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Astron. Astrophys. 345, 869-883 (1999)


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RX J0720.4-3125: Implications for neutron star cooling and the dense matter equation of state

John C.L. Wang  *  1, Bennett Link 2, Kenneth VanRiper 3, Keith A. Arnaud 1,4 and Juan A. Miralles 5

1 Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
2 Department of Physics, Montana State University, Bozeman, MT 59717-0350, USA
3 White Rock Science, P.O. Box 4729, Los Alamos, NM 87545, USA
4 Laboratory for High Energy Astrophysics, Goddard Space Flight Center, Greenbelt, MD 20771, USA
5 Department d'Astronomia i Astrofísica, Universitat de Valéncia, Valencia, Spain

Received 9 October 1998 / Accepted 15 January 1999

Abstract

The soft X-ray source RX J0720.4-3125 appears to be a clean example of a cooling neutron star. Its X-ray emission is well-fit by a [FORMULA] eV blackbody and displays a periodic modulation with period [FORMULA] sec and semi-amplitude [FORMULA]%. The observational upper limit to the period derivative implies a minimum spindown age of [FORMULA] yr if the star was born rapidly rotating. The absence of a visible supernova remnant independently suggests an age for this source of [FORMULA] yr. With the interpretation of this source as a cooling-driven, magnetized, rotating neutron star, we explore the implications for the dense matter equation of state (EOS), the mode of energy loss (modified URCA vs. direct URCA), and the rate of internal heating due to superfluid friction. For the standard (modified URCA) cooling models, we study two types of stars: those born slowly rotating, with relatively small spin-down rates and conventional dipole magnetic fields ([FORMULA]), and those born rapidly rotating with large spin-down rates and magnetar-scale dipole fields ([FORMULA]). We find that standard cooling with a stiff or moderately stiff EOS is consistent with the observations of RX J0720.4-3125 provided the star's age is [FORMULA] yr. If the EOS is very stiff, the star must be born with a short rotation period and significant internal heating by superfluid friction is required. More moderate heating suffices only if the star is very massive ([FORMULA]) and has an age [FORMULA]. Stars with [FORMULA] and a moderately stiff EOS give modulations about a factor of five below that observed. However, the inclusion of atmospheric effects or more complex field geometries could increase the modulation to a level consistent with the observations. Stars with a stiff EOS give modulations close to that observed. As an illustration of the effects of accelerated cooling processes, we consider direct quark URCA cooling. We find that these models cool too fast and are cleanly ruled out for this source. Hence, exotic matter is an insignificant component in the stellar core, or does not participate in accelerated cooling. Direct URCA reactions in nucleonic matter are similarly ruled out. A measure of RX J0720.4-3125's spin-down age would afford crucial tests of our conclusions.

Key words: stars: neutron – stars: evolution – stars: magnetic fields – dense matter – X-rays: stars – stars: individual: RX J0720.4-3125

* Deceased

Send offprint requests to: B. Link (blink@dante.physics.montana.edu)

© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999

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