arXiv subjects

Monica Colpi

Publications and source records attributed to Monica Colpi.

105 records · Page 6Linked to original sources

arXiv2000

Spin and magnetism in old neutron stars

The thermal, spin and magnetic evolution of neutron stars in the old low mass binaries is first explored. Recycled to very short periods via accretion torques, the neutron stars lose their magnetism progressively. If accretion proceeds undisturbed for 100 Myrs these stars can rotate close to break up with periods far below the minimum observed of 1.558 ms. We investigate their histories using population synthesis models to show that a tail should exist in the period distribution below 1.558 ms. The search of these ultrafastly spinning neutron stars as pulsars can help discriminating among the various equations of state for nuclear matter, and can shed light into the physics of binary evolution. The evolution of isolated neutron stars in the Galaxy is explored beyond the pulsar phase. Moving through the tenuous interstellar medium, these old solitary neutron stars lose their rotational energy. Whether also their magnetism fades is still a mystery. A population synthesis model has revealed that only a tiny fraction of them is able to accrete from the interstellar medium, shining in the X-rays. There is the hope that these solitary stars will eventually appear as faint sources in the Chandra sky survey. This might give insight on the long term evolution of the magnetic field in isolated objects.

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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.

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arXiv2000

Tidal stirring and the origin of dwarf spheroidals in the Local Group

N-Body/SPH simulations are used to study the evolution of dwarf irregular galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on plunging orbits. We propose a new dynamical mechanism driving the evolution of gas rich, rotationally supported dIrrs, mostly found at the outskirts of the Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs) or dwarf ellipticals (dEs), observed to cluster around the two giant spirals. The initial model galaxies are exponential disks embedded in massive dark matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the pericenter of their orbit partially strip their halo and disk and trigger dynamical instabilities that dramatically reshape their stellar component. After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This evolutionary mechanism naturally leads to the morphology-density relation observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest and most dark matter dominated among LG dSphs. If disks include a gaseous component, this is both tidally stripped and consumed in periodic bursts of star formation. The resulting star formation histories are in good qualitative agreement with those derived using HST color-magnitude diagrams for local dSphs.

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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.

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arXiv1999

Isolated Neutron Stars: Accretors and Coolers

As many as $10^9$ neutron stars populate the Galaxy, but only $\approx 10^3$ are directly observed as pulsars or as accreting sources in X-ray binaries. In principle also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at $\sim 200-400 {\rm km s}^{-1}$, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales $\sim 10^8-10^9$ yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes.

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arXiv1999

Recycling NSs to ultrashort periods: a statistical analysis of their evolution in the "magnetic moment vs period" plane

We investigate the statistical evolution of magnetic neutron stars recycled in Low Mass Binary (LMB) systems, simulating synthetic populations. Irrespective to the details of the physical models, we find to be significant the fraction of neutron stars spinning close to their mass shedding limit relative to the millisecond pulsar population. The simulated neutron stars show a tail in their period distribution at periods shorter than 1.558 ms, the minimum detected so far. Crustal magnetic field decay models predict also the existence of massive rapidly spinning neutron stars with very low magnetic moment.

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arXiv1999

Recycling neutron stars to ultra short periods: a statistical analysis of their evolution in the mu-P plane

We investigate the statistical evolution of magnetic neutron stars, recycled in binary systems, simulating synthetic populations. To bracket uncertainties, we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear matter and explore the hypothesis that the magnetic field is confined in the stellar crust. We follow the magneto-rotational evolution within a simple recycling scenario, including the possibility of magnetospheric propeller. We find the presence of a tail in the period distribution of the synthetic populations at periods shorter than 1.558 ms, the minimum detected so far. For the soft EoS the recycling gives rise to a spin distribution which is increasing monotonically toward short periods and a clear ``barrier'' forms at the minimum period for the onset of mass shedding. For the stiff EoS the distribution is flatter displaying a broad maximum about 2-4 ms. The estimated fraction of neutron stars spinning close to their shedding limit over the millisecond pulsar population is found to be significant. Crustal magnetic field decay models predict the existence of massive (M>1.4 M_sun) rapidly spinning neutron stars with very low magnetic moment.

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arXiv1999

Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response

The evolution of a small satellite inside a more massive truncated isothermal spherical halo is studied using both the Theory of Linear Response for dynamical friction and N-Body simulations. The analytical approach includes the effects of the gravitational wake, of the tidal deformation and the shift of the barycenter of the primary, so unifying the local versus global interpretation of dynamical friction. Sizes, masses, orbital energies and eccentricities are chosen as expected in hierarchical clustering models. We find that in general the drag force in self-gravitating backgrounds is weaker than in uniform media and that the orbital decay is not accompanied by a significant circularization. We also show that the dynamical friction time scale is weakly dependent on the initial circularity. We provide a fitting formula for the decay time that includes the effect of mass and angular momentum loss. Live satellites with dense cores can survive disruption up to an Hubble time within the primary, notwithstanding the initial choice of orbital parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A and Fornax, have already suffered mass stripping and, with their present masses, the sinking times exceed 10 Gyr even if they are on very eccentric orbits.

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arXiv1999

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.

