BeppoSAX Wide Field Cameras observations of six type I X-ray bursters

Cornelisse, R.; Verbunt, F.; Zand, J. J. M. in ’t; Kuulkers, E.; Heise, J.; Remillard, Ronald A.; Cocchi, M.; Natalucci, L.; Bazzano, A.; Ubertini, P.

doi:10.1051/0004-6361:20020707

Cited by 89 publications

(120 citation statements)

References 27 publications

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“…Although the average 2-10 keV X-ray luminosity of this source is lower than is typically seen for active LMXBs (e.g., Chen et al 1997), there is a growing group of sub-luminous sources that accrete at similar X-ray intensities to AX J1538.3-5541 (e.g., AX J1754.2-2754 discussed above, but see also Cornelisse et al 2002;Muno et al 2005;Wijnands et al 2006;in't Zand et al 2005in't Zand et al , 2007Del Santo et al 2007b;Degenaar & Wijnands 2009;Campana 2009;Degenaar et al 2010).…”

Section: Ax J15383-5541mentioning

confidence: 82%

Swiftfollow-up observations of unclassified ASCA sources

Degenaar

Starling

Evans

et al. 2012

A&A

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We present Swift follow-up observations of a sample of 35 unclassified faint X-ray sources drawn from the ASCA Galactic centre and Galactic plane surveys. Our short, pointed XRT observations allow detections down to a limiting 0.3-10 keV flux of F X ∼ 10 −13 erg cm −2 s −1 , which translates into a luminosity of L X ∼ 10 33 erg s −1 for an assumed distance of D = 8 kpc. The brightest source in our sample reaches a maximum 0.3-10 keV luminosity of L X ∼ 2 × 10 36 (D/8 kpc) 2 erg s −1 during our Swift observations. We detect 16 (46%) of the ASCA sources with the XRT, while 19 were not detected during our program. Since we are probing the faint end of the ASCA source populations, we expect a large fraction of the non-detections to be due to the Eddington bias. This is strengthened by the fact that we find the observed XRT count rates to be predominantly lower than expected based on the reported ASCA intensities. Nevertheless, investigation of the ASCA properties and any possible long-term X-ray variability leads us to conclude that the nondetections likely include two spurious ASCA detections and three objects that are highly variable or transient X-ray sources. For the 16 XRT-detected sources we obtain positional accuracies of ∼2−4 , which significantly improves upon their ASCA uncertainties of ∼1 . We use the X-ray spectra and variability to characterise these objects. Most appear to be faint, persistent X-ray emitters that have highly absorbed spectra. Based on their X-ray properties we identify three accreting compact objects: one confirmed (AX J1754.2-2754) and one candidate (AX J1538.3-5541) X-ray binary, and one possible magnetically accreting white dwarf (AX J1620.1-5002). Furthermore, we use the improved positions of XRT-detected sources to search for counterparts in simultaneously obtained Swift/UVOT images and possible associations with catalogued sources at various wavelengths. This reveals three possible main sequence stars amongst our sample. The other sources remain unclassified, but our improved XRT positions provide good prospects for dedicated follow-up observations that have the potential to elucidate the nature of these faint, unclassified ASCA sources.

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Section: Ax J15383-5541mentioning

confidence: 82%

Swiftfollow-up observations of unclassified ASCA sources

Degenaar

Starling

Evans

et al. 2012

A&A

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“…If the source emits isotropically during and between bursts, and if the peak flux during the bursts F peak represents emission at the Eddington limit, then the parameter γ (≡F pers /F peak , where both fluxes are bolometric) gives the source persistent luminosity in terms of the Eddington limit (see, e.g., Cornelisse et al 2002b). Using the values from Table 2 and using only those bursts where F peak > 15 × 10 −8 erg s −1 cm −2 (see Sect.…”

Section: Persistent Emissionmentioning

confidence: 99%

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

Kuulkers¹,

Zand

Atteia

et al. 2010

A&A

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103

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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.

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“…A few other LMXB transients have shown flashes after and/or in between X-ray outbursts when their X-ray emission was below the detection thresholds and the systems were inferred to be in their quiescent phase: 2S 1711−339, SAX J1808.4−3658 and GRS 1747−312 (Cornelisse et al 2002a, in 't Zand et al 2001, 2003b. Also Cen X-4 may have shown a flash ∼2 years after its 1969 X-ray outburst (Gorenstein et al 1974;see Appendix A).…”

Section: Introductionmentioning

confidence: 99%

Restless quiescence: thermonuclear flashes between transient X-ray outbursts

Kuulkers¹,

Zand

Lasota

2009

A&A

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For thermonuclear flashes to occur on neutron-star surfaces, fuel must have been accreted from a donor star. However, sometimes flashes are seen from transient binary systems when they are thought to be in their quiescent phase, during which no accretion, or relatively little, is expected to occur. We investigate the accretion luminosity during several such flashes, including the first-ever and brightest detected flash from Cen X-4 in 1969. We infer from observations and theory that immediately prior to these flashes the accretion rate must have been between about 0.001 and 0.01 times the equivalent of the Eddington limit, which is roughly 2 orders of magnitude less than the peak accretion rates seen in these transients during an X-ray outburst and 3-4 orders of magnitude more than the lowest measured values in quiescence. Furthermore, three such flashes, including the one from Cen X-4, occurred within 2 to 7 days followed by an X-ray outburst. A long-term episode of enhanced, but low-level, accretion is predicted near the end of the quiescent phase by the disk-instability model, and may thus have provided the right conditions for these flashes to occur. We discuss the possibility of whether these flashes acted as triggers of the outbursts, signifying a dramatic increase in the accretion rate. Although it is difficult to rule out, we find it unlikely that the irradiance by these flashes is sufficient to change the state of the accretion disk in such a dramatic way.

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BeppoSAX Wide Field Cameras observations of six type I X-ray bursters

Cited by 89 publications

References 27 publications

Swiftfollow-up observations of unclassified ASCA sources

Swiftfollow-up observations of unclassified ASCA sources

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

Restless quiescence: thermonuclear flashes between transient X-ray outbursts

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