CURRICULUM VITAE PROF. DR. LUCIANO BURDERI

FULL PROFESSOR - ASTRONOMY AND ASTROPHYSICS - UNIVERSITY OF CAGLIARI

EDUCATION:

July 2018: National Scientific Qualification for the professorships of the I band for the Concession Sector 02 / C1 Astronomy, Astrophysics, Physics of the Earth and the Planets, 2016 Call (D.D. 1532/2016);

July 2013: National Scientific Qualification for the professorships of the I band for the Concession Sector 02 / C1 Astronomy, Astrophysics, Physics of the Earth and the Planets, 2012 Call (DD No. 222/2012);

1994: PhD degree obtained at the University of Palermo discussing a PhD thesis entitled: Temporal variability in neutron star systems;

1989: Degree in Physics obtained at the University of Palermo with a score of 110/110 cum laude. Title of degree thesis: A timing method for a millisecond pulsar in a binary system.

ACADEMIC TITLES:

March 2020: Winner of competition for Full Professor in Astronomy at the University of Cagliari

2005 – today: Associate Professor at the Department of Physics at the University of Cagliari;

1999 – 2005: Researcher Astronomer at the Astronomical Observatory of Rome;

1996 – 1999: Post-doc position at the Italian Space Agency;

1994 – 1996: Post-doc position on a rolling grant at the University of Leicester.

TEACHING EXPERIENCE:

Significant teaching experience. In particular:

University of Cagliari:

2006 – today: General Astronomy (Bachelor Degree);

2006 – 2009: Astrophysics (Master Degree);

2006 – 2009: Radio Astronomy (Master Degree);

2006 – 2009: Experimental Astronomy (Master Degree);

2006 – today: High Energy Astrophysics (Master Degree);

2006 – 2009: Coordinator of the PhD School in Nuclear Physics and Astrophysics;

2016 – 2018: Introduction to Special and General Relativity (Progetto Lauree Scientifiche, for high-school teachers).

University of Palermo:

1997 – 1999: Fisica Generale (Facoltà di Agraria, Università di Palermo);

1999 – 2000: Didattica e Storia dell' Astronomia (Scuola Interuniversitaria Siciliana -S.I.S.S.I.S., Università di Palermo);

2000 – 2002: Laboratorio di Didattica di Astrofisica e Cosmologia (Scuola Interuniversitaria Siciliana -S.I.S.S.I.S., Università di Palermo).

NUMBER OF PUBLICATIONS AND H-INDEX:

SAO/NASA Astrophysics Data System (http://adsabs.harvard.edu/abstract_service.html):

Publications: 387

202 publications in international ISI journals with referees, 185 publications non-refereed.

Total number of citations: 6535, Hirsch Index (H-index): 43;

Google Scholar (https://scholar.google.it):

Publications: 445

Total number of citations: 8470, Hirsch Index (H-index): 48;

Web of Science (https://login.webofknowledge.com)

Publications: 264

Total number of citations: 5451, Hirsch Index (H-index): 40.

REVIEWING AND EDITORIAL EXPERIENCE:

Editor of the book:  Interacting binaries: accretion, evolution, and outcomes, AIP Conference Proceedings, Vol. 797, http://dx.doi.org/10.1063/v797;

Editor of the book:  The multicolored landscape of compact objects and their explosive origins, AIP Conf. Proc. 924, http://dx.doi.org/10.1063/v924;

2017 – today: Referee of international scientific journal Heliyon, Elsevier Publishing Company;

2016 – today: Reviewer for the University of Rome Tor Vergata for proposals of the program Consolidate the Foundations to support fundamental research;

2015 – today: Referee for the Italian Ministry of Education for the program Programma per Giovani Ricercatori "Rita Levi Montalcini";

2015 – today: Referee of international scientific journal Advances in Astronomy;

2013 – today: International Reviewer for the Marie Curie COFUND Programme PISCOPIA within the EU 7th Framework Programme;

2013 – today: Reviewer for the Italian Ministry of Education (MIUR) Albo MIUR dei Revisori;

2010 – today: Referee for the Netherlands Organisation for Scientific Research (NWO) for funding of research proposals in the Netherlands;

2002 – today: Referee of international scientific journal Astronomy & Astrophysics;

2000 – today: Referee of international scientific journal Monthly Notices of the Royal Astronomical Society;

1997 – today: Referee of international scientific journal The Astrophysical Journal.

