule out, at ∼95 per cent confidence, the simple hypothesis that the intrinsic UV and optical variations of all quasars are fully correlated. Our results therefore imply the existence of physical mechanism(s) that can generate uncorrelated optical and UV flux variations
Testing the relativistic Doppler boost hypothesis for supermassive black hole binary candidates
Supermassive black hole binaries (SMBHBs) should be common in galactic nuclei as a result of frequent galaxy mergers. Recently, a large sample of sub-parsec SMBHB candidates was identified as bright periodically variable quasars in optical surveys. If the observed periodicity corresponds to the redshifted binary orbital period, the inferred orbital velocities are relativistic (v/c ≈ 0.1). The optical and ultraviolet (UV) luminosities are expected to arise from gas bound to the individual BHs, and would be modulated by the relativistic Doppler effect. The optical and UV light curves should vary in tandem with relative amplitudes which depend on the respective spectral slopes. We constructed a control sample of 42 quasars with aperiodic variability, to test whether this Doppler colour signature can be distinguished from intrinsic chromatic variability. We found that the Doppler signature can arise by chance in ∼20 per cent (∼37 per cent) of quasars in the nUV (fUV) band. These probabilities reflect the limited quality of the control sample and represent upper limits on how frequently quasars mimic the Doppler brightness+colour variations. We performed separate tests on the periodic quasar candidates, and found that for the majority, the Doppler boost hypothesis requires an unusually steep UV spectrum or an unexpectedly large BH mass and orbital velocity. We conclude that at most approximately one-third of these periodic candidates can harbor Doppler-modulated SMBHBs
Correlation between optical and UV variability of a large sample of quasars
The variability of quasars across multiple wavelengths is a useful probe of physical conditions in active galactic nuclei. In particular, variable accretion rates, instabilities, and reverberation effects in the accretion disc of a supermassive black hole are expected to produce correlated flux variations in ultraviolet (UV) and optical bands. Recent work has further argued that binary quasars should exhibit strongly correlated UV and optical periodicities. Strong UV–optical correlations have indeed been established in small samples of (N ≲ 30) quasars with well-sampled light curves, and have extended the ‘bluer-when-brighter’ trend previously found within the optical bands. Here, we further test the nature of quasar variability by examining the observed-frame UV–optical correlations among bright quasars extracted from the Half Million Quasars (HMQ) catalogue. We identified a large sample of 1315 quasars in HMQ with overlapping UV and optical light curves from the Galaxy Evolution Explorer and the Catalina Real-time Transient Survey, respectively. We find that strong correlations exist in this much larger sample, but we rule out, at ∼95 per cent confidence, the simple hypothesis that the intrinsic UV and optical variations of all quasars are fully correlated. Our results therefore imply the existence of physical mechanism(s) that can generate uncorrelated optical and UV flux variations
«Η Εξέλιξη του συστήματος της Τραπεζικής Εποπτείας στην Ευρωπαϊκή Ένωση»
Οι προκλήσεις που πηγάζουν από τη δημιουργία του Ευρωπαϊκού συστήματος Κεντρικών Τραπεζών και η ανάγκη αντιμετώπισής τους με ένα κοινό σύστημα εποπτείας.Το νέο εποπτικό ρυθμιστικό πλαίσιο και η ικανότητά του να διαφυλάξει τη σταθερότητα του Ευρωπαϊκού χρηματοπιστωτικού τομέα σε συνθήκες αυξημένου ανταγωνισμού.The challenges stemming from the creation of the European System of Central Banks and the need to address them with a common supervisory system. The new supervisory regulatory framework and its ability to preserve the stability of the European financial sector in conditions of increased competition
Efficient large-scale, targeted gravitational-wave probes of supermassive black-hole binaries
Supermassive black hole binaries are promising sources of low-frequency
gravitational waves (GWs) and bright electromagnetic emission. Pulsar timing
array searches for resolved binaries are complex and computationally expensive
and so far limited to only a few sources. We present an efficient approximation
that empowers large-scale targeted multi-messenger searches by neglecting GW
signal components from the pulsar term. This Earth-term approximation provides
similar constraints on the total mass and GW frequency of the binary, yet is
times more efficient.Comment: Comments welcom
Supermassive Black-hole Demographics & Environments With Pulsar Timing Arrays
Precision timing of large arrays (>50) of millisecond pulsars will detect the
nanohertz gravitational-wave emission from supermassive binary black holes
within the next ~3-7 years. We review the scientific opportunities of these
detections, the requirements for success, and the synergies with
electromagnetic instruments operating in the 2020s.Comment: Submitted to the Astro2020 Decadal Survey. One of 5 core white-papers
authored by members of the NANOGrav Collaboration. 9 pages, 2 figure
Testing the relativistic Doppler boost hypothesis for supermassive black hole binary candidates
Supermassive black hole binaries (SMBHBs) should be common in galactic nuclei as a result of frequent galaxy mergers. Recently, a large sample of sub-parsec SMBHB candidates was identified as bright periodically variable quasars in optical surveys. If the observed periodicity corresponds to the redshifted binary orbital period, the inferred orbital velocities are relativistic (v/c ≈ 0.1). The optical and ultraviolet (UV) luminosities are expected to arise from gas bound to the individual BHs, and would be modulated by the relativistic Doppler effect. The optical and UV light curves should vary in tandem with relative amplitudes which depend on the respective spectral slopes. We constructed a control sample of 42 quasars with aperiodic variability, to test whether this Doppler colour signature can be distinguished from intrinsic chromatic variability. We found that the Doppler signature can arise by chance in ∼20 per cent (∼37 per cent) of quasars in the nUV (fUV) band. These probabilities reflect the limited quality of the control sample and represent upper limits on how frequently quasars mimic the Doppler brightness+colour variations. We performed separate tests on the periodic quasar candidates, and found that for the majority, the Doppler boost hypothesis requires an unusually steep UV spectrum or an unexpectedly large BH mass and orbital velocity. We conclude that at most approximately one-third of these periodic candidates can harbor Doppler-modulated SMBHBs
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