Isolated Neutron Stars

Figure 1:XMM EPIC MOS image of RBS 1774 in small window mode.

In the last decade, the detection of emission from isolated neutron star (INSs) in the IR, optical and X-ray bands unveiled a complex and diverse phenomenology. In particular, the very interesting class of radio-quiet INSs include sources with extremely different properties, from the dim soft X-ray emitters discovered with ROSAT to the soft gamma-ray repeaters, whose giant flares are the most energetic transient events ever observed in the Galaxy. The ROSAT INSs are characterized by blackbody-like emission at 100 eV and spin periods of about 5-10 s, with no trace of radio activity. These X-ray dim INSs offer an unprecedented opportunity to study in detail the neutron star surface properties, thanks to their clean thermal spectra, unmarred by the emission from a companion, a surrounding supernova remnant or the magnetosphere. X-ray spectroscopy and timing may reveal the details of the star temperature and magnetic field surface distributions. If the distance is known, this will provide a measure of the star radius and, in case spectral features are detected at the same time, also of its mass. With present-day astronomical techniques the simultaneous measurement of the neutron star mass and radius is within reach. This will make it possible to place tight constraints on the matter equation of state at ultra-high densities.  

Plot
Figure 2:Spectral energy distribution of RBS 1774. Diamonds represent the XMM-Newton spectrum (Zane et al. 2005, ApJ, 627, 397). The solid line shows the unabsorbed blackbody that best fits the X-ray data, while arrows represent the 5 sigma upper limits reported by Rea et al. (2007, MNRAS, 379, 1484). The new VLT measurement is shown as a cross (Zane et al. 2008).

 We have first identified the seventh X-ray dim INS in the Rosat Bright Survey, RBS 1774 (Zampieri et al. 2001, A&A).  A subsequent XMM-Newton observation (Figure 1) confirms this identification and shows evidence of both a periodicity (9.437 s) in the X-ray flux and an absorption feature (at 0.7 keV) in the X-ray spectrum. The periodicity is associated with the neutron star spin period, while the energy of the absorption feature may be interpreted as an electron or a proton cyclotron line, giving an estimate of the surface magnetic field of about 1.0e11 or 1.0e14 G, respectively. Recently we identified a candidate optical counterpart of this INS in the B band (B=27.4±0.2). The infrared-through-optical spectral energy distribution of RBS 1774 is shown in Figure 2. The optical flux exceeds the extrapolation of the X-ray blackbody at optical wavelengths by a factor ~35. This is barely compatible with thermal emission from the neutron star surface but, at the same time, appears also difficult to reconcile with rotation-powered magnetospheric emission, unless the source has an extremely large optical emission efficiency.

At present we are working at a long-term investigation aimed at confronting the light curves and spectra of this unique class of objects with theoretical models. This will allow us to constrain the star thermal map and hence the magnetic field structure. To this end, we plan to build-up an archive of models for the detailed analysis of X-ray spectra of INS atmospheres, including the effects of magnetic fields and of general relativity on the propagation of radiation.

People: L. Zampieri

Collaboration: A. Treves (Insubria Univ.), R. Turolla (Padova Univ.), M. Cropper, R. P. Mignani, S. Zane (MSSL-UCL, UK)

Publications: Cropper et al. (2007), Ap&SS 308,161; Zane et al. (2008), ApJ 682,487

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