PARTICIPATION IN INTERNATIONAL RESEARCH GROUPS:

2019 – today: Science Working Group coordinator of the Scientific Space Mission Theseus (Working Group 6: Additional and GO Science);

2018 – today: Principal Investigator of the Scientific Space Mission HERMES Scientific Pathfinder funded by Italian Ministry of Education, Italian Space Agency, European Community (HORIZON2020) with a total budget of  € 8,184,450;

2015 – today: Member of the International Research Group: X-ray Imaging Polarimetry Explorer - XIPE - Science Study Magnetic Fields working group in compact objects, WG.1-2.4;

2014 – today: Member of an international research group funded by the International Space Science Institute in Bern, on the subject “The disk-magnetosphere interaction around transitional millisecond pulsars” (PI: A. Papitto, webpage: www.issibern.ch/teams/millisecpulsars/);

2013 – today: Member of the International Research Groups: Strong Gravity and Observatory Science for the Athena + X-ray observatory mission, a Large Class science mission selected by ESA for the ESA's Cosmic Vision 2015-25 program;

2013 – today: Member for Research Group on Low Mass X-ray Binaries (WG1) within NewCompStar Collaboration, MPNS COST Action MP1304 “Exploring fundamental physics with compact stars (NewCompStar)” (web site: http://compstar.uni-frankfurt.de/);

2011 – today: Member of the International Research Groups: Science Observatory and Dense Matter for the evaluation phase of the LOFT (Large Observatory For X-ray Timing) satellite;

2006 – today: Member of the International Astronomical Union (IAU - Member Reference # 11727).

SCIENTIFIC RESPONSIBILITY FOR INTERNATIONAL AND NATIONAL RESEARCH PROJECTS, ELIGIBLE  FOR FUNDING ON THE BASIS OF COMPETITIVE CALLS FOR PEER REVIEW (MOST RELEVANT SINCE  1999):

1) 2019: PRIN (PROGETTI DI RICERCA DI RILEVANTE INTERESSE NAZIONALE) 2017: Scientific Responsible of the research unit of the University of Cagliari for the PRIN: – Bando 2017, The new frontier of Multi-Messenger Astrophysics: follow-up of electromagnetic transient counterparts of gravitational wave sources (Prot. 20179ZF5KS), admitted to funding and funded with € 896,100.00;

2) 2019: Scientific Responsible of the HERMES Project, funded by internal Italian Space Agency proposal with € 1,900,000.00;

3) 2018: Member of Project Manager Board in the EU Horizon 2020 project – HERMES: High Energy Rapid Modular Ensemble of Satellites, Scientific Pathfinder, PI Fiore, funded with € 3,318,450;

4) 2017: Scientific Responsible and Scientific Coordinator of the HERMES Project, funded by the Ministry of Education with € 1,650,915.00;

5) 2016: Scientific Responsible and Scientific Coordinator of the Research Project “H.E.R.M.E.S. High Energy Rapid Modular Experiment Scintillator”, funded by Italian Space Agency with € 500,000.00;

6) 2012: Scientific Responsible and Scientific Coordinator of the Research Project of Fundamental Science 2012 annuality of the Autonomous Region of Sardinia: “Neutrons Stars as a laboratory of Physics of the Ultra-dense matter: a Multifrequency study” (Code: CRP-60529);

7) 2007: Scientific Coordinator of European Community Funds: FP7-PEOPLE-2007-1-1-ITN: “Multiwavelength Studies of Galactic Black Holes” - ITN 215212 (http://www.black-hole.eu);

8) PRIN 2005: “Osservazioni e ricerca di pulsar e pulsar al millisecondo”;

9) PRIN 2004: “Ambienti estremi nella nostra Galassia: campi gravitazionali forti, getti relativistici e campi magnetici critici in stelle di neutroni in accrescimento”;

10) PRIN 2003: “Un nuovo impulso allo studio teorico delle pulsar al millisecondo”;

11) PRIN 2002: “Collaudo Scientifico del Telescopio REM: Gamma-Ray Bursts, Oggetti Compatti Galattici e Stelle Variabili”;

12) PRIN 2001: “Evoluzione delle pulsar al millisecondo nella Galassia e negli Ammassi Globulari”;

13) PRIN 2000: “Progettazione e realizzazione dell'Ottica del telescopio e della camera infrarossa del progetto REM. Sviluppo del software di processo, telemetria ed analisi dei risultati”;

14) PRIN 1999: “Osservazioni multifrequenze di radio pulsar e di associazioni pulsar/SNR e studio dei processi non termici connessi”.

