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

TLDR: In this article, the authors describe the burst properties in relation to the persistent emission of the low-mass X-ray binary 4U 0614+091 and find bursts with a wide variety of characteristics in serendipitous wide-field X-r ay 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

Abstract: The low-mass X-ray binary 4U 0614+091 is a source of sporadic thermonuclear (type I) X-ray burs ts. We find bursts with a wide variety of characteristics in serendipitous wide-field X-r ay 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 a bout 15 Crab, but a few are weak and only reach a peak flux below a Crab. One of the bursts shows a ver y 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 sec t o 5 min. However, after one of the intermediate-duration bursts, a f aint tail is seen to at least about 2.4 hours 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 diffi cult 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 correlation s are apparent, except that the intermediate-duration burs ts occurred when 4U 0614+091’s persistent emission was lowest and calm, and when burs ts were infrequent (on average roughly one every month to 3 months). The average burst rate increased significantly af ter this period. The maximum average burst recurrence rate i s about once every week to 2 weeks. The burst behaviour may be partly understood if there is at least an appreciable amount of helium pre sent 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 h elium, we find that the energy production per accumulated mas s is about 2.5 times less than expected for pure helium matter.