Thermal emission from radio-quiet Isolated Neutron Stars: probing the central compact object

Neutron stars are numerous but observationally very diverse. In particular, direct emission from the neutron star surface layers (the only source of information on the physical conditions of the star) is only accessible in a handful of sources. This is because the surface of old neutron stars (age >10^6 yr) is too cold to emit X-rays, while the radiation from young active radio pulsars (age < 10^4 yr) is dominated by non-thermal emission from the magnetosphere surrounding the star. In the last decade ROSAT satellite observations led to the discovery of seven dim sources, which were later associated with radio-silent, isolated, middle-age (age ~10^5-10^6 yr) neutron stars. They are characterized by clean thermal emission at energies of about 0.1 keV without any trace of contamination from a surrounding supernova remnant or magnetospheric activity. They are located within a few hundred pc, close enough to be studied with the latest X-ray orbiting observatories and therefore represent important targets for the study of neutron star surface emission (see Treves, Zane et al., 2000 for a review).

These dim isolated neutron stars (DINSs) are key objects in compact object astrophysics. They offer a unique laboratory for investigating the properties of matter under extreme conditions, such as the equation of state at supra-nuclear densities, or the interaction of highly relativistic plasmas with radiation in the presence of Giga- or Tera-Gauss magnetic fields. Detailed X-ray spectra of DINSs have been recently obtained with Chandra and XMM-Newton, and show quite unexpected characteristiscs. The prototype of the class, RX J1856.5-3754, exhibits a featureless spectrum extremely close to a pure blackbody. Broad absorption features have been detected in four pulsating sources with evidence of a spectral variation with phase. Very recently, spectral evolution on timescale of ~yrs have been reported for the second most luminous source, RX J0720.4-3125. In addition, when detected the optical counterpart lies a factor ~5-10 above the extrapolation of the X-ray blackbody at optical wavelenghts. All these new findings represent a challenge for conventional atmospheric models, typically based on surface temperature distributions induced by a dipolar magnetic field.

MSSL's research is aimed to determine the fundamental physical parameters of isolated neutron stars, including the star mass, radius and magnetic field. We have an extensive programme of neutron star studies, which combines observations from the latest, sensitive X-ray observatories with a long-standing expertise in the development of sophisticated radiative transfer codes. Particular emphasis is on the study of high-energy, highly magnetized plasmas. By fitting the thermal spectrum with atmospheric models, we can measure the star mass and radius, probing the equation of state at supra-nuclear densities and the superfluid properties of the stars interior. Also, the presence/absence of absorption edges and lines provides information on the chemical composition and magnetic field strength of the star, further constraining our theoretical models.

Some of our recent results are summarized below.

1) We observe pulsations at the neutron star spin period in a few members of the class of DINSs. This allows an accurate timing analysis and pulse-phase spectroscopy to be carried on, revealing among other things the link between magnetic field decay and spin evolution of isolated pulsars: one of the major unresolved issues in compact object astrophysics. We lead timing studies of the brightest of such pulsating neutron stars, RXJ0720.4-3125. To monitor its spin period evolution, we have incorporated 7 years of data from Rosat, BeppoSAX, Chandra and XMM-Newton, obtaining the first estimate of the period derivative and hence of the star magnetic field (Zane et al., 2002, Cropper et al., 2004). RXJ0720.4-3125 is a star is of uppermost importance, since it is one of the few that shows a possible proton cyclotron line at low energies (Cropper et al., 2001, Treves, Zane et al., 2000, Haberl et al., 2004): by studying the phase-dependent spectral variation of such a line we can derive the morphology of the magnetic field. Features of the same kind are also observed in two of the faintest objects which show pulsations, RXJ 0420.0-5022 and RXJ 0806.4-4123 (Haberl et al., 2004).

2) We are currently analyzing XMM-Newton observations aimed at an accurate spectral and timing analysis of the isolated neutron star RBS1774. This source was identified only last year, and so far we know only the barest minimum about it. Since the small number of well-studied sources seriously hinders our overall understanding of isolated neutron stars, it is very important to make deep exposures of each new member.

These observations are comlemented with theoretical studies of several aspects of pulsar emission models

3) We are modelling the variation of the cyclotron line with spin phase, by constructing spectral models at different viewing angles and following the ray tracing in the neutron star gravitational field (Zane, Turolla, et al., 2004 in preparation. See also the presentation given by Silvia Zane at the 35th Cospar Symposium, Paris, July 2004). This will provide a powerful numerical tool, which we will make available for fitting pulse-phase resolved lines in XSPEC (the standard X-ray spectral fitting package). Models computed at high field strengths will also be applied to the cyclotron lines observed in soft gamma-ray repeaters and anomalous X-ray pulsars.

4) We have computed the emissivity of the neutron star crust, incorporating the electron-phonon interactions between electro-magnetic radiation and ion lattice. These models can shed light on whether or not the absence of spectral lines observed in the coldest neutron stars is due to a phase transition of their atmospheric gaseous layers into a condensate (Turolla et al., 2004 ). Results can also been used to compute the refractive index of radiation propagating within the crust and thus to upgrade the calculation of the photon thermal conductivities of ultra-magnetized neutron stars (see Potekhin et al., 2003). The ultimate goal is an accurate determination of the cooling and evolutionary history of such objects.

Our recent publications on the subject include the following:

1) Haberl et al., 2004 The isolated neutron star X-ray brulsars RX J0420.0-5022 and RX J0806.4-4123: new X-ray and obrtical observation
2) Cropper et al., 2004 Timing Analysis of the Isolated Neutron Star RX J0720.4-3125 Revisited
3) Turolla et al., 2004 Bare Quark Stars or Naked Neutron Stars ? The Case of RX J1856.5-3754
4) Zane et al, 2004, Is RX J1856.5-3754 a naked neutron star ? Advances in Space Research, Volume 33, Issue 4, p. 531-536
5) Zane et al., 2002 RX J1856.5-3754: Bare Quark Star or Naked Neutron Star ?
6) Zane et al. 2002, Timing study of the isolated neutron star RX J0720.4-3125
7) Zane et al., 2002 Timing analysis of the isolated neutron star RX J0720.4-3125
8) Zane et al., 2001 Proton Cyclotron Features in Thermal Spectra of Ultra-magnetized Neutron Stars
9) Pearels, et al., 2001 First XMM-Newton observations of an isolated neutron star: RXJ0720.4-3125
10) Cropper, et al., 2001 Modelling the spin pulse profile of the isolated neutron star RX J0720.4--3125 observed with XMM-Newton
11) Zane, et al., 2000 Magnetized Atmospheres around Accreting Neutron Stars
12) Treves, et al., 2000 Isolated Neutron Stars: Accretors and Coolers"

You can see the links to some presentations we gave at various international meetings and get the powerpoint files from here