HIGHLIGHTS OF SCIENTIFIC ACTIVITY:

Luciano Burderi carries out his research activity in the field of High Energy Astrophysics. His studies are focused on binary systems that harbour a compact object (neutron star, NS, or black hole, BH) accreting matter from a companion star (secondary). In these sources, matter accretes on the compact (primary) object and releases the potential gravitational energy mostly in the X-ray band. For NS and BH the efficiency of the accretion process is about 10% of the rest mass. The analysis of the emission in the X-ray band allows to study the geometry of the accretion flow, the physical parameters of the primary and the behavior of matter in extreme conditions: strong gravitational fields and intense magnetic fields.

The research activity is based on both a theoretical and an observational approach, mainly through the analysis of data from satellites for X-ray astronomy. In particular, the scientific activity of Burderi has focused on the so-called Low Mass X-ray Binaries (LMXBs), in which the secondary has a mass smaller than that of the Sun.

Most of these systems are transients with time-scales ranging from days to years. The transience is difficult to explain with variations in the contact conditions between the surface of the secondary and its Roche Lobe (RL), since these occur on much longer time-scales. Indeed, the nuclear evolution of the secondary goes-on at least for tens or hundreds of millions of years, and comparable or longer time-scales are associated with other processes that maintain the contact with the RL thanks to losses of angular momentum: Magnetic braking or emission of gravitational waves. One possible solution is that instabilities in the accretion flow determines the transience. In particular, the thermal instabilities of the accretion disk occurring where its surface temperature falls below the plasma ionization temperature, are a promising mechanism.

Indeed, unstable disks are emptied on viscous time-scales, which are on the desired range (from days to years, depending on the viscosity and the extension of the disk). In this case it is important to take into account the irradiation of the disk by the X-rays from the accreting source. This has stabilizing effects for compact binary systems that accommodate less extended disks (the extent of the disk depends on the size of the RL of the primary, which increases with the orbital period ). The proposed scenario is in broad agreement with the observations (van Paradijs 1996, King, Kolb, and Burderi 1996), although it needs ad hoc values for the dimensionless viscosity parameter of Shakura & Sunyaev (1973) to be reconciled with experimental data. In particular it is necessary to assume significant variations of this parameter between the Hot and Cold states of the disk. It is believed that these more complex models, known as Disk Instability Models (DIMs), operate in most systems.

Encouraged by this success, Burderi, King and Szuszkiewicz (1998) studied the disks that feed the super-massive BHs of the Active Galactic Nuclei (AGN), examining all the possible models of optically thick and geometrically thin stationary discs relevant to AGNs, identifying the schemes in which they are stable compared to the DIMs. The result of this study shows that most AGN disks are unstable.

Burderi, King and Szuszkiewicz (1998) have therefore hypothesized that each galaxy hosts a super-BH at its center and that the AGNs are only the fraction of supermassive BH currently subject to an unstable growth episode (outburst). In this context only some, if not all, Quasars could contain a population of stable discs, having growth rates much higher than the other AGNs. The scale times of the DIM, suitably rescaled, range from several tens to several hundred years depending on the viscosity parameter adopted. In this context we should mention the editorial News and Views of the prestigious Nature magazine signed by Siemiginovska & Elvis (1999) which discusses the implications of the results of Burderi, King and Szuszkiewicz (1998). The same work was cited by Virginia Trimble in its review Astrophysics in 1999 (Trimble and Aschwanden 2000), where the most significant results in astrophysics of the year are illustrated. In the same review, another work of Burderi (Burderi, King, and Wynn 1998) is quoted on the discrepancy between the cooling timescale and the spin-down timescale of a Millisecond Radio Pulsar (MSP, a class of about 200 radio pulsars with spin periods below ten milliseconds).

