117 research outputs found
On Low Mass X-ray Binaries and Millisecond Pulsar
The detection, in 1998, of the first Accreting Millisecond Pulsar, started an
exciting season of continuing discoveries in the fashinating field of compact
binary systems harbouring a neutron star. Indeed, in these last three lustres,
thanks to the extraordinary performances of astronomical detectors, on ground
as well as on board of satellites, mainly in the Radio, Optical, X-ray, and
Gamma-ray bands, astrophysicists had the opportunity to thoroughly investigate
the so-called Recycling Scenario: the evolutionary path leading to the
formation of a Millisecond Radio Pulsar. The most intriguing phase is certainly
the spin-up stage during which, because of the accretion of matter and angular
momentum, the neutron star accumulates an extraordinary amount of mechanical
rotational energy, up to one percent of its whole rest-mass energy. These
millisecond spinning neutron stars are truly extreme physical objects: General
and Special Relativity are fully in action, since their surfaces, attaining
speeds close to one fifth of the speed of light, are located just beyond their
Schwartzscild Radius, and electrodynamical forces, caused by the presence of
huge surface magnetic fields of several hundred million Gauss, display their
spectacular properties accelerating electrons up to such energies to promote
pair creation in a cascade process responsible for the emission in Radio and
Gamma-ray. The rotational energy is swiftly converted and released into
electromagnetic power which, in some cases, causes the neutron star to outshine
with a luminosity of one hundred Suns. In this paper I will review some of the
most recent discoveries on (accreting) millisecond pulsars.Comment: 7 pages, 1 table, proceedings of the conference: "Reading the book of
Globular Clusters with the lens of stellar evolution", Rome Astronomical
Observatory, 26-28 November 201
Quantum clock: A critical discussion on spacetime
We critically discuss the measure of very short time intervals. By means of a
Gedankenexperiment, we describe an ideal clock based on the occurrence of
completely random events. Many previous thought experiments have suggested
fundamental Planck-scale limits on measurements of distance and time. Here we
present a new type of thought experiment, based on a different type of clock,
that provide further support for the existence of such limits. We show that the
minimum time interval that this clock can measure scales as the
inverse of its size . This implies an uncertainty relation between
space and time: ; where G, and c
are the gravitational constant, the reduced Planck constant, and the speed of
light, respectively. We outline and briefly discuss the implications of this
uncertainty conjecture.Comment: 10 pages, published in Physical Review
Spectral changes during six years of Scorpius X-1 monitoring with BeppoSAX Wide Field Cameras
We analyse a sample of fifty-five observations of Scorpius X-1 available in
the BeppoSAX Wide Field Camera public archive and spanning over the six years
of BeppoSAX mission life. Spectral changes are initially analysed by inspection
of colour-colour and colour-intensity diagrams, we also discuss the shift of
the Z tracks in these diagrams. Then we select two long observations for
spectral fitting analysis, a secular shift is evident between the tracks in
these observations. We finally extract spectra along the tracks and discuss the
best fit model, the parameter variations along the track and between tracks,
and their link to the accretion rate.Comment: 6 pages, 11 postscrpt figures.To appear in the conference proceedings
of `Interacting Binaries: Accretion, Evolution & Outcomes' (Cefalu', July
4-10 2004
3D MHD Simulations of accreting neutron stars: evidence of QPO emission from the surface
3D Magnetohydrodynamic simulations show that when matter accretes onto
neutron stars, in particular if the misalignment angle is small, it does not
constantly fall at a fixed spot. Instead, the location at which matter reaches
the star moves. These moving hot spots can be produced both during stable
accretion, where matter falls near the magnetic poles of the star, and unstable
accretion, characterized by the presence of several tongues of matter which
fall on the star near the equator, due to Rayleigh-Taylor instabilities.
Precise modeling with Monte Carlo simulations shows that those movements could
be observed as high frequency Quasi Periodic Oscillations. We performed a
number of new simulation runs with a much wider set of parameters, focusing on
neutron stars with a small misalignment angle. In most cases we observe
oscillations whose frequency is correlated with the mass accretion rate
. Moreover, in some cases double QPOs appear, each of them showing the
same correlation with .Comment: 2 pages, 1 figure, to appear in the Proceedings of the Bologna x-ray
conference 2009, uses aipproc.cls, aip-6s.clo,
Neutron stars with submillisecond periods: a population of high mass objects?
