Precise optical timing of PSR J1023+0038, the first millisecond pulsar detected with Aqueye+ in Asiago

We report the first detection of an optical millisecond pulsar with the fast photon counter Aqueye+ in Asiago. This is an independent confirmation of the detection of millisecond pulsations from PSR J1023+0038 obtained with SiFAP at the Telescopio Nazionale Galileo. We observed the transitional millisecond pulsar PSR J1023+0038 with Aqueye+ mounted at the Copernicus telescope in January 2018. Highly significant pulsations were detected. The rotational period is in agreement with the value extrapolated from the X-ray ephemeris, while the time of passage at the ascending node is shifted by $11.55 \pm 0.08$ s from the value predicted using the orbital period from the X-rays. An independent optical timing solution is derived over a baseline of a few days, that has an accuracy of $\sim 0.007$ in pulse phase ($\sim 12$ $\mu$s in time). This level of precision is needed to derive an accurate coherent timing solution for the pulsar and to search for possible phase shifts between the optical and X-ray pulses using future simultaneous X-ray and optical observations.Comment: 6 pages, 4 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Letter

Spin-down rate of the transitional millisecond pulsar PSR J1023+0038 in the optical band with Aqueye+

We present a timing analysis of the transitional millisecond pulsar PSR J1023+0038 using observations taken between January 2018 and January 2020 with the high time resolution photon counter Aqueye+ mounted at the 1.82 m Copernicus telescope in Asiago. We report the first measurement of the timing solution and the frequency derivative of PSR J1023+0038 based entirely on optical data. The spin-down rate of the pulsar is $(-2.53 \pm 0.04) \times 10^{-15}$ Hz$^2$, which is $\sim$20% slower than that measured from the X-ray observations taken in 2013-2016 and $\sim$5% faster than that measured in the radio band during the rotation-powered state.Comment: 6 pages, 4 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Letter

Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes

The results of gamma-ray observations of the binary system HESS J0632 + 057 collected during 450 hr over 15 yr, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these observations were accompanied by measurements of the Hα emission line. A significant detection of the modulation of the very high-energy gamma-ray fluxes with a period of 316.7 4.4 days is reported, consistent with the period of 317.3 0.7 days obtained with a refined analysis of X-ray data. The analysis of data from four orbital cycles with dense observational coverage reveals short-timescale variability, with flux-decay timescales of less than 20 days at very high energies. Flux variations observed over a timescale of several years indicate orbit-to-orbit variability. The analysis confirms the previously reported correlation of X-ray and gamma-ray emission from the system at very high significance, but cannot find any correlation of optical Hα parameters with fluxes at X-ray or gamma-ray energies in simultaneous observations. The key finding is that the emission of HESS J0632 + 057 in the X-ray and gamma-ray energy bands is highly variable on different timescales. The ratio of gamma-ray to X-ray flux shows the equality or even dominance of the gamma-ray energy range. This wealth of new data is interpreted taking into account the insufficient knowledge of the ephemeris of the system, and discussed in the context of results reported on other gamma-ray binary systems

MAGIC and H.E.S.S. detect VHE gamma rays from the blazar OT081 for the first time: a deep multiwavelength study

https://pos.sissa.it/395/815/pdfPublished versio

Studio delle pulsars e del loro ambiente attraverso osservazioni gamma alle altissime energie