In recent years the research of Burderi and his group have focused on theoretical and observational studies of a new class of X-ray sources, the so-called Accreting Millisecond Pulsars (AMPs): binary systems that host a rapidly rotating NS (which emits pulsations in the X-ray band), with a weak magnetic field undergoing transient accretion episodes from a very low mass secondary. To date, 18 AMS are known (the first SAXJ1808.4-3658, was discovered in 1998, Wijnands and van der Klis 1998), and it is believed that these systems are closely related to MSPs. The theoretical framework that link these two classes, revealed in two very different bands of the electromagnetic spectrum, is the so-called Recycling Scenario. According to this evolutionary scheme, the spin of an NS is firstly slowed down - below the threshold required to trigger the non-thermal emission of the radio pulsar - from the conversion of rotational energy into electromagnetic radiation according to the Larmor’s formula. Subsequently, the NS is reactivated as radio pulsar (recycled) following a phase of acceleration of the spin determined by the accretion of matter and angular momentum from the secondary (see eg Bhattacharya and van den Heuvel 1991, for a review). In a broad sense, the Recycling Scenario suggests that AMPs are at least a part of the progenitors of MSPs. From the observational point of view, Burderi and his group have developed temporal analysis techniques (Timing) specifically designed to analyze data of accreting AMPs collected in the X-ray band, which allowed them to determine the spin of the NS and the orbital parameters with enormous precision (see for example Burderi et al 2007, Papitto et al 2007, Riggio et al., 2007). This allowed them to observe the behavior of the NS in response to accretion over a time-scale of dozens of days (Burderi et al., 2006, Papitto et al., 2008).

Furthermore, the analysis of subsequent outbursts of the same AMP allowed them to study the secular evolution of the spin and of the orbital parameters (see for example Burderi et al 2009, Hartman et al 2009, Papitto et al., 2011). The theoretical interpretation of these high quality data allowed Burderi and his group to verify different assumptions of the Recycling Scenario and to outline, also, new evolutionary phases of these systems. In particular, the secular spin-down of the NS spin, found in some AMP, has allowed them to determine that the intensity of the (dipolar) magnetic field is of some hundreds of millions of Gauss (see for example Riggio et al., 2011), demonstrating how the NSs in the AMPs have magnetic fields comparable to those of the MSPs, as predicted by the Recycling Scenario.

On the other hand, the significant orbital expansion discovered in SAXJ1808.4-3658 (Burderi et al., 2009) seems to indicate that mass transfer is highly non-conservative: short accretion episodes (outburst), are followed by long phases of quiescence during which the matter, which continues to overflow from the secondary RL, is swept away by the radiation pressure exerted by the rapidly rotating NS which acts as a magneto-dipole rotator (i.e. a rotating magnetic dipole that emits electromagnetic waves and high energy relativistic particles, see e.g. the classical model of Goldreich and Julian (1969), developed immediately after the discovery of the first pulsar radio). Burderi and collaborators called this evolutionary phase Radio-Ejection (Burderi et al., 2001). Since the pressure exerted by the magneto-dipole rotator increases as the cube of the spin frequency, the Radio-Ejection is very effective in fastly spinning NSs. This is therefore a promising mechanism to inhibit the formation of magnetic NSs with spin less than one millisecond, which, in fact, have never been observed, although thoroughly searched. In this context, in order to effectively limit the NS spin-up process, it is not necessary to invoke ad hoc limiting factors such as the occurrence of non-axi-symmetric rotational instabilities and the consequent emission of gravitational waves from the rapidly rotating NSs. Therefore, the onset of the Radio-Ejection phase allows to stop the spin-up process before the NS attains its centrifugal limit, which is below one millisecond for realistic models of NSs.

