Breaking the AMSP mould: the increasingly strange case of HETE J1900.1-2455

TL;DR

This paper reports on over three years of RXTE observations of the pulsar HETE J1900.1-2455, revealing persistent activity, variable pulsations, and unusual spectral features, challenging existing models of accretion-powered pulsars.

Contribution

It provides the first long-term monitoring of HETE J1900.1-2455, documenting its evolving pulsation and spectral properties, and highlights the complexity of its accretion behavior.

Findings

Pulsations only within the first 70 days of activity.

Detected extremely weak pulsations during nondetection intervals.

Unusual spectral variations in the brightest burst, favoring a comptonisation model.

Abstract

We present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues in this system more than 3 years after discovery, at a mean luminosity of 4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the first 70 days. X-ray variability has increased each year, notably with a brief interval of nondetection in 2007, during which the luminosity dropped to below 1e-3 of the mean level. A deep search of data from the intervals of nondetection in 2005 revealed evidence for extremely weak pulsations at an amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen early in the outburst. X-ray burst activity continued through 2008, with bursts typically featuring strong radius expansion. Spectral analysis of the most intense burst detected by RXTE early in the outburst revealed unusual variations in the inferred photospheric radius, as well as significant deviations from a blackbody. We obtained much better fits instead with a comptonisation model.