Questa Tesi illustra le attività ed i risultati che ho ottenuto durante il mio corso di dottorato presso l’Università degli Studi di Padova. Il mio lavoro si è concentrato principalmente sullo studio di alcune classi di sorgenti Galattiche che emettono nella parte più energetica dello spettro elettromagnetico, le altissime energie VHE (>100 GeV): pulsars, mil- lisecond pulsars e transitional millisecond pulsars. Le pulsars sono stelle di neutroni estremamente dense (circa 1018 kg/m3) e in rapida rotazione che si formano durante l’esplosione di una stella massiccia ed altamente magnetizzata (107-1015G). Una pulsar standard ha una massa di 1.44 M⊙ e un raggio di circa 10 km. Le pulsars più conosciute emettono nella banda radio ma possono anche produrre pulsazioni ottiche, X e gamma, fino al range delle VHE. Mostrano comportamenti complessi e, ad oggi, il meccanismo con cui emettono non è stato ancora del tutto compreso. Grazie alla loro radiazione coerente emessa sotto forma di fasci collimati uscenti dai due poli magnetici, le pulsar possono essere sfruttate come speciali laboratori per studiare la Fisica fondamentale. Alcune di queste pulsars ruotano ad una velocità incredibile, alcune centinaia di volte al secondo, diventando così le sor- genti che ruotano più velocemente nell’Universo e che per questo vengono definite pulsars al millisecondo, MSPs. In generale hanno periodi di rotazione di 1−30 ms e dei campi magnetici più deboli rispetto alle pulsars normali (107-1010 G). Sono pulsars “riciclate” che acquistano materia e momento angolare dalla stella compagna. Esiste poi una partico- lare classe di MSP: le tMSPs. Sono sorgenti che periodicamente passano da una fase in cui si comportano come MSP radio a una fase in cui la pulsazione radio si spegne e si comportano come sistemi binari in accrescimento in cui la stella di neutroni inizia a risucchiare materia dalla compagna, cambiando radicalmente la sua fenomenologia. La transizione tra i due stati avviene in tempi brevi, dell’ordine di quache settimana. Il mio lavoro è principalmente dedicato allo studio dell’emissione VHE e alla ricerca di pulsazione gamma da parte di due sorgenti candidate ad emettere questo tipo di emissione: PSR J0218+4232 (una MSP) e PSR J2229+6114 (una pulsar regolare). Ad oggi sono state rilevate solo tre pulsars nel range delle altissime energie: Crab Pulsar (Aliu et al., 2008), Vela Pulsar (H. E. S. S. Collaboration et al., 2018a) e, molto recentemente, Geminga Pulsar (MAGIC Collabo- ration et al., 2020). In particolare, ho lavorato sui dati raccolti con i telescopi MAGIC situati presso l’Osservatorio di Roque de Los Muchachos nell’isola delle Canarie di La Palma. I dati sono stati presi con il Sum-Trigger-II, un sistema di trigger che, con una soglia di energia di decine di GeV, è in grado di aumentare l’efficienza dei telescopi MAGIC alle più basse energie. Questo sistema è utile per rilevare sorgenti gamma estremamente deboli, come le pulsar. Purtroppo con i telescopi MAGIC non è stato trovato alcun segnale, o emissione pulsata, superiore a 20 GeV, proveniente dalle due sorgenti. Le due sorgenti sono state inoltre osservate e studiate anche utilizzando i dati raccolti dal satellite Fermi-LAT e da quest’analisi risulta che PSR J0218+4232 ha un’emissione pulsata superiore ai 25 GeV, mentre PSR J2229+6114 superiore ai 20 GeV. Una parte del dottorato l’ho dedicata allo studio delle prospettive di emissione delle tMSPs nel range delle altissime energie, in particolare da parte di PSR J1023+0038 e XSS J12270-4859 durante il loro stato di accrescimento. Ho studiato i dati raccolti dal satellite Fermi-LAT considerando diversi modelli spettrali per poi studiare la possibilità di rilevarli con il futuro CTA.This thesis illustrates the activities and the results that I have performed during my PhD course at the University of Padova. My work was mainly focused on the study of some classes of Galactic sources: pulsars, millisecond pulsars and transitional millisecond pulsars in the most energetic part of the electromagnetic spectrum, the Very-High-Energy (VHE), >100 GeV, gamma ray regime. Pulsars are very dense (about 1018 kg/m3), extremely magnetized (107−1015 G) and rapidly rotating neutron stars, formed after supernova explosions. A standard pulsar has a mass of 1.44 M⊙ and a radius of about 10 km. Most known pulsars are observed in radio but they can also produce detectable optical, X-ray and gamma-ray pulsations, up to the VHE range. Pulsars show complex behaviors and in particular the underlying mechanism by which they emit electromagnetic radiation is still not fully understood. Thanks to their coherent radiation, emitted in the form of collimated beams from the two magnetic poles, they can be exploited as special natural laboratories for fundamental Physics. Some pulsars rotate at an incredible rate of a few hundred times per second, making them the fastest-spinning stars known in the Universe. They are the so-called Millisecond Pulsars (MSPs). Generally, they have spin-periods of 1−30 ms and lower magnetic fields than standard pulsars (107 −1010 G). They are “recycled” pulsars that accrete matter and angular momentum from a companion star. In particular, there is a class of MSPs which belong to binary systems: the Transitional MSPs (tMSPs). They are neutron stars rotating at a period of a few milliseconds which undergo transi- tions between two states: a bright X-ray pulsar regime powered by the accretion onto the neutron star surface of matter transferred by the companion star and a radio (and possibly gamma-ray) pulsar regime powered by the energy loss due to the fast rotation of the neutron star magnetic field. The transitions between the two regimes take place on short time-scales of less than a few weeks. My research is mainly focused on studying the VHE emission and gamma-ray pulsation from two of the best pulsars candidates for VHE gamma-ray emission: PSR J0218+4232 (a MSP) and PSR J2229+6114 (a regular pulsar). Up to now, only three pulsars have been detected at VHE: the Crab Pulsar (Aliu et al., 2008), the Vela Pulsar (H. E. S. S. Collaboration et al., 2018a) and, very recently, the Geminga Pulsar (MAGIC Collaboration et al., 2020). In particular I used data taken with the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes located at the Observatory of the Roque de Los Muchachos in the Canary Island of La Palma. The data were taken with the Sum- Trigger-II system which has an energy threshold of about tens of GeV, in order to increase the detection efficiency at the lowest energies reached by MAGIC. This system is extremely useful to detect soft gamma-ray sources, such as pulsars. In the search for pulsed emission from both sources, PSR J0218+4232 and PSR J2229+6114, no signal or pulsed emis- sion above 20 GeV was found with the MAGIC telescopes. The sources were also studied using Fermi-LAT and PSR J0218+4232 shows high energy pulsed emission above 25 GeV and PSR J2229+6114 above 20 GeV. During my PhD I dedicated a part of the time also to the study of the prospects for VHE gamma-ray emission from tMSPs, in particular from the two systems PSR J1023+0038 and XSS J12270-4859, during their accretion disk state. I studied their Fermi-LAT emission considering different spectral models and then I investigated the feasibility to detect them with the future Cherenkov Telescope Array (CTA) and the prospect for studying them

