392 research outputs found
Transitional Millisecond Pulsars
No full textMillisecond pulsars in tight binaries have recently challenged our understanding of physical processes governing the evolution of binaries and the interaction between astrophysical plasma and electromagnetic fields. Transitional systems that showed changes from rotation-powered to accretion-powered states and vice versa have bridged the populations of radio and accreting millisecond pulsars, eventually demonstrating the tight evolutionary link envisaged by the recycling scenario. A decade of discoveries and theoretical efforts have just grasped the complex phenomenology of transitional millisecond pulsars from the radio to the gamma-ray bands. This chapter summarizes the main properties of the three transitional millisecond pulsars discovered so far, as well as of candidates and related systems, discussing the various models proposed to cope with their multifaceted behaviour
Analysis of the rotational behaviour and evolutionary scenarios of Accreting Millisecond Pulsars
No full textIn questo studio viene presentata un'analisi
dell'evoluzione rotazionale ed orbitale delle Pulsar X al millisecondo (AMSP). Queste sorgenti sono delle stelle di neutroni (NS) che emettono energia in raggi X accrescendo la materia trasferita da una stella compagna, tramite un disco di accrescimento. Poichè le AMSP poseggono una magnetosfera che interrompe il flusso di materia nel disco prima della superficie, concentrando l'accrescimento nelle vicinanze dei poli magnetici, l'emissione X è pulsata al periodo di rotazione della stella, che è di qualche millisecondo per una AMSP.
Il mio progetto di ricerca si basa sull'utilizzo di questa proprietà fondamentale per valutare la reazione della NS all'accrescimento di materia. Infatti le particelle di un disco di accrescimento posseggono un elevato momento angolare specifico, specialmente nelle vicinanze della NS; quando la materia viene accresciuta il suo momento angolare viene rilasciato alla stella, che tende quindi ad accellerare. E' proprio questo meccanismo che ha condotto le AMSP alla estrema velocità di rotazione attuale (un decimo della velocità della luce). Avendo a disposizione un tale orologio solidale alla stella, ho utilizzato le pulsazioni X provenienti dalla superficie per misurare le piccole variazioni della frequenza di rotazione della NS mentre accresce massa. Se ne trae quindi una misura delle coppie che agiscono su di essa e la stima delle grandezze che le regolano, principalmente il tasso di accrescimento di massa e l'intensità del campo magnetico della NS.
Tali misure sono estremamente delicate, specialmente nel caso delle AMSP. Infatti queste sorgenti accrescono massa al più per qualche mese ed, a causa dell'elevata inerzia di una NS, la variazione di frequenza attesa è solamente di poche parti su dieci miliardi. Innanzi tutto le tecniche standard di timing sono state quindi adattate al caso particolare di questi oggetti, permettendo per la prima volta una misura affidabile del loro stato rotazionale. Sono state prese in considerazione sei delle dieci AMSP scoperte sin ora. In particolare, le due alle quali mi sono dedicato maggiormente mostrano adeguatamente come il semplice schema di accellerazione della NS delineato sopra non valga in ogni caso, e come una AMSP possa anche decelerare durante l'accrescimento. La causa di tale decelerazione è individuata dalla teoria dell'accrescimento nell'interazione tra il campo magnetico e il disco di accrescimento, interazione che può quindi rallentare la stella specialmente se questa ruota molto rapidamente. In questo lavoro mostro come questi rallentamenti vengono effettivamente osservati e come consentano di ottenere stime del campo magnetico della NS.
Gli elementi basilari delle teorie dell'accrescimento su un rotatore veloce sono stati testati non solo sulla base dei risultati dell'analisi temporale. Si mostra infatti come le attese teoriche siano supportate anche alla luce dell'informazione spettrale. In particolare l'osservazione di una AMSP mostra la presenza di una riga del ferro molto larga nel suo spettro in raggi X. Essendo la regione interna del disco di accrescimento l'unica possibile regione di formazione di una riga così larga, è stato possibile misurare, per la prima volta nel caso di una pulsar, l'estensione del bordo interno del disco. Il valore misurato è perfettamente in accordo con il ristretto intervallo permesso dalla teoria, rappresentando così una verifica fondamentale della sua consistenza.
L'analisi temporale consente inoltre di valutare l'evoluzione orbitale del sistema binario al quale appartiene la NS. Nell'unico caso di un sistema che abbia mostrato più di un episodio di attività, si è trovata evidenza di una evoluzione molto più rapida di quella attesa. Questo comportamento può essere spiegato solo in termini di rilevanti perdite di massa, massa che porta con sé la quantità di momento angolare necessaria per rendere conto dell'evoluzione misurata. Ciò supporta inoltre l'ipotesi che tali perdite di massa siano dovute all'accensione di una pulsar alimentata dalla rotazione durante le sue fasi di quiete del sistema. Questo può in definitiva essere considerato uno dei pochi casi astrofisici in cui viene osservata in tempo reale un'evoluzione altamente non conservativa.
