VLBI detection of an AGN pair in the binary black hole candidate SDSS J1536+0441

We present first pc-scale radio imaging of the radio-quiet candidate binary black hole system SDSS J1536+0441. The observations were carried out by the European VLBI Network at the frequency of 5 GHz and allowed to image SDSS J1536+0441 with a resolution of about 10 mas (50 pc). Two compact radio cores are detected at the position of the kpc-scale components VLA-A and VLA-B, proving the presence of two compact active nuclei with radio luminosity about 10^{40} erg/s, thus ruling out the possibility that the two radio sources are both powered by one 0.1 pc binary black hole. From a comparison with published 8.5 GHz flux densities we derived an estimate of the radio spectral index of the two pc-scale cores. Both cores have flat or inverted spectral index and, at least for the case of VLA-A, we can rule out the possibility that synchrotron self-absorption is responsible for the inverted radio spectrum. We suggest that thermal free-free emission from an X-ray heated disk wind may be powering the radio emission in VLA-A.Comment: Accepted for publication in ApJ Letters

Massive black hole binaries: dynamical evolution and observational signatures

The study of the dynamical evolution of massive black hole pairs in mergers is crucial in the context of a hierarchical galaxy formation scenario. The timescales for the formation and the coalescence of black hole binaries are still poorly constrained, resulting in large uncertainties in the expected rate of massive black hole binaries detectable in the electromagnetic and gravitational wave spectra. Here we review the current theoretical understanding of the black hole pairing in galaxy mergers, with a particular attention to recent developments and open issues. We conclude with a review of the expected observational signatures of massive binaries, and of the candidates discussed in literature to date.Comment: 4 Figures. Accepted for publication in Advances in Astronom

AGN pairs: chance superpositions or black hole binaries?

Several active galactic nuclei (AGN) with multiple sets of emission lines separated by over 2000 km/s have been observed recently. These have been interpreted as being due to massive black hole (MBH) recoil following a black hole merger, MBH binaries, or chance superpositions of AGN in galaxy clusters. Moreover, a number of double-peaked AGN with velocity offsets of ~ a few 100 km/s have also been detected and interpreted as being due to the internal kinematics of the narrow line regions or MBH binary systems. Here we reexamine the superposition model. Using the Millennium Run we estimate the total number of detectable AGN pairs as a function of the emission line offset. We show that AGN pairs with high velocity line separations up to ~2000 km/s are very likely to be chance superpositions of two AGN in clusters of galaxies for reasonable assumptions about the relative fraction of AGN. No superimposed AGN pairs are predicted for velocity offsets in excess of ~3000 km/s as the required AGN fractions would violate observational constraints. The high velocity AGN pair numbers predicted here are competitive with those predicted from the models relying on MBH recoil or MBH binaries. However, the model fails to account for the largest emission line velocity offsets that require the presence of MBH binaries.Comment: 4 pages, 2 figures, submitted to ApJ Letter

A quasar companion to the puzzling quasar SDSS J0927+2943

We report the discovery of a quasar close to SDSS J0927+2943 (z = 0.713), which is a massive binary / recoiling black hole candidate. The companion quasar is at a projected distance of 125 h_70^{-1} kpc and exhibits a radial velocity difference of ~1400 km/s with respect to the known quasar. We discuss the nature of this peculiar quasar pair and the properties of its environment. We propose that the overall system is caught in the process of ongoing structure formation.Comment: 4 pages, 3 figures. Accepted for publication in A&

Downsizing of supermassive black holes from the SDSS quasar survey (II). Extension to z~4

Starting from the quasar sample of the Sloan Digital Sky Survey (SDSS) for which the CIV line is observed, we use an analysis scheme to derive the z-dependence of the maximum mass of active black holes, which overcomes the problems related to the Malmquist bias. The same procedure is applied to the low redshift sample of SDSS quasars for which Hbeta measurements are available. Combining with the results from the previously studied MgII sample, we find that the maximum mass of the quasar population increases as (1+z)^(1.64+/-0.04) in the redshift range 0.1<z<4, which includes the epoch of maximum quasar activity.Comment: 9 pages, 8 figures. To appear in MNRA