Analisi temporale di osservazioni IquEye della Vela pulsar

In questa tesi sono stati analizzati i dati ottici della Vela pulsar ottenuti nel 2009 con il il contatore veloce di fotoni IquEye montato al telescopio NTT in Cile. Tramite il software di analisi temporale XRONOS abbiamo determinato la curva di luce e lo spettro di potenza di tutte le osservazioni; 3 osservazioni su 15 mostrano segnale statisticamente significativo che abbiamo analizzato per ricavare frequenza, periodo di rotazione e forma dell’impulso della Vela pulsar. I dati ottici di IquEye sono state confrontate con le con le effemeridi di radio ottenuti nello stesso periodio con i telescopi di Hobart (Australia) e di Hartebeesthoek (Sud Africa

Very High Energy emission in Galactic transient millisecond pulsars and prospects of detection with the Cherenkov Telescope Array

Le sorgenti PSR J1023+0038 e PSR J1227-4853 alternano due fasi: nella prima sono una pulsar radio al millesecondo (alimentata dall'energia rotazionale), mentre nella seconda una Low Mass X-Ray Binary (alimentata dall'accrescimento da una stella compagna). Con i parametri spettrali ottenuti dall'analisi dei dati gamma del satellite Fermi/LAT, presi tra il 2008 e il 2017, sono state effettuate delle simulazioni spettrali nella banda di energia tra circa 100 GeV e 300 TeV per capire se tali sorgenti potranno essere osservate con il Cherenkov Telescope Array e studiarne le proprietà di emissione. PSR J1023+0038 e` stata recentemente osservata anche nella banda ottica con Aqueye+ al telescopio Copernico di Asiago. La Tesi riporta anche l'analisi delle curve di luce relative a queste osservazioni, che mostrano significativa variabilità, come riscontrato in precedenti osservazioni in banda ottica ed X

Timing analysis and pulse profile of the Vela pulsar in the optical band from Iqueye observations

The Vela pulsar is among a number of pulsars which show detectable optical pulsations. We performed optical observations of this pulsar in 2009 January and December with the Iqueye instrument mounted at the ESO 3.5 m New Technology Telescope. Our aim was to perform phase fitting of the Iqueye data, and to measure the optical pulse profile of the Vela pulsar at high time resolution, its absolute phase, and rotational period. We calculated for the first time an independent optical timing solution and obtained the most detailed optical pulse profile available to date. Iqueye detected a distinct narrow component on the top of one of the two main optical peaks, which was not resolved in previous observations, and a third statistically significant optical peak not aligned with the radio one. The quality of the Iqueye data allowed us to determine the relative time of arrival of the radio-optical-gamma-ray peaks with an accuracy of a fraction of a millisecond.We compare the shape of the Iqueye pulse profile with that observed in other energy bands and discuss its complex multiwavelength structure

Prospects for Galactic transient sources detection with the Cherenkov Telescope Array

Several types of Galactic sources, like magnetars, microquasars, novae or pulsar wind nebulae flares, display transient emission in the X-ray band. Some of these sources have also shown emission at MeV–GeV energies. However, none of these Galactic transients have ever been detected in the very-high-energy (VHE; E>100 GeV) regime by any Imaging Air Cherenkov Telescope (IACT). The Galactic Transient task force is a part of the Transient Working group of the Cherenkov Telescope Array (CTA) Consortium. The task force investigates the prospects of detecting the VHE counterpart of such sources, as well as their study following Target of Opportunity (ToO) observations. In this contribution, we will show some of the results of exploring the capabilities of CTA to detect and observe Galactic transients; we assume different array configurations and observing strategies

Prospects for Galactic transient sources detection with the Cherenkov Telescope Array

International audienceSeveral types of Galactic sources, like magnetars, microquasars, novae or pulsar wind nebulae flares, display transient emission in the X-ray band. Some of these sources have also shown emission at MeV–GeV energies. However, none of these Galactic transients have ever been detected in the very-high-energy (VHE; E>100 GeV) regime by any Imaging Air Cherenkov Telescope (IACT). The Galactic Transient task force is a part of the Transient Working group of the Cherenkov Telescope Array (CTA) Consortium. The task force investigates the prospects of detecting the VHE counterpart of such sources, as well as their study following Target of Opportunity (ToO) observations. In this contribution, we will show some of the results of exploring the capabilities of CTA to detect and observe Galactic transients; we assume different array configurations and observing strategies