The 2001 work by Burderi et al. was quoted by Virginia Trimble in his review Astrophysics in 2002 (Trimble and Aschwanden 2003). Burderi quickly realized that this evolutionary phase could play a crucial role in determining the transient nature of the AMPs. In fact, the application of the DIM to the AMPs raises great difficulties. Observations indicate periods of quiescence of several years, but since all matter, poured during quiescence from the secondary into the RL of the primary, should accrete on the NS during the outbursts, the mass transfer rates mediated over time are very low (often only lower limits can be placed for the duration of the quiescence). Such low rates are marginally compatible, or not compatible, at least in one case (Marino et al. 2017), with the predicted rates assuming that the RL of the secondary shrinks because of losses of angular momentum due to the emission of gravitational waves, particularly intense in the AMPs of short orbital periods. Furthermore, if the orbital expansion of SAXJ1808.4-3658 is the result of a substantial mass loss rate from the secondary, only a tenth of this flow of matter would have been seen to accrete on the NS during the seven outbursts observed so far. This apparent paradox is solved if DIMs do not play a significant role in the transience of AMS. On the other hand, transient behavior can be easily explained by long phases of Radio-Ejection alternating with short accretion episodes (Burderi et al., 2009).

A further evidence of the presence of a magneto-dipole rotator in the AMPs has been suggested by Burderi and collaborators with a study of the optical counterpart of SAXJ1808.4-3658 in quiescence (Burderi et al., 2003, A & A, 404, L43). Observations indicated that the secondary was over-bright. This extra brightness is perfectly compatible with the reprocessing of the radiation of a magneto-dipole rotator by the secondary surface, when a magnetic field of some hundreds of millions of Gauss is assumed. This value is consistent with the constraints imposed by the presence of pulses during outbursts, (Di Salvo and Burderi 2003) and with the aforementioned secular spin evolution. Subsequently, this method of investigation was applied successfully by other researchers (e.g. Campana et al 2004, Campana et al 2005, D'Avanzo et al., 2009) to determine the intensity of the magnetic field in AMPs. The idea is to use the secondary as a bolometer that measures the power of the (otherwise elusive) rotating magneto-dipole radiation.

A spectacular confirmation of the existence of a phase of Radio-Ejection, during the evolution of a compact binary system that harbours a NS with a rapidly rotating magnetic field, was obtained with the discovery of PSR J1740, an eclipsing MSP (D'Amico et al., 2001). The long and variable eclipses, the widening of the shape of the pulsed profile, the high and variable dispersion, suggest the presence of plasma on the orbital plane. Furthermore, an ellipsoidal periodic modulation of the optical counterpart indicates that the secondary is not spherical and therefore fills its RL, although the absence of X-ray emission and the presence of an active MSP indicate that the matter is not accreted by the NS.

Burderi, D'Antona and Burgay (2002) have interpreted this peculiar observational picture as evidence of a phase of Radio-Ejection. They have indeed calculated that the pressure of the magnetic rotator is sufficient to halt the mass transfer from the Inner Lagrangian Point, determining the mass losses in the orbital plane required to explain the observations in the radio band. This work was cited by Virginia Trimble in her review Astrophysics in 2002 (Trimble and Aschwanden 2003).

The Radio-Ejection phase was invoked to explain the scarcity of systems with orbital periods between 20 and 60 days in the distribution of binaries that contain a MSP. Also this work was cited by Virginia Trimble in Astrophysics in 2006 (Trimble, Aschwanden, and Hansen 2007).

Over the last few years, Burderi and his group have underlined the importance of highly non-conservative mass transfer phases during the evolution of LMXBs. In these phases significant losses of angular momentum, caused by the loss of mass, induce a greater contraction of the RL around the surface of the secondary, which increases the mass-transfer rate. This feedback can dramatically amplify the secular mass-transfer rate. If the NS is not able to accrete this huge flow of matter, either because it reaches the Eddington limit or because of the onset of a Radio-Ejection phase, the flow in excess is expelled from the system. This can result in a brief phase of unstable mass transfer, during which accretion occurs at the Eddington limit or does not happen at all.

