(News & views on the paper by Tong, Xu & Song 2011, vol.11, 1371-1376)
Toward an understanding of the thermal radiation
from dead pulsars
Guojun Qiao (Department of Astronomy, School of Physics, Peking University)
(Department of Astronomy, School of Physics, Peking University)
Since the discovery of pulsars in 1967, more and more kinds of pulsar-like compact objects have been found. The seven X-ray dim isolated neutron stars (XDINSs) are among the interesting manifestations, which are characterized by their very simple timing and spectra behaviors (Turolla 2009). Their periods are about 10 seconds, characteristic surface magnetic field ~1013G, and characteristic age ~106years (Kaplan & van Kerkwijk 2009). Very interestingly, their featureless X-ray spectra can be fitted very well by a single blackbody spectra (Burwitz et al. 2001). Up to now no high energy tails are found (van Kerkwijk & Kaplan 2007). Meanwhile, they are also radio silent. Therefore, the seven XDINSs are suggested to be dead pulsars, i.e. ones without magnetospheric activities, which are good specimen for studies of neutron star cooling, atmosphere, as well as equation of state.
A long standing problem in the studies is: how we can model XDINSs' broad band spectra energy distributions. It is really a puzzle that the optical/UV emissions are in excess of their X-ray blackbody extrapolations (i.e. optical/UV excess). This problem could also be related to the equation of state of neutron stars (Xu 2002). Previously, only two sources (RX J1856.5-3754 and RX J0720.4-3125) have optical/UV observations (van Kerjwijk & Kaplan 2007), and Tong et al. (2010) pointed out that the optical/UV excess of XDINSs may be due to resonant cyclotron scattering (RCS) in their magnetospheres. Tong et al. (2010) applied the RCS model to the case of RX J1856.5-3754 and RX J0720.4-3125, and had shown that the optical/UV excess of RX J1856.5-3754 and RX J0720.4-3125 can be explained quite well in the RCS model.
Recently, Kaplan et al. (2011) measured the optical/UV emissions of all seven XDINSs. They found that all seven XDINSs have optical/UV excess of order ten. Moreover, the source RX J2143.0+0654 may have a relatively flat optical/UV spectrum. These new observations provide challenges to existing theoretical models. Considering this observational progresses, Tong, Xu & Song (2011) apply their RCS model to all seven XDINSs. The bremsstrahlung emissions of the electron system are also included in addition to the RCS process. They found that for most sources, the spectral energy distributions of XDINSs can be explained quite well in the RCS model, while the flat spectrum of RX J2143.0+0654 may result from the bremsstrahlung emissions of the electron system.
XDINSs could be quark stars if the optical/UV emissions are due to magnetospheric activity (Tong, Xu & Song 2011) rather than of atmospheric origin (Trümper 2005). The non-atomic feature of the thermal X-ray spectra would be naturally understood if XDINSs are bare quark stars (Xu 2002), which can also provide the seed X-ray photons in the RCS model. In addition, this study may also deepen the understanding of pulsar magnetospheres along with other pulsar-like objects, e.g. magnetars and RRATs (rotating radio transients).
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