I risultati presentati in questa tesi coprono quindi molti aspetti della fisica di questi sistemi, mostrando come l'unione dell'analisi temporale e spettrale possa fornire una gran quantità di informazioni su questi sistemi estremi e per certi versi sconcertanti. In definitiva sono state confermate le attese teoriche di base sull'accrescimento su NS veloci, ma si aprono anche diverse questioni che promettono di gettare maggiore luce sulla fisica dell'ambiente immediatamente circostante la stella e sull'effettiva linea evolutiva delle AMSP.I present in this study an analysis of the spin and orbital evolution of
Accreting Millisecond Pulsars (AMSP). These sources are neutron stars (NS) emitting X-rays because of the accretion of mass transferred
by a nearby companion star through an accretion disc. As AMSP owns a magnetosphere that truncates the disc before the NS, thus channelling accreted matter in the vicinity of the magnetic poles, their X-ray emission is pulsed at the NS
spin period, which is of few milliseconds in an AMSP.
My scientific project relies on the use this invaluable property to
evaluate the rotational reaction of the NS to the accretion of mass. As a matter
of fact, mass orbiting in an accretion disc has a large specific angular
momentum especially close to the NS; when this matter is accreted, it releases
its angular momentum to the NS that is therefore expected to accelerate. It is
indeed through this mechanism that AMSP have been spun up to their extreme
rotational velocities (up to 0.1 times the speed of light in vacuum). I therefore used the X-ray
pulsations coming from the NS surface as a clock to precisely
measure the tiny variations of the accretor spin frequency as it accretes.
This is ultimately a measure of the accretion torques acting on the NS and
allows a model dependent estimate of the physical quantities regulating these
torques, mainly the rate at which mass is accreted on the NS and the magnetic field
straight.
Such measurements can be very tricky especially for AMSP. They accrete mass for
at most few months, and because of to the large inertia of a NS, the expected
frequency variations are of only few parts on ten billions. Standard timing
techniques were therefore first tailored to the particular case of these
sources, allowing for the first time reliable estimates of their spin state. Six
among the ten AMSP discovered so far are considered in this work. In particular,
the two sources I focused on the most show how the simple picture of the NS
spin-up outlined above does not hold in every case, as the outcome of the
accretion can also be the deceleration of the NS. The reason for this behaviour
is interpreted by the accretion theory in terms of the interaction between the
magnetic field and the accretion disc. This interaction may then brake of the
compact object especially if it is very fast. I show in this work how these
spin-down are effectively observed and how this allows an estimate of the NS
magnetic field.
The basics of the accretion picture onto a fast object are tested not only on
the basis of a temporal analysis. I show in fact how the spectral information
also supports the theoretical expectations. In particular a high spectral
resolution observation of a AMSP shows the presence of a broadened iron line in
its X-ray spectrum. The only viable location for the formation of a line so
broadened is the inner part of the accretion disc, thus allowing for the first
time the measure of the size of the inner disc rim of a pulsar. This measure is
perfectly consistent with the small range allowed by theory, thus representing a
fundamental test of their consistency.
Temporal analysis also allows to enlighten the evolution of the binary system
the NS belongs to. In the only case of a system which recurred more than once,
we could find evidence of a faster than expected evolution. We interpret such
behaviour as an indication of relevant mass lost which carries
away the angular momentum needed to match the observations. This supports the hypothesis that a rotation powered pulsar switches on during the quiescent phases of the binary. Moreover, this observation can
be considered as one of the few astrophysical cases in which a highly non
conservative evolution was directly observed.
The results presented in this thesis cover many aspects of the
physics of these fast accretors, and show how X-ray temporal and spectral
analysis can jointly supply a wealth of information on the physical state of
these extreme and puzzling systems. These results confirm the basic theoretical
expectations but open also several issues which are very promising to shed some
light in particular on the environment surrounding these fast rotating NS and on
their actual evolutionary progeny
Discovery of polarized X-ray emission from the accreting millisecond pulsar SRGA J144459.2–604207
No full textAlessandro Papitto et al.We report the discovery of polarized X-ray emission from an accreting millisecond pulsar. During a 10-day-long coverage of the February 2024 outburst of SRGA J144459.2−604207, the Imaging X-ray Polarimetry Explorer (IXPE) detected an average polarization degree of the 2–8 keV emission of 2.3%±0.4% at an angle of 59° ±6° (east of north; the uncertainties quoted are at the 1σ confidence level). The polarized signal shows a significant energy dependence with a degree of 4.0%±0.5% between 3 and 6 keV and < 1.5% (90% c.l.) in the 2–3 keV range. We used NICER, XMM–Newton, and NuSTAR observations to obtain an accurate pulse-timing solution and to perform a phase-resolved polarimetric analysis of IXPE data. We did not detect any significant variability in the Stokes parameters Q and U with the spin and orbital phases. We used the relativistic rotating-vector model to show that a moderately fan-beam emission from two point-like spots at low magnetic obliquity (≃10°) is