breaking the amsp mould: the increasingly strange case of hete j1900.1-2455

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


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


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)


…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


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)


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


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


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 (?)


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


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?


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?)


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…


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…


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!


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

breaking the amsp mould: the increasingly strange case of hete j1900.1-2455

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