The aperiodic timing behaviour of XTE J1814-338

ATel #165; Rudy Wijnands (University of St Andrews) and Jeroen Homan (OAB, Merate)
on 11 Jun 2003; 20:06 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

Tweet

We report on the aperiodic timing behaviour of the accretion-driven millisecond X-ray pulsar XTE J1814-338 (IAUC 8144 ) as observed with the Rossi X-ray Timing Explorer between 8 and 11 June 2003. These observations were already used by Markwardt et al. (ATEL #164) to report on the improved orbital period of the system and on the discovery of burst oscillations.

We combined all observations (~50 ksec of data after removal of the type-I X-ray bursts) to increase the sensitivity. The overall shape of the 3-60 keV power spectrum is dominated by a strong broad band-limited noise component (23.7+/-0.3 % r.m.s. amplitude for the frequency range 0.01-100 Hz). A broken power law fit to this component yielded a break frequency of 0.27+/-0.01 Hz and indices of -0.06+/-0.03 and 0.88+/-0.01, respectively, below and above the break. On top of this noise component, a bump is present which could be fitted with a Lorentzian with a frequency of 1.8+/-0.1 Hz, a width of 1.6+/-0.3 Hz, and a strength of 6.9+/-0.7 % r.m.s. These characteristics make the power spectrum of XTE J1814-338 very similar to that observed in the non-pulsing low-luminosity neutron-star low-mass X-ray binaries, the atoll sources, when they are observed at relatively low X-ray luminosities (i.e., in the so-called island state). This is consistent with the hard power-law X-ray spectrum of the source reported in ATEL #164. This resemblance of XTE J1814-338 to the atoll sources is further strengthened by the fact that the source falls on the relation between the break frequency and the frequency of the bump found by Wijnands & van der Klis (1999, ApJ 514, 939).

We searched for QPOs above 100 Hz (the kHz QPOs), but none were found (see plot). However, the amplitude upper limits (3-60 keV) on the presence of such QPOs are not very stringent; depending on the width, which was chosen to be between 50 and 100 Hz, we obtained upper limits of 6-7 % r.m.s. (for frequencies above 600 Hz) and 9-11 % (below 600 Hz). Those upper limits are comparable to the amplitudes of the recently discovered kHz QPOs in the accretion-driven millisecond X-ray pulsar SAX J1808.4-3658 (Wijnands et al. 2003, Nature, in press), so the presence of similar kHz QPOs in XTE J1814-338 cannot be excluded.