Revised Orbit and Burst Oscillations from the Millisecond Pulsar XTE J1814-338

ATel #164; C. B. Markwardt (U. Maryland & GSFC), T. E. Strohmayer and J. H. Swank (GSFC)
on 11 Jun 2003; 14:36 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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C. B. Markwardt, University of Maryland and Goddard Space Flight Center (GSFC), T. E. Strohmayer and J. H. Swank (GSFC) report on further observations of the newly discovered accreting millisecond pulsar XTE J1814-338. In IAUC 8144 , we reported that the position of the source, as derived from RXTE PCA scans on Jun 5.1 (UT) was R.A. = 18h13m40s, Dec. = -33o46' (equinox 2000.0) with a 99% confidence radius of 2'. The Jun 5.1 observation also contained pointed data which revealed coherent pulsations with a period of 3.2 ms, making this the fifth known millisecond pulsar, and the fourth to be discovered in the past fourteen months.

In IAUC 8144 , we reported a preliminary orbital period of ~2 hr, based on a short orbservation of 3400 s. By combining observations from Jun 5-10, we have determined a phase-connected timing solution including binary Doppler motion, and find a rest-frame pulsar frequency of 314.35610(2) Hz with a best fit orbital period is 4.27462(9) hr. This determination is now based on a much increased time baseline which fully samples the orbital phases. The projected pulsar semimajor axis, a_x sin(i), is 390.3(3) lt-ms, which yields a mass function of 0.002016(4) solar masses. This system has a significantly longer period, larger orbit and larger companion (minimum companion mass of 0.16 solar masses), compared to the four previously known accreting millisecond pulsar systems. The companion is likely to be a hydrogen dwarf.

At least three thermonuclear X-ray bursts have been seen from XTE J1814-338. All of them have exhibited nearly coherent oscillations during the burst, at a frequency close to the persistent pulsation frequency. There is no apparent evolution in burst oscillation frequency during the burst. This is very similar behavior to the burst oscillations originally discovered from the first accreting millisecond pulsar, SAX J1808.4-3658 (Chakrabarty et al. 2003, Nature, in press).

The X-ray flux has been slowly and steadily rising from about 9 mCrab on Jun 5, to 12 mCrab on Jun 10. The source is either very underluminous (~1% of Eddington at the galactic center), or very distant. However, if XTE J1814-338 is very distant, the high galactic latitude (12 deg) would place it several kiloparsecs off the galactic plane. The energy spectrum is somewhat harder than the previously known millisecond pulsar systems, consistent with a power law with photon index ~1.5, and a high energy cut-off above 20 keV.