What ignites on the neutron star of 4U 0614+091?

Abstract

The low-mass X-ray binary 4U 0614+091 is a source of sporadic thermonuclear (type I) X-ray bursts. We find bursts with a wide variety of characteristics in serendipitous wide-field X-ray observations by the WATCH on EURECA, the ASM on RXTE, the WFCs on BeppoSAX, the FREGATE on HETE-2, the IBIS/ISGRI on INTEGRAL, and the BAT on Swift, as well as pointed observations with the PCA and HEXTE on RXTE. Most of the bursts are bright, i.e., they reach a peak flux of about 15 Crab, but a few are weak and only reach a peak flux below a Crab. One of the bursts shows a very strong photospheric radius-expansion phase. This allows us to evaluate the distance to the source, which we estimate to be 3.2 kpc. The burst durations vary generally from about 10 s to 5 min. However, after one of the intermediate-duration bursts, a faint tail is seen to at least about 2.4 h after the start of the burst. One very long burst was observed, which lasted for several hours. This superburst candidate was followed by a normal type-I burst only 19 days later. This is, to our knowledge, the shortest burst-quench time among the superbursters. The observation of a superburst in this system is difficult to reconcile if the system is accreting at about 1% of the Eddington limit. We describe the burst properties in relation to the persistent emission. No strong correlations are apparent, except that the intermediate-duration bursts occurred when 4U 0614+091's persistent emission was lowest and calm, and when bursts were infrequent (on average roughly one every month to 3 months). The average burst rate increased significantly after this period. The maximum average burst recurrence rate is about once every week to 2 weeks. The burst behaviour may be partly understood if there is at least an appreciable amount of helium present in the accreted material from the donor star. If the system is an ultra-compact X-ray binary with a CO white-dwarf donor, as has been suggested, this is unexpected. If the bursts are powered by helium, we find that the energy production per accumulated mass is about 2.5 times less than expected for pure helium matter.

The Fermi–gbm Three-Year X-Ray Burst Catalog

“…This is the same number of bursts detected from 4U0614+09 with nine different instruments over the course of 15 years (1992( -2007( , Kuulkers et al 2010, which shows the drastic improvement in detection efficiency gained by GBM. Given GBM's 50% observing duty cycle, we measure a burst recurrence time of t rec = 17±2 day (1σ Poissonian uncertainty), 5 day longer than, but consistent with, the results of Linares et al (2012).…”

“…Lower ṁ implies a colder neutron star envelope, a longer fuel accumulation time, and a greater ignition depth. However, ignition models still have problems in reproducing these recurrence times and burst energies quantitatively (Kuulkers et al 2010;Linares et al 2012). Assuming solar metallicity, the pure helium ignition models from Cumming & Bildsten (2000) require large amounts of deep crustal heating to reproduce the recurrence times that we measure in 2S0918-549 and 4U0614+09: more than 3MeV per accreted nucleon (see Figure 7 and further discussion in Linares et al 2012).…”

“…Due to its location far from other bursters and its proximity (Kuulkers et al 2010 measured a distance of d = 3.2 kpc, which we adopt in this work), it is an ideal source in which to study thermonuclear bursts at low accretion rates. During the first year of the Fermi-GBM X-ray burst monitor we detected 15 bursts from 4U0614+09 (Linares et al 2012).…”

Evidence of heavy-element ashes in thermonuclear X-ray bursts with photospheric superexpansion

“…Fourth panels: black body radius assuming isotropic emission. Assumed distances: 3.2 kpc (4U 0614+09; Kuulkers et al 2010Kuulkers et al ), 9.5 kpc (4U 1722Kuulkers et al 2003) and 7.6 kpc (4U 1820-30; Kuulkers et al 2003); Fifth and sixth panels: absorption edge energy E edge in keV and optical depth τ opt . Bottom panels: reduced χ 2 value for each fit to the black body + reflection + edge model.…”

“…X-ray bursts are thermonuclear shell flashes that result from the unstable burning of fuel accreted on the surface of a neutron star (NS) from a Roche-lobe filling companion star (Grindlay et al 1976;Woosley & Taam 1976;Maraschi & Cavaliere 1977;Lamb & Lamb 1978; for reviews, see Lewin et al 1995;Strohmayer & Bildsten 2006). Approximately 20% of all bursts exhibit some degree of photospheric radius expansion (PRE; see, e.g., Galloway et al 2008a) due to a flux exceeding the Eddington limit, resulting in a radiatively driven expansion of the photosphere (Lewin et al 1984;Ebisuzaki et al 1983;Tawara et al 1984a,b).…”