type-I X-ray Bursts Research Articles

High Resolution and Broad Band Spectra of Low Mass X-ray Binaries: a Comparison between Black Holes and Neutron Stars

A common question about compact objects in high energy astrophysics is whether it is possible to distinguish black hole from neutron star systems with some other property that is not the mass of the compact object. Up to now a few characteristics have been found which are typical of neutron stars (like quasi periodic oscillations at kHz frequencies or type-I X-ray bursts), but in many respects black hole and neutron star systems show very similar behaviors. We present here a spectral study of low mass X-ray binaries containing neutron stars and show that these systems have spectral characteristics that are very similar to what is found for black hole systems. This implies that it is unlikely we can distinguish between black holes and neutron stars from their X-ray spectra, except for the fact that black hole systems show sometimes a more extreme behavior with respect to neutron star systems.

The superburst recurrence time in luminous persistent LMXBs

Theory and observations favor stable helium burning as the most important means to produce fuel for superbursts on neutron star surfaces. However, all known superbursters exhibit unstable burning as well. This ambiguity prompted us to search for superbursts in data from the BeppoSAX Wide Field Cameras of ten luminous LMXBs, most of which do not exhibit normal type-I X-ray bursts. We found no superbursts and determine a lower limit on the recurrence time which varies between 30 and 76 days (90% confidence). All recurrence time limits except one are longer than the observed recurrence time for GX 17+2. This difference can be understood if the mass accretion rate in GX 17+2 is several tens of percent higher than in the other sources; alternatively, the accreted material in GX 17+2 might be hydrogen deficient, leading to larger carbon yields than in the other sources. We compare our results to the latest models of superbursts. As our search method is indiscriminate of the burst ignition scenario, the recurrence time limits may also be applied to other bursts of similar duration and brightness.

On the possibility of a helium white dwarf donor in the presumed ultracompact binary 2S 0918–549

2S 0918-549 is a low-mass X-ray binary (LMXB) with a low optical to X-ray flux ratio. Probably it is an ultracompact binary with an orbital period shorter than 60 min. Such binaries cannot harbor hydrogen rich donor stars. As with other (sometimes confirmed) ultracompact LMXBs, 2S 0918-549 is observed to have a high neon-to-oxygen abundance ratio (Juett et al. 2001, ApJ, 560, L59) which has been used to argue that the companion star is a CO or ONe white dwarf. However, types-I X-ray bursts have been observed from several of these systems implying the presence of hydrogen or helium on the neutron star surface. In this paper, we argue that the companion star in 2S 0918-549 is a helium white dwarf. We first present a type-I X-ray burst from 2S 0918-549 with a long duration of 40 min. We show that this burst is naturally explained by accretion of pure helium at the inferred accretion rate of ∼0.01 times the Eddington accretion rate. At higher accretion rates of ∼0.1 Eddington, hydrogen is required to explain long duration bursts. However, at low rates the long duration is due to the large amount of helium that accumulates prior to the burst. We show that it is possible to form a helium white dwarf donor in an ultracompact binary if accretion starts during the first ascent of the giant branch, when the core is predominantly made of helium. Furthermore, this scenario naturally explains the high neon-to-oxgen ratio, without requiring a CO or ONe white dwarf companion. The only observational aspect of 2S 0918-549 that we cannot explain is the absence of helium lines in the optical spectrum. Model calculations of optical accretion disk spectra need to be carried out in order to obtain limits on the helium abundance.

INTEGRAL detection of a long powerful burst from SLX 1735-269

We present results of an analysis of the bursting behavior of the low mass X-ray binary system SLX 1735-269 during INTEGRAL observations of the Galactic Center region in 2003. Six type-I X-ray bursts were detected in total with one being much longer and more powerful then the others. A strong dependence of the burst recurrence time on the mass accretion rate is observed, that is likely caused by a change in the burning regime. The long burst demonstrated a photospheric radius expansion. We discuss possible scenarios of this long burst and show that it is unlikely a carbon burning flash but rather burning of a large pile of hydrogen and helium accelerated by electron capture processes in a dense accumulated layer.

Superbursts at near-Eddington mass accretion rates

Models for superbursts from neutron stars involving carbon shell flashes predict that the mass accretion rate should be anywhere in excess of one tenth of the Eddington limit. Yet, superbursts have so far only been detected in systems for which the accretion rate is derived to be between 0.1 and 0.25 times that limit. The question arises whether this is a selection effect or an intrinsic property. Therefore, we have undertaken a systematic study of data from the BeppoSAX Wide Field Cameras on the luminous source GX 17+2, comprising 10 Msec of effective observing time on superbursts. GX 17+2 contains a neutron star with regular Type-I X-ray bursts and accretes matter within a few tens of percents of the Eddington limit. We find four hours-long flares which reasonably match superburst characteristics. Two show a sudden rise (i.e., faster than 10 s), and two show a smooth decay combined with spectral softening. The implied superburst recurrence time, carbon ignition column and quenching time for ordinary bursts are close to the predicted values. However, the flare decay time, fluence and the implied energy production of (2-4) × 10 17 erg g −1 are larger than expected from current theory.

