arXiv subjects

Dany Page

Publications and source records attributed to Dany Page.

70 records · Page 4Linked to original sources

arXiv2002

Thermal Evolution and Light Curves of Young Bare Strange Stars

The cooling of a young bare strange star is studied numerically by solving the equations of energy conservation and heat transport for both normal and superconducting strange quark matter inside the star. We show that the thermal luminosity from the strange star surface, due to both photon emission and e+e- pair production, may be orders of magnitude higher than the Eddington limit, for about one day for normal quark matter but possibly for up to a hundred years for superconducting quark matter, while the maximum of the photon spectrum is in hard X-rays with a mean energy of ~ 100 keV or even more. This differs both qualitatively and quantitatively from the photon emission from young neutron stars and provides a definite observational signature for bare strange stars. It is shown that the energy gap of superconducting strange quark matter may be estimated from the light curves if it is in the range from ~ 0.5 MeV to a few MeV.

astro-ph↗

arXiv2001

Neutron star cooling in transiently accreting low mass binaries: a new tool for probing nuclear matter

We explore, using an exact cooling code, the thermal evolution of a neutron star undergoing episodes of intense accretion, alternated by long periods of quiescence (e.g. Soft X-Ray Transients). We find that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. In the case of Cen X-4 strong neutrino emission from the inner core is necessary to explain the observation: this may indicate that the neutron star of Cen X-4 is heavier than 1.4 Msun. This study opens the possibility of using the quiescent emission of Soft X-Ray Transients as a tool for probing the core superfluidity in relation to the mass of the neutron star.

astro-ph↗

arXiv2000

Charting the temperature of the Hot Neutron Star in a Soft X-ray Transient

We explore the thermal evolution of a neutron star undergoing episodes of intense accretion, separated by long periods of quiescence. By using an exact cooling code we follow in detail the flow of heat in the star due to the time-dependent accretion-induced heating from pycno-nuclear reactions in the stellar crust, to the surface photon emission, and the neutrino cooling. These models allow us to study the neutron stars of the Soft X-Ray Transients. In agreement with Brown, Bildsten and Rutledge (1998) we conclude that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522, and of the recently discovered SAX J1808.4, can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. However, we show that in the case of Cen X-4, despite its long recurrence time, strong neutrino emission from the neutron star inner core is necessary to understand the observed low ratio of quiescent to outburst luminosity. This result implies that the neutron star in Cen X-4 is heavier than the one in the other systems and the pairing critical temperature in its center must be low enough (well below 10^9 K) to avoid a strong suppression of the neutrino emission.

astro-ph↗

arXiv2000

Prospects of Detecting Baryon and Quark Superfluidity from Cooling Neutron Stars

Baryon and quark superfluidity in the cooling of neutron stars are investigated. Observations could constrain combinations of the neutron or Lambda-hyperon pairing gaps and the star's mass. However, in a hybrid star with a mixed phase of hadrons and quarks, quark gaps larger than a few tenths of an MeV render quark matter virtually invisible for cooling. If the quark gap is smaller, quark superfluidity could be important, but its effects will be nearly impossible to distinguish from those of other baryonic constituents.

hep-ph↗

arXiv1999

Period clustering of the Anomalous X-ray Pulsars and magnetic field decay in magnetars

We confront theoretical models for the rotational, magnetic, and thermal evolution of an ultra-magnetized neutron star, or magnetar, with available data on the Anomalous X-ray Pulsars (AXPs). We argue that, if the AXPs are interpreted as magnetars, their clustering of spin periods between 6 and 12 seconds, observed at present in this class of objects, their period derivatives, their thermal X-ray luminosities, and the association of two of them with young supernova remnants, can only be understood globally if the magnetic field in magnetars decays significantly on a time scale of the order of 10^4 years.

astro-ph↗

arXiv1998

Thermal Evolution of Isolated Neutron Stars. II

After a brief summary of neutron star cooling theory I present results which emphasize the importance of baryon pairing in the neutron star core. I show how the thermal evolution may be totally controlled by pairing for models which include only nucleons and models containing also hyperons. Finally, I consider the thermal evolution of ultramagnetized neutron stars whose existence has been inferred from the soft gamma repeaters.