compatible with the observed pulse profile and polarization properties. IXPE also detected 52 type I X-ray bursts whose recurrence time Δtrec increased from 2 to 8 h as a function of the observed count rate C as Δtrec ∝ C−0.8. We stacked the emission observed during all the bursts and obtained an upper limit on the polarization degree of 8.5% (90% c.l.).We warmly thank the Directors and the Science Operations Team of IXPE, NICER, NuSTAR, and XMM for promptly scheduling the observations reported here and Bas Dorsman, Matteo Bachetti and Anna Watts for useful discussions. The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2022-13-I.0, agreements ASI-INAF-2022-19-HH.0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). NICER is a 0.2–12 keV X-ray telescope operating on the International Space Station, funded by NASA. The NuSTAR mission is a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. Data analysis was performed using the NuSTAR Data Analysis Software (NuSTARDAS), jointly developed by the ASI Science Data Center (SSDC, Italy) and the California Institute of Technology (USA). XMM-Newton is an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. MN and this work were supported by NASA under grant 80NSSC24K1175. AP, GI, FA, RLP, CM and LS are supported by INAF Research Grant FANS and the Italian Ministry of University and Research PRIN 2020 Grant 2020BRP57Z (GEMS). AP acknowledges support from the Fondazione Cariplo/CDP, grant no. 2023-2560. JP thanks the Ministry of Science and Higher Education grant 075-15-2024-647 for support. AB acknowledges support from the Finnish Cultural Foundation grant 00240328. MCB acknowledges support from the INAF-Astrofit fellowship. FCZ is supported by a Ramón y Cajal fellowship (grant agreement RYC2021-030888-I). TS acknowledges support from ERC Consolidator Grant No. 865768 AEONS (PI: Watts).Peer reviewe
A model to interpret pulse phase shifts in AMXPs: SAX J1808.4-3658 as a proof of concept
No full textObservational evidences of erratic 1st harmonic pulse phase shifts in accreting millisecond X-ray pulsars pulse phase evolution was reported by several authors. This effect always go together with much more stable 2nd harmonics pulse phase delays. Different possible explanations of these phase shifts have been given in literature. But all these interpretations do not explain why the 2nd harmonic are more stable than the 1st harmonic. The explanation of such a behaviour is of fundamental importance in order to gain an insight on the NS rotational behaviour and to remove the still present interpretative ambiguity on the results of timing analysis. We propose a simple toy-model to interpret these phenomena as effect of small variations of the ratio between the fluxes from the two hot-spots. We show how it affects the phase of the overall pulse profile, when the two amplitudes are similar and the two hot-spots are not perfectly antipodal. Using this model, we give a detailed explanation of the phase jump observed in the 2002 outburst of SAX J1808.4-3658
The Equation of State of Neutron Star Matter
No full textNeutron stars are remarkable natural laboratories that allow us to investigate the fundamental constituents of matter and their interactions under extreme conditions that cannot be reproduced in terrestrial laboratories. This chapter gives a brief pedagogical introduction to the physics of matter at very high densities (i.e. up to several times the density of atomic nuclei) that hopefully could be useful to researchers in pulsars’ astrophysics and related areas
The pulse profile and spin evolution of the accreting pulsar in Terzan 5, IGR J17480-2446, during its 2010 outburst
No full textWe analyse the spectral and pulse properties of the 11 Hz transient accreting pulsar, IGR J17480-2446, in the globular cluster Terzan 5, considering all the available Rossi X-Ray Timing Explorer, Swift and INTEGRAL observations performed during the outburst shown between 2010 October and November.
By measuring the pulse phase evolution we conclude that the neutron star (NS) spun up at an average rate of ? Hz s-1, compatible with the accretion of the Keplerian angular momentum of matter at the inner disc boundary. This confirms the trend previously observed by Papitto et al., who considered only the first few weeks of the outburst. Similar to other accreting pulsars, the stability of the pulse phases determined by using the second harmonic component is higher than that of the phases based on the fundamental frequency. Under the assumption that the second harmonic is a good tracer of the NS spin frequency, we successfully model its evolution in terms of a luminosity-dependent accretion torque. If the NS accretes the specific Keplerian angular momentum of the in-flowing matter, we estimate the inner disc radius to lie between 47 and 93 km when the luminosity attains its peak value. Smaller values are obtained if the interaction between the magnetic field lines and the plasma in the disc is considered.
The phase-averaged spectrum is described by thermal Comptonization of photons with energy of ≈1 keV. A hard to soft state transition is observed during the outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an electron temperature of ≈20 keV towards an optically denser cloud at kTe≈ 3 keV. At the same time, the pulse amplitude decreases from 27 per cent to few per cent, as already noted by Papitto et al., and becomes strongly energy dependent. We discuss various possibilities to explain such a behaviour, proposing that at large accretion luminosities a significant fraction of the in-falling matter is not channelled towards the magnetic poles, but rather accretes more evenly on to the NS surface
XMM-Newton gives spectral modeling and updated ephemeris of the Accreting Millisecond Pulsar IGR J17511-3057
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