Fast spinning neutron stars, recycled in low mass binaries, may have accreted
a substantial amount of mass. The available relativistic measurements of
neutron star masses, all clustering around 1.4 M_sun, however refer mostly to
slowly rotating neutron stars which accreted a tiny amount of mass during
evolution in a massive binary system. We develop a semi-analytical model for
studying the evolution of the spin period P of a magnetic neutron star as a
function of the baryonic mass load M_{ac}; evolution is followed down to
submillisecond periods and the magnetic field is allowed to decay significantly
before the end of recycling. We use different equations of state and include
rotational deformation effects, the presence of a strong gravitational field
and of a magnetosphere. For the non-magnetic case, comparison with numerical
relativistic codes shows the accuracy of our description. The minimum accreted
mass requested to spin-up a magnetized 1.35M_sun-neutron star at a few
millisecond is 0.05 M_sun, while this value doubles for an unmagnetized neutron
star. Below 1 millisecond the request is of at least 0.25 M_sun. There may
exist a yet undetected population of massive submillisecond neutron stars. The
discovery of a submillisecond neutron star would imply a lower limit for its
mass of about 1.7M_sun.Comment: To appear in the Astrophysical Journal, June 199
Radio ejection in the evolution of X-ray binaries: the bridge between low mass X-ray binaries and millisecond pulsars
We present a scenario for the spin-up and evolution of binary millisecond
pulsars. This can explain the observational properties of the recently
discovered binary millisecond pulsar PSR J1740-5340, with orbital period 32.5
hrs, in the Globular Cluster NGC 6397. The optical counterpart of this system
is a star as luminous as the cluster turnoff stars, but with a lower Teff (a
larger radius) which we model with a star of initial mass compatible with the
masses evolving in the cluster (~0.85 Msun). This star has suffered Roche lobe
overflow while evolving off the main sequence, spinning up the neutron star to
the present period of 3.65 ms. There are evidences that at present, Roche lobe
overflow is still going on. Indeed Roche lobe deformation of the mass losing
component is necessary to be compatible with the optical light curve. The
presence of matter around the system is also consistent with the long lasting
irregular radio eclipses seen in the system. We propose that this system is
presently in a phase of `radio-ejection' mass loss. The radio-ejection phase
can be initiated only if the system is subject to intermittency in the mass
transfer during the spin-up phase. In fact, when the system is detached the
pulsar radio emission is not quenched, and may be able to prevent further mass
accretion due to the action of the pulsar pressure at the inner Lagrangian
point.Comment: 6 pages, including 3 figures. To appear in the proceedings of the
XXII Moriond Astrophysics Meeting "The Gamma-Ray Universe" (Les Arcs, March
9-16, 2002), eds. A. Goldwurm, D. Neumann, and J. Tran Thanh Van, The Gioi
Publishers (Vietnam
Discovery of hard phase lags in the pulsed emission of GRO J1744-28
We report on the discovery and energy dependence of hard phase lags in the
2.14 Hz pulsed profiles of GRO J1744-28. We used data from XMM-Newton and
NuSTAR. We were able to well constrain the lag spectrum with respect to the
softest (0.3--2.3 keV) band: the delay shows increasing lag values reaching a
maximum delay of 12 ms, between 6 and 6.4 keV. After this maximum, the
value of the hard lag drops to 7 ms, followed by a recovery to a plateau at 9
ms for energies above 8 keV. NuSTAR data confirm this trend up to 30 keV, but
the measurements are statistically poorer, and therefore, less constraining.
The lag-energy pattern up to the discontinuity is well described by a
logarithmic function. Assuming this is due to a Compton reverberation
mechanism, we derive a size for the Compton cloud 120
, consistent with previous estimates on the magnetospheric radius.
In this scenario, the sharp discontinuity at 6.5 keV appears difficult
to interpret and suggests the possible influence of the reflected component in
this energy range. We therefore propose the possible coexistence of both
Compton and disk reverberation to explain the scale of the lags and its energy
dependence.Comment: Accepted for publication in MNRAS Letters on 2016 June 0
A Complex Environment around Circinus X-1
We present the results of an archival 54 ks long Chandra observation of the peculiar source Cir X-1 during the phase passage 0.223-0.261. We focus on the study of detected emission and absorption features using the HETGS. A comparative analysis of X-ray spectra, selected at different flux levels of the source, allows us to distinguish between a very hard state, at a low count rate, and a brighter, softer, highly absorbed spectrum during episodes of flaring activity. The spectrum of the hard state clearly shows emission lines of highly ionized elements, while, during the flaring state, the spectrum also shows strong resonant absorption lines. The most intense and interesting feature in this latter state is present in the Fe K alpha region: a very broadened absorption line at energies similar to 6.5 keV that could result from a smeared blending of resonant absorption lines of moderately ionized iron ions (Fe XX-Fe XXIV). We also observe strong resonant absorption lines of Fe XXV and Fe XXVI, together with a smeared absorption edge above 7 keV. We argue that the emitting region during the quiescent/hard state is constituted of a purely photoionized medium, possibly present above an accretion disk, or of a photoionized plasma present in a beamed outflow. During the flaring states the source undergoes enhanced turbulent accretion that modifies both the accretion geometry and the optical depth of the gas surrounding the primary X- ray source
A method to constrain the neutron star magnetic field in Low Mass X-ray Binaries
We describe here a method to put an upper limit to the strength of the magnetic field of neutron stars in low mass X‐ray binaries for which the spin period and the X‐ray luminosity during X‐ray quiescent periods are known. This is obtained using simple considerations about the position of the magnetospheric radius during quiescent periods. We applied this method to the accreting millisecond pulsar SAX J1808.4‐3658, which shows coherent X‐ray pulsations at a frequency of ∼ 400 Hz and a quiescent X‐ray luminosity of ∼ 5 × 1031 ergs/s, and found that B ⩽ 5 × 108 Gauss in this source. Combined with the lower limit inferred from the presence of X‐ray pulsations, this constrains the SAX J1808.4‐3658 neutron star magnetic field in the quite narrow range (1 – 5) × 108 Gauss. Similar considerations applied to the case of Aql X‐1 and KS 1731‐260 give neutron star magnetic fields lower than ∼ 109 Gauss
The Zoo of emission lines in the spectrum of Cir X-1 observed by XMM-Newton
We present the preliminary analysis of a 10 ks XMM-Newton EPIC/pn observation of Cir X-1 immediately after the zero phase. The continuum emission is modeled using a blackbody component partially absorbed by neutral matter probably located around the binary system. We detect a forest of emission lines associated to highly ionized ions
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