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arXiv1999

The morphological evolution of galaxy satellites

We study the evolution of galaxy satellites with high resolution N-body simulations. Satellites are modeled as replicas of typical low and high surface brightness galaxies (LSBs and HSBs). Encounters on high eccentricity orbits (as typical in hierarchical models of galaxy formation) strip LSBs of most of their stars and tend to decrease their surface brightness. On the contrary, bar instability in HSBs leads to substantial loss of angular momentum of the stellar component and to an increase of central surface brightness. In both cases the remnant resembles a spheroidal galaxy with an exponential surface brightness profile. A simple modeling of color evolution and interactions driven star formation gives M/L ratios for the remnants that are roughly consistent with observations. These results suggest an evolutionary scenario for the dwarf galaxies in our Local Group, faint dSphs being the descendents of LSBs and brighter dSphs/dEs being the final state of HSB satellites.

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arXiv1998

Will a Black Hole Soon Emerge from SN 1997D ?

Observations combined with theoretical modeling of the light curve of the recently discovered supernova 1997D in NGC 1536 suggest that it might host a black hole formed in the aftermath of the explosion. We consider some observable consequences of a black hole in SN 1997D and estimate the late--time accretion luminosity of the material which falls back onto the hole. We find that this luminosity, decaying with a characteristic power--law dependence on time, may emerge above the emission of the envelope in just a few years. Its detection would thus provide unmistakable evidence for the presence of a black hole.

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arXiv1998

Supernova Fallback and the Emergence of a Black Hole

We present the first fully relativistic investigation of matter fallback in a supernova. We investigate spherically symmetric supernova fallback using a relativistic radiation hydrodynamics Lagrangian code that handles radiation transport in all regimes. Our goal is to answer the fundamental question: did SN1987A produce a black hole and, if so, when will the hole become detectable ? We compute the light curve, assuming that a black hole has been formed during the explosion, and compare it with the observations. Our preliminary calculations lack radioactive energy input and adopt a very simple chemical composition (pure hydrogen). As a result, our computed models cannot fit the observed data of SN1987A in detail. Nevertheless, we can show that, during the first hours, the accretion flow is self--regulated and the accretion luminosity stays very close to the Eddington limit. The light curve is completely dominated, during the first few weeks, by the emission of the stellar envelope thermal energy, and resembles that obtained in ``standard'' supernova theory. Only long after hydrogen recombination takes place is there even a chance to actually detect radiation from the accreting black hole above the emission of the expanding envelope. The presence of a black hole is thus not inconsistent with observations to date. Because of the exponential decay of the $^{44}$Ti radioactive heating rate, the date of the emergence of the black hole is not very sensitive to the actual parameters of the models and turns out to be about 1000 years. The bulk of the emission then is expected to be in the visible band, but will be unobservable with present instrumentation. We discuss the implications of our results in connection with the possible emergence of a black hole in other supernovae.

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arXiv1998

The Emergence of a Black Hole in Supernova Explosions

We present results from a fully relativistic investigation of matter fallback in a supernova, assuming that the central compact star is a black hole (BH). Our goal is to answer the following questions: can we infer from the light curve whether a stellar black hole (BH) has formed in the aftermath of a supernova explosion ? Did SN1897A produce a BH and, if so, when will the hole become detectable ? We show that it should not be possible to infer the presence of a BH in the remnant for another several hundred to a thousand years. In the case of SN1987A, we estimate the BH luminosity to be $L\simeq 5\times 10^{34}$ erg s$^{-1}$, which is well below the present day bolometric luminosity of the remnant ($\sim 10^{36}erg/s$; Suntzeff 1997).

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arXiv1995

OLD ISOLATED ACCRETING NEUTRON STARS: CONTRIBUTION TO THE SOFT X--RAY BACKGROUND IN THE 0.5--2 KEV BAND

The issue of the observability of Old Isolated Neutron Stars (ONSs) accreting from the interstellar gas, is reconsidered using the spectra presented in Zampieri {\it et al.\/} (1995). In particular, we focus our attention on the overall soft X--ray emission of ONSs, which may provide a substantial contribution to the X--ray background in the 0.5--2 keV band. It is found that if accretion is channeled into polar caps by a magnetic field of $\sim 10^9$ G, up to $\sim$ $12-25 \%$ of the unresolved soft excess observed at high latitudes in the X--ray background can be explained in terms of emission from accreting isolated ONSs. However, this would imply the detection with ROSAT of $\sim 10$ sources deg$^{-2}$ above a threshold of $\sim 10^{-14} \, {\rm erg\, cm^{-2} s^{-1}}$. A brief reconsideration of the observability of isolated ONSs as individual X--ray sources is also presented.

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arXiv1994

Spherical Accretion onto Neutron Stars Revisited: Are Hot Solutions Possible ?

Stationary, spherical accretion onto an unmagnetized neutron star is here reconsidered on the wake of the seminal paper by Zel'dovich \& Shakura (1969). It is found that new ``hot'' solutions may exist for a wide range of luminosities. These solutions are characterized by a high temperature, $10^{9}÷10^{11}$ K, and arise from a stationary equilibrium model where the dominant radiative mechanisms are multiple Compton scattering and bremsstrahlung emission. For low luminosities, $\mincir 10^{-2} \ L_{E}$, only the ``cold'' (à la Zel'dovich and Shakura) solution is present.

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