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

2. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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

3. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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)

4. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 …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 June 14 November 1 Thermonuclear bursts

5. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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)

6. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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 Only detections after MJD 53600; not included in the fit Although the pulse amplitude is highly correlated with time since the last burst, pulsations appeared in one case ~2h BEFORE the burst

7. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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

8. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 To summarize: “classical” AMSPsHETE J1900.1-2455 OutburstTypically 2 weeks, max 40 d Ongoing Presence of pulsationsConsistently present throughout outburst Present only for first part of outburst Fractional pulse amplitude Constant or declining slightly; ≈5-10% Variable <3% Dependence on bursts?NoYes, decaying on ~10d timescale following bursts Pulse shapeFundamental & second harmonic Fundamental only (?)     

9. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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

10. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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?

11. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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  10 32 erg/s, 2-10 keV); subsequently recovered (ATel #1106) -> not seen in other AMSPs (cf. with SAX J1808.4-3658?)

12. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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…

13. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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…

14. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 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!

15. Galloway, Breaking the AMSP mould: HETE J1900.1-2455 Summary and future prospects Pulsations in HETE J1900.1-2455 are closely tied to both the burst activity and the outburst duration. t decay ~ ∆t burst 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