astro-ph↗

arXiv1997

Thermal Evolution of Isolated Neutron Stars

I give a general overview of the theory of neutron star cooling, emphasizing the intuitive understanding of the effects of the various physical ingredients, including the recently proposed accreted envelopes and neutrino emission from Cooper pair breaking and formation. I describe how the present data may be compatible with both the `standard model' and the fast cooling scenarios, with or without invoking extensive presence of baryon pairing.

astro-ph↗

arXiv1996

Fast Cooling of Neutron Stars: Superfluidity vs. Heating and Accreted Envelope

It is generally considered that the neutron star cooling scenarios involving fast neutrino emission, from a kaon or pion condensate, quark matter, or the direct Urca process, require the presence of baryon pairing in the central core of the star to control the strong neutrino emission and produce surface temperatures compatible with observations. I show here that within the kaon condensate scenario pairing is not necessary if: 1) the equation of state is stiff enough for the star to have a thick crust in which sufficient friction can occur to heat the star and 2) a thin layer, of mass larger than 10^{-12} Msol, of light elements (H and He) is present at the stellar surface. The effect of the light elements is to increase the heat flow and thus produce a higher surface temperature. Both the occurrence of heating and the presence of H and/or He at the surface (deposited during the late post-supernova accretion) can possibly be confirmed or infirmed by future observations.

astro-ph↗

arXiv1996

Strange stars: Which is the ground stage of QCD at finite baryon number

Witten's conjecture about strange quark matter (`Strange Matter') being the ground state of QCD at finite baryon number is presented and stars made of strange matter (`Strange Stars') are compared to neutron stars. The only observable way in which a strange star differs from a neutron star is in its early thermal history and a detailed study of strange star cooling is reported and compared to neutron star cooling. One concludes that future detection of thermal radiation from the compact object produced in the core collapse of SN 1987A could present the first evidence for strange matter.

astro-ph↗

arXiv1996

Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. II

We complete our study of pulsars' non-uniform surface temperature and of its effects on their soft X-ray thermal emission. Our previous work had shown that, due to gravitational lensing, dipolar fields cannot reproduce the strong pulsations observed in Vela, Geminga, PSR 0656+14, and PSR 1055-52. Assuming a standard neutron star mass of 1.4 Msol, we show here that the inclusion of a quadrupolar component, if it is suitably oriented, is sufficient to increase substantially the pulsed fraction, Pf, up to, or above, the observed values if the stellar radius is 13 km or even 10 km. For models with a radius of 7 km the maximum pulsed fraction obtainable with (isotropic) blackbody emission is of the order of 15% for orthogonal rotators (Vela, Geminga and PSR 1055-52) and only 5% for an inclined rotator as PSR 0656+14. Given the observed values this indicates that the neutron stars in Geminga and PSR 0656+14 have radii significantly larger than 7 km and, given the very specific quadrupole components required to increase Pf, even radii of the order of 10 km may be unlikely in all four cases. We confirm our previous finding that the pulsed fraction always increases with photon energy, below about 1 keV, when blackbody emission is used and show that it is due to the hardenning of the blackbody spectrum with increasing temperature. The observed decrease of pulsed fraction may thus suggest that the emitted spectrum softens with increasing temperature. Finally, we apply our model to reassess the magnetic field effect on the outer boundary condition used in neutron star cooling models and show that, in contradistinction to several previous claims, it is very small and most probably results in a slight reduction of the heat flow through the envelope.

astro-ph↗

arXiv1996

Surface Temperature of Magnetized Neutron Stars

We show that the expected inhomogeneous temperature distribution induced at the surface of a neutron star by the anisotropy of heat transport in the magnetized envelope allows us to understand quite well the observed pulse profiles of the four nearby pulsars for which surface thermal emission has been detected. However, due to gravitational lensing, dipolar magnetic fields are not adequate and the observed high pulsed fractions force us to include a quadrupolar component.

