Galloway, Breaking the AMSP mould: HETE J1900.1-2455
Breaking the AMSP mould:the increasingly strange case of HETE J1900.1-2455
• Duncan GallowayMonash University
• Ed Morgan• Deepto ChakrabartyKavli Institute, MIT
Ten Years of Accretion-powered Millisecond Pulsars, UvA, April 2008
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 1.99810-6 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
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
…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
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)
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
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
Galloway, Breaking the AMSP mould: HETE J1900.1-2455
To summarize:“classical” AMSPs HETE J1900.1-2455
Outburst Typically 2 weeks, max 40 d
Ongoing
Presence of pulsations
Consistently present throughout outburst
Present only for first part of outburst
Fractional pulse amplitude
Constant or declining slightly; ≈5-10%
Variable <3%
Dependence on bursts?
No Yes, decaying on ~10d timescale following bursts
Pulse shape Fundamental & second harmonic
Fundamental only (?)
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
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?
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; <51032 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
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
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
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
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