Burderi et al. (2010) argue that this is a promising framework to interpret the phenomenology of the Bright X-ray Sources of the Galactic Bulge, the so-called Z sources, a group of less than ten LMXBs, all with luminosity close to the Eddington limit. On the other hand, if growth is prevented by Radio-Ejection, unstable mass transfer can lead to the expulsion of most of the secondary mass (Di Salvo et al., 2008). This evolutionary channel for the formation of isolated MSPs is an alternative to the Black Widow scenario proposed by Tavani (Tavani 1991, Tavani 1991, Tavani and Brookshaw 1991) in which the magneto-dipole radiation is capable to "evaporate" the secondary of small mass. The peculiarity of the model proposed by Burderi lies in the fact that, once the unstable transfer begins, NS rotational energy is no more required to destroy the secondary since this occurs at the expense of the enormous binding energy released by the secondary during the dramatic orbital shrinking. Once most of the secondary mass has been expelled, the small residual core, now in a very close orbit, can be easily "evaporated" by the magneto-dipole radiation emitted by the rapidly rotating pulsar. This makes the formation of isolated MSPs much easier.

Recently, the research group of Burderi and collaborators has obtained very relevant experimental results in the study of the connections between the MSPs and the AMPs. In particular, in 2013 (Papitto et al., 2013), these researchers have discovered the source IGR J18245-2452, which alternates outburst phases in which it is visible in the X band as a AMP to phases of quiescence in which it is detectable in the radio band as a MSP. This source seems to be the connecting link that shows that AMPs and MSPs can be different phases in the evolution of the same system.

In 2017 (Ambrosino et al., 2017), these researchers have discovered millisecond optical pulsations from PSR J1023+0038, a transitional millisecond pulsar, i.e. a LMXB harboring a fast spinning NS that shows transitions from X-ray to radio phases. This discovery shows that NSs in an MSP phase can emit pulsed optical emission. The opening of this new window in the pulsed electromagnetic spectrum of rapidly spinning NS is of paramount importance for our understanding of the complex rotating magneto-dipole phenomenon.

In the last years the research interests of Burderi have extended to fundamental physics issues and, in particular, to the problems concerning an operative definition for the measurements of arbitrarily short space and time intervals. In particular Burderi et al. (2016) have published a work in which they propose a critical discussion on the measurement of these fundamental quantities through a Gedankenexperiment that illustrates the construction of an appropriate clock that the authors have named Quantum Clock . Measuring very small-time intervals means experimenting with very high-energy particles, but we do not know if in this case the two fundamental theories, quantum mechanics and general relativity theory, can get along. Indeed, at very high energies (technically speaking at the Planck scale) it is necessary to consider both quantum effects and general relativity (which is still a theory based on classical mechanics). While we know that, for not too high energies, quantum mechanics and general relativity are more or less consistent, for energies at the Planck scale the problem is still far from being solved.

The work starts from an ideal experiment designed by the authors, a Quantum Clock that, in principle, could measure small time intervals with all the accuracy?requested. The "trick" is to measure, rather than the time elapsed, the number of decays of a large mass of radioactive atoms. This type of measures, in principle, can be done with a suitable Geiger counter. It must be considered, however, that the mass of this "clock"

influences the structure of the space-time in which it is inserted, and, in particular, that this "clock" cannot have arbitrarily small spatial dimensions (because otherwise the photons that signal the emission of radioactive particles could not escape from the radioactive mass that would be subjected to a complete gravitational collapse). It follows that it is not possible to simultaneously determine the spatial and temporal coordinates of an event with arbitrary accuracy. The indeterminacy relationship that is obtained is the main result of the work.

The interest in Quantum Gravity and on its possible experimental consequences, have recently led Burderi and collaborators to propose an ambitious experiment of High Energy Astrophysics, capable of revealing possible effects of a granular structure of space-time on a very small scale. In particular, this structure could imply a dispersion law for photons, whose group velocity would tend only asymptotically to the constant value postulated by Einsteinian theories, for photonic energies tending to zero. This miniscule effect, would be detectable if we consider high energy photons that have traveled distances comparable with the radius of the visible Universe. The astrophysical objects that are the best candidates for this type of investigation are the Gamma Ray Bursts that emit flashes of light in the gamma ray band from the edge of the visible Universe (cosmological redshift up to 8). To carry out this type of measurements, we intend to use a modular network of tens/hundreds of nano-satellites (up to a few kilograms of weight) in a low orbit. The project called HERMES (High Energy Rapid Modular Ensemble of Satellites) is currently in a feasibility study phase, and a pathfinder experiment involving six satellites will be carried out by the end of 2021, fully financed by the Italian Space Agency and the European commission. This project has received considerable interest from the international astrophysical community.

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