Models of hydrostatic magnetar atmospheres at high luminosities

TL;DR

This study models hydrostatic atmospheres of magnetars at high luminosities, concluding that strong magnetic fields prevent large atmospheric expansions during bursts, challenging previous interpretations of burst spectra.

Contribution

It provides the first detailed modeling showing that strong magnetic fields inhibit extended atmospheres in magnetars, revising understanding of burst mechanisms and spectral features.

Findings

Extended atmospheres are impossible in strong magnetic fields (~10 m extent).

Maximum luminosity in magnetar atmospheres is lower than previously estimated.

Proximity of polarization mode photospheres questions blackbody spectral fits.

Abstract

We investigate the possibility of Photospheric Radius Expansion (PRE) during magnetar bursts. Identification of PRE would enable a determination of the magnetic Eddington limit (which depends on field strength and neutron star mass and radius), and shed light on the burst mechanism. To do this we model hydrostatic atmospheres in a strong radial magnetic field, determining both their maximum extent and photospheric temperatures. We find that spatially-extended atmospheres cannot exist in such a field configuration: typical maximum extent for magnetar-strength fields is ~10 m (as compared to 200 km in the non-magnetic case). Achieving balance of gravitational and radiative forces over a large range of radii, which is critical to the existence of extended atmospheres, is rendered impossible in strong fields due to the dependence of opacities on temperature and field strength. We conclude that high luminosity bursts in magnetars do not lead to expansion and cooling of the photosphere, as in the non-magnetic case. We also find the maximum luminosity that can propagate through a hydrostatic magnetar atmosphere to be lower than previous estimates. The proximity and small extent of the photospheres associated with the two different polarization modes also calls into question the interpretation of two blackbody fits to magnetar burst spectra as being due to extended photospheres.