A systematic search for massive black hole binaries in SDSS spectroscopic sample

We present the results of a systematic search for massive black hole binaries in the Sloan Digital Sky Survey spectroscopic database. We focus on bound binaries, under the assumption that one of the black holes is active. In this framework, the broad lines associated to the accreting black hole are expected to show systematic velocity shifts with respect to the narrow lines, which trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929 galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture of two quasars at two different redshifts. The spectral model is a data-driven dimensionality reduction of the SDSS quasar spectra based on a matrix factorization. We identified 32 objects with peculiar spectra. Nine of them can be interpreted as black hole binaries. This doubles the number of known black hole binary candidates. We also report on the discovery of a new class of extreme double-peaked emitters with exceptionally broad and faint Balmer lines. For all the interesting sources, we present detailed analysis of the spectra, and discuss possible interpretations.Comment: 10 pages, 2 figures, accepted for publication in Ap

Positive ideologies ain’t enough! (Dis)junctions and paradoxes in minority language protection

In 2018, I conducted research on the application of protection measures for two minority language groups (Cimbrian and Mocheno) in Northern Italian schools. The study aimed at describing linguistic learning programs but it slowly turned into something more. The anthropological holistic perspective unveiled profound contradictions and paradoxes in the implementation of minority language policies. Such discrepancies suggest that law can be both a “present and absent” social actor that influences reality in non-immediately perceptible ways. On the one side, purposes of a legislative manifesto can be soundlessly inhibited by bureaucrats’ deliberate inaction or lack of competence. On the other side, an inhibited legislative text still produces undetectable effects and ontologically impacts on individuals. Consequently, positive ideologies ain’t enough if little is known about what happens in everyday operational terms and this cannot be ignored, especially by those who are sincere in their support for linguistic and educational rights. This article endorses a critique of ‘the justice possible through law’ upheld by early law and society research.Nel 2018 ho condotto una ricerca sull’applicazione delle misure di tutela per due gruppi linguistici minoritari (cimbro e mocheno) nelle scuole dell’Italia settentrionale. Lo studio aveva la finalità di descrivere i programmi di apprendimento linguistico ma si è lentamente trasformato in qualcosa di più. La prospettiva olistica dell’antropologia ha svelato profonde contraddizioni e paradossi nell’attuazione delle politiche linguistiche delle minoranze. Tali discrepanze suggeriscono che il diritto possa essere letto al contempo come un attore sociale “presente e assente” che influenza la realtà in modi non immediatamente percepibili. Da un lato, gli scopi di un dettato legislativo possono essere inibiti silenziosamente dall’inerzia deliberata o dalla mancanza di competenza dei burocrati. Dall’altro, un testo normativo, pur inibito, produce comunque effetti non rilevabili e impatta ontologicamente sugli individui. Di conseguenza, le ideologie positive non bastano se si sa poco di ciò che accade in termini operativi quotidiani e ciò non può essere ignorato, soprattutto dai sinceri sostenitori dei diritti linguistici ed educativi. Questo articolo sostiene una critica della “giustizia possibile attraverso il diritto” sostenuta dalle prime ricerche su diritto e società

On phase-based localization with narrowband backscatter signals

Abstract Backscatter communication is widely adopted for radio-frequency identification (RFID). Recently, the possibility of localizing passive tags or readers, exploiting phase measurements from backscatter signals, received large attention. In particular, several applications with standard ultra-high frequency (UHF) RFID were proposed, thanks to the availability of the phase information in many commercial readers, without requiring any hardware modification. In this paper, the problem of localizing a tag or a reader using phase measurements is addressed from the estimation theory point of view. The derived structure for the maximum likelihood estimator is compared with other approaches proposed in the literature, showing its enhanced performance in a typical application context