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

1. Breaking the AMSP mould:the increasingly strange case ofHETE J1900.1-2455 Duncan Galloway Monash University Ed Morgan Deepto Chakrabarty Kavli Institute, MIT Ten Years of Accretion-powered Millisecond Pulsars, UvA, April 2008 Galloway, Breaking the AMSP mould: HETE J1900.1-2455

2. A remarkable transient: HETE J1900.1-2455 • A thermonuclear burst from this source detected by the HETE-2 satellite June 2005 (ATel #516) • Subsequent PCA observations revealed 377.3 Hz pulsations and Doppler variations from an 83.3 min orbit (ATel #523, 538; Kaaret et al. 2006, ApJ 638, 963) • Mass function is 1.99810-6 M so that minimum companion mass (assuming a 1.4 M neutron star) is 0.016 M • Several more thermonuclear bursts detected, also by RXTE/PCA Galloway, Breaking the AMSP mould: HETE J1900.1-2455

3. A quasi-persistent AMSP… • Flux highly variable (34% RMS) between observations • has been continuously active (with one hiccup) for almost 3 years • Recall the longest outburst from the other AMSPs was ≈50 d (XTE J1814-338) Galloway, Breaking the AMSP mould: HETE J1900.1-2455

4. November 1 …with quasi-persistent pulsations • HETE J1900.1-2455 showed pulsations intermittently, only for the first few months of the outburst • All 6 other millisecond pulsars discovered up until then showed pulsations consistently throughout ~2 week outbursts • We have not detected pulsations now since Aug ‘06; since then the source more closely resembles a low-luminosity, non-pulsing, persistent LMXB Thermonuclear bursts June 14 Galloway, Breaking the AMSP mould: HETE J1900.1-2455

5. Transient pulses with decaying rms Following three bursts detected early in the outburst, the pulsations appeared at ~2% rms and then decayed away on a timescale of ~10 d Bursts which occurred later in the outburst did not trigger pulsations (Galloway et al. 2007, ApJL 654, L73) Galloway, Breaking the AMSP mould: HETE J1900.1-2455

6. Only detections after MJD 53600; not included in the fit Pulsations leading a burst • On average the pulse amplitude decreased with time elapsed since the previous burst • Decay constant 11 ± 2 d • Pulsations were only detected twice following the 5th burst, and were not detected afterwards • Although the pulse amplitude is highly correlated with time since the last burst, pulsations appeared in one case ~2h BEFORE the burst Galloway, Breaking the AMSP mould: HETE J1900.1-2455

7. Pulsations trailing a burst • We detected three thermonuclear bursts with the PCA, one early in the outburst before pulsations ceased altogether • No pulsations were detected in the observation taken as a whole • However, the pulse amplitude rose beginning ~30 min after the burst onset Galloway, Breaking the AMSP mould: HETE J1900.1-2455

8. To summarize:      Galloway, Breaking the AMSP mould: HETE J1900.1-2455

9. A colourful character • Weekly RXTE observations continue, with a linked TOO to trigger on recovery of pulsations • In 2007, the degree of variability increased dramatically, accompanied by color changes (solid symbols are colors from Feb-May 2007 -> not seen in other systems Galloway, Breaking the AMSP mould: HETE J1900.1-2455

10. An evolutionary trigger? • Podsiadlowski ‘02 computed orbital period & mass transfer sequences for variously evolved binaries. • For one case a local peak in the accretion rate corresponds to passage through the ~80 min period • This seems rather improbable, but might explain the unusually long active period? Galloway, Breaking the AMSP mould: HETE J1900.1-2455

11. A brief quiescence… • Drop in flux in 2007 May (ATel #1086) triggered Swift observations • In one of those, the source was no longer detected (ATel #1098; <51032 erg/s, 2-10 keV); • subsequently recovered (ATel #1106) -> not seen in other AMSPs (cf. with SAX J1808.4-3658?) Galloway, Breaking the AMSP mould: HETE J1900.1-2455

12. Nonstandard cooling? • Compare with cooling measurements from other AMSPs and LMXBs (Heinke et al. 2007) • Flux limit is about middle-of-the-range, but time-averaged accretion rate (in outburst) is rather higher than other systems • This may not be a meaningful comparison right now, but if activity continues… Galloway, Breaking the AMSP mould: HETE J1900.1-2455

13. Some very odd burst profiles • All three bursts observed with the PCA have fast rises and exhibit strong radius-expansion • All three bursts exhibit double (or triple) peaks in the X-ray flux • The first burst exhibits multiple peaks and extremely unusual variation of blackbody radius and temperature with time • Second & third less energetic, commensurate with lower peak flux -> best comparison is 1808, but… Galloway, Breaking the AMSP mould: HETE J1900.1-2455

14. Accretion rate -> pulsations? • The two observations where we can constrain the relative timing of the pulsations and the bursts were at accretion rates differing by a factor of two • Perhaps this can impose some constraints on thermomagnetic (etc.) effects in the burning layer • More theoretical work is required! Galloway, Breaking the AMSP mould: HETE J1900.1-2455

15. Summary and future prospects • Pulsations in HETE J1900.1-2455 are closely tied to both the burst activity and the outburst duration. tdecay ~ ∆tburst • Not the case in any other millisecond pulsar! (two others also burst) -> pulse mechanism • For most of the time it’s been active, HETE J1900.1-2455 has been indistinguishable from a non-pulsing, low-accretion rate LMXB -> a “missing link” with the larger population • Lots and lots of other unexplained differences Galloway, Breaking the AMSP mould: HETE J1900.1-2455