Discovery of the shortest recurrence time between thermonuclear X-ray bursts

ATel #1979; M. Linares, A. Watts, D. Altamirano, A. Patruno, P. Casella, N. Rea, P. Soleri, M. van der Klis, R. Wijnands (Amsterdam), T. Belloni (INAF-Brera), J. Homan (MIT), M. Mendez (Groningen)
on 19 Mar 2009; 17:21 UT
Credential Certification: Manuel Linares (mlinares@science.uva.nl)

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An RXTE observation of 4U 1636-536 taken on March 10th, 2009, revealed two type I X-ray bursts from this well studied neutron star low-mass X-ray binary (NS-LMXB). The recurrence time (wait time) between the two events, 5.4 minutes, is the shortest observed in any NS-LMXB.

The average spectrum exhibited by both bursts was thermal. We performed time-resolved spectroscopy along the bursts, using a black body model and an absorbing column density of 0.3e22 cm-2 (Fiocchi et al. 2006, ApJ, 651 416) to fit the 2.5-25.0 keV energy spectrum. We find clear signatures of cooling during the decay of both events, identifying them as type I (thermonuclear) X-ray bursts. The first burst (B1) reached a peak bolometric luminosity of ~5e37 erg/s with a black body temperature of ~1.6 keV, which later decayed to ~1 keV. The second burst (B2) reached a bolometric luminosity of ~1e38 erg/s at the peak and a black body temperature of ~1.9 keV, which declined to about 1 keV. The rise time (3.8s for B1, 1.2s for B2), exponential decay timescale (11.2s for B1, 5.2s for B2) and total duration (about 47s in B1 and about 28s in B2) show that the second type I burst was faster and more luminous. This resulted in a similar total energy output of [7.6-9]e38 erg for both bursts [B1-B2]. All luminosities and energies quoted herein use a distance of 6 kpc (Galloway et al. 2006, ApJ, 639 1033) and the burst timescales are calculated as in Galloway et al. (2008, ApJS, 179 360). We also detect burst oscillations at approximately 581 Hz in the rise of B2, with a fractional rms amplitude close to 10%. From broadband spectral fits to the persistent (non-burst) emission during this observation we find a bolometric accretion luminosity of ~2.2e37 erg/s, i.e. about 9% of the Eddington luminosity for a 1.4Mo neutron star.

The shortest recurrence time between type I bursts previously known was 4.3-6.4 min (from 4U 1608-52; Galloway et al. 2008), where the time range is due to the presence of a data gap. We re-analyzed that double burst (including event data, which do not have the data gap) and found a wait time of 6 min. We also note that a time separation between bursts of ~4 min was observed with SAS-3 near the Galactic center (Lewin et al. 1976, MNRAS, 177 83L), but source confusion and the lack of type I identification make that result highly uncertain (e.g. the bursting pulsar, which shows frequent type II bursts, is only ~0.6 deg. away). Hence to our knowledge the March 10th. double burst from 4U 1636-536 constitutes the shortest wait time between thermonuclear bursts observed in more than 30 years of type I X-ray burst studies. It also constitutes the shortest wait time to a burst with oscillations, which was previously 13.6 min (Galloway et al. 2008). Short wait time bursts are difficult to explain, since the recurrence times are too short (given the inferred accretion rate) to build up the column depth required for unstable ignition (e.g. Lewin et al. 1993, SSRv, 62, 223L). They may be associated with hydrodynamical mixing of unburnt fuel, waiting points in the nuclear burning reactions, or transitions between different burning regimes. The fact that this pair occurs with such a short wait time and at high accretion rate (compared to most other burst pairs; Galloway et al. 2008) is a new constraint on models.