Optical pulsations from the anomalous X-ray pulsar 1E 1048.1-5937

TL;DR

This study reports the detection of optical pulsations from the anomalous X-ray pulsar 1E 1048.1-5937, confirming its brightness increase during an X-ray flare and showing similar pulse profiles in optical and X-ray wavelengths.

Contribution

First detection of optical pulsations from 1E 1048.1-5937 with simultaneous X-ray comparison, linking optical and X-ray emission properties in this pulsar.

Findings

Optical pulsations match X-ray period of 6.458 s.

Optical pulsed fraction is approximately 21%.

Optical and X-ray pulses are nearly in phase.

Abstract

We present high-speed optical photometry of the anomalous X-ray pulsar 1E 1048.1-5937 obtained with ULTRACAM on the 8.2-m Very Large Telescope in June 2007. We detect 1E 1048.1-5937 at a magnitude of i'=25.3+/-0.2, consistent with the values found by Wang et al. (2008) and hence confirming their conclusion that the source was approximately 1 mag brighter than in 2003-2006 due to an on-going X-ray flare that started in March 2007. The increased source brightness enabled us to detect optical pulsations with an identical period (6.458 s) to the X-ray pulsations. The rms pulsed fraction in our data is 21+/-7%, approximately the same as the 2-10 keV X-ray rms pulsed fraction. The optical and X-ray pulse profiles show similar morphologies and appear to be approximately in phase with each other, the latter lagging the former by only 0.06+/-0.02 cycles. The optical pulsations in 1E 1048.1-5937 are very similar in nature to those observed in 4U 0142+61. The implications of our observations for models of anomalous X-ray pulsars are discussed.