astro-ph↗

arXiv1996

Temperature, distance and cooling of the Vela pulsar

We use models of magnetized hydrogen atmospheres to fit the soft X-ray thermal spectrum of the Vela pulsar obtained by ROSAT. The distance and hydrogen column density required by our fits are in good agreement with other independents estimates and can be considered as an argument for the presence of hydrogen at the surface of this pulsar. The low temperature obtained, T = 7.85 +/- 0.25 10^5 K, is below the predictions of the `standard' model of neutron star cooling and strengthens previous claims for the necessity of the occurrence of fast neutrino cooling in this neutron star.

astro-ph↗

arXiv1995

GEMINGA'S SOFT X-RAY EMISSION AND THE STRUCTURE OF ITS SURFACE

We present a model to explain the decrease in the amplitude of the pulse profile with increasing energy observed in Geminga's soft X-ray surface thermal emission. We assume the presence of plates surrounded by a surface with very distinct physical properties: these two regions emit spectra of very distinct shapes which present a crossover, the warm plates emitting a softer spectrum than the colder surrounding surface. The strongly pulsed emission from the plates dominates at low energy while the surroundings emission dominates at high energy, producing naturally a strong decrease in the pulsed fraction. In our illustrative example the plates are assumed to be magnetized while the rest of the surface is field free. This plate structure may be seen as a schematic representation of a continuous but very nonuniform distribution of the surface magnetic field or as a quasi realistic structure induced by past tectonic activity on Geminga.

astro-ph↗

arXiv1995

THERMAL RADIATION FROM MAGNETIZED NEUTRON STARS: A look at the Surface of a Neutron Star.

Surface thermal emission has been detected by ROSAT from four nearby young neutron stars. Assuming black body emission, the significant pulsations of the observed light curves can be interpreted as due to large surface temperature differences produced by the effect of the crustal magnetic field on the flow of heat from the hot interior toward the cooler surface. However, the energy dependence of the modulation observed in Geminga is incompatible with blackbody emission: this effect will give us a strong constraint on models of the neutron star surface.

astro-ph↗

arXiv1994

Surface Temperature of a Magnetized Neutron Star and Interpretation of the ROSAT Data. I. Dipolar Fields

We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observable X-ray spectra and light curves, taking fully into account gravitational lensing. Only dipolar fields are considered here. General features are studied and compared with the ROSAT data from the pulsars 0833-45 (Vela), 0656+14, 0630+178 (Geminga), and 1055-52. The composite blackbody spectra obtained are not very different from a black- body spectrum at the star's effective temperature: temperature estimates using single temperature models are practically equivalent to composite model results. Gravitational lensing strongly suppresses the light curve pulsations: this may be a new tool to constrain the size of neutron stars. Dipolar surface fields yield many features somewhat similar to the observed ones but are not sufficient for an accurate description. The pulsed fractions obtained in all our models increase with photon energy: the strong decrease observed in Geminga at energies 0.3 - 0.5 keV is definitely a genuine effect of the atmospheric magnetic field on the locally emitted spectrum in contradistinction to the magnetic effects on the surface temperature. A complete model will have to consider complex surface field structures and include effects of the atmospheric magnetic field on the emitted spectrum.

astro-ph↗

arXiv1994

Geminga: A Cooling Superfluid Neutron Star

We show that Geminga's surface temperature can be understood by both types of neutrino cooling scenarios, i.e, slow neutrino cooling by the modified Urca process or fast neutrino cooling by the direct Urca process or by some exotic matter, and thus does not allow us to discriminate between these two competing schemes. However, for both types of scenarios, agreement with the observed temperature can only be obtained if BARYON PAIRING IS PRESENT IN MOST, IF NOT ALL, OF THE CORE OF THE STAR. We also comment on the recent temperature estimates for PSR 0656+14 and PSR 1055-52, which pertain to the same photon cooling era. We argue though that observational evidence for the slow neutrino cooling model (the ``standard'' model) is in fact very dim and that the interpretation of the surface temperature of all neutron stars could be done with a reasonable theoretical a priori within the fast neutrino cooling scenarios only. In this case, Geminga, PSR 0656+14, and PSR 1055-52 all show evidence of baryon pairing down to their very centers. xxx----To be published in Ap.J. (June 10 isue).---xxx

astro-ph↗