Outburst of the 2 s Anomalous X-ray Pulsar 1E 1547.0-5408

TL;DR

This paper reports a significant X-ray outburst from the Anomalous X-ray Pulsar 1E 1547.0-5408, analyzing its spectral, temporal, and radio properties, and suggesting a deep crustal heating event as the cause.

Contribution

It presents the first detailed multi-wavelength observation of an outburst in AXP 1E 1547.0-5408, linking X-ray and radio emissions and proposing a crustal heating model.

Findings

X-ray flux increased by an order of magnitude during outburst.

Spectral analysis indicates a hot region of ~0.5 keV covering 16% of the neutron star surface.

Radio emission appeared with a broad pulse, suggesting a nearly aligned rotator.

Abstract

Following our discovery of radio pulsations from the newly recognized Anomalous X-ray Pulsar (AXP) 1E 1547.0-5408, we initiated X-ray monitoring with the Swift X-ray Telescope, and obtained a single target-of-opportunity observation with the Newton X-ray Multi-Mirror Mission (XMM-Newton). In comparison with its historic minimum flux of 3e-13 ergs cm^-2 s^-1, the source was found to be in a record high state, f_X(1-8 keV) = 5e-12 ergs cm^-2 s^-1, or L_X = 1.7e35(d/9 kpc)^2 ergs s^-1, and declining by 25% in 1 month. Extrapolating the decay, we bound the total energy in this outburst to 1e42 < E < 1e43 ergs. The spectra (fitted with a Comptonized blackbody) show that an increase in the temperature and area of a hot region, to 0.5 keV and ~16% of the surface area of the neutron star, respectively, are primarily responsible for its increase in luminosity. The energy, spectrum, and timescale of decay are consistent with a deep crustal heating event, similar to an interpretation of the X-ray turn-on of the transient AXP XTE J1810-197. Simultaneous with the 4.6 hour XMM-Newton observation, we observed at 6.4 GHz with the Parkes telescope, measuring the phase relationship of the radio and X-ray pulse. The X-ray pulsed fraction of 1E 1547.0-5408 is only ~7%, while its radio pulse is relatively broad for such a slow pulsar, which may indicate a nearly aligned rotator. As also inferred from the transient behavior of XTE J1810-197, the only other AXP known to emit in the radio, the magnetic field rearrangement responsible for this X-ray outburst of 1E 1547.0-5408 is probably the cause of its radio turn-on.