An observational review of accretion-driven millisecond X-ray pulsars

I present an observational review of the five currently known accretion-driven millisecond X-ray pulsars. A prominent place in this review is given to SAX J1808.4-3658; it was the first such system discovered and currently four outbursts have been observed from this source. This makes SAX J1808.4-3658 the best studied example of the group. Its most recent outburst in October 2002 is of particular interest because of the discovery of two simultaneous kilohertz quasi-periodic oscillations and nearly coherent oscillations during type-I X-ray bursts. This is the first time that such phenomena are observed in a system for which the neutron star spin frequency is exactly known. The other four systems were discovered within the last two years and only limited results have been published. Since new exiting results are to be expected in the future for all five sources, this review will only represent a snap-shot of the current observational knowledge of accretion-driven millisecond X-ray pulsars. A more extended and fully up-to-date review can be found at this http URL

X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature

SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was the longest and brightest. We have analyzed X-ray data sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data cover unabsorbed 2–10 keV fluxes between 0.1 and $5.3\times10^{-9}$ erg s -1 cm -2 . The equivalent luminosity range is $6\times10^{35}$ to $2\times10^{37}$ erg s -1 . The 0.3–10 keV spectrum is well described by a combination of a multi-temperature disk blackbody, a hot Comptonization component and a narrow Fe–K emission line at 6.5 to 6.8 keV with an equivalent width of up to 285 eV. The hydrogen column density in the line of sight is $(8.8\pm0.5)\times10^{22}$ cm -2 . The most conspicuous spectral changes in this model are represented by variations of the temperature and radius of the inner edge of the accretion disk, and a jump of the equivalent width of the Fe–K line in one observation. Furthermore, 45 type-I X-ray bursts were unambiguously detected between 1998 and 2001 which all occurred during or close to outbursts. We derive a distance of $7.5\pm1.3$ kpc which is consistent with previous determinations. Our failure to detect bursts for prolonged periods outside outbursts provides indirect evidence that the source returns to quiescence between outbursts and is a true transient.

A superburst from 4U 1254-69

We report the detection with the BeppoSAX Wide Field Cameras of a superburst from 4U 1254-69. The superburst is preceded by a normal type-I X-ray burst, has a decay time that is the longest of all eight superbursts detected so far and a peak luminosity that is the lowest. Like for the other seven superbursts, the origin is a well-known type-I X-ray burster with a persistent luminosity level close to one tenth of the Eddington limit. Based on WFC data of all persistently bright X-ray bursters, the average rate of superbursts is per year per persistently bright X-ray burster. Some systems may have higher superburst rates. For all superbursters, we present evidence for a pure helium layer which is burnt in an unstable as well as a stable manner.

The Effect of the Mass Accretion Rate on the Burst Oscillations in 4U 1728−34

We present a comprehensive study of the properties of nearly coherent brightness oscillations (NCOs) in a large sample of type-I X-ray bursts observed from low mass X-ray binary 4U 1728-34. We have analyzed ~545ks of data of this source obtained with the Rossi X-ray Timing Explorer over a 3 year span from 1996-1999. The data contain 38 bursts, 16 of which show NCOs. We find no NCOs present when the inferred mass accretion rate of the system is lowest. Furthermore, we define a measure of the strength of the oscillations and find that this strength increases with increasing mass accretion rate. This correlation is particularly evident within bursts detected only a few weeks apart, and becomes less clear for bursts separated in time by several months to years. The correlation we find for 4U 1728-34 between the NCOs and the inferred accretion rate of the system is similar to that found for KS 1731-260 by Muno et al. (2000), where the NCOs are only present at relatively high mass accretion rates. However, unlike the case for KS 1731-260, we find an anti-correlation in the bursts of 4U 1728-34 between the existence of episodes of photospheric radius expansion and the inferred mass accretion rate of the system. Moreover, we distinguish between NCOs present in the rise and in the decay phase of the burst and find that the bursts with NCOs only in the decay phase occur at intermediate accretion rates, while those with NCOs in both the rise and the decay phases are concentrated at high inferred mass accretion rates. We discuss these results in the context of the theory of thermonuclear bursts and propose intrinsic differences in the neutron stars in KS 1731-260 and 4U 1728-34 as the origin of the different behaviors.

The first outburst of SAX J1808.4-3658 revisited

Data of the 1996 outburst of the single-known accreting millisecond pulsar SAX J1808.4-3658, taken with the Wide Field Cameras (WFCs) on BeppoSAX , are revisited with more complete data coverage and more comprehensive analysis techniques than in a previous report. An additional type-I X-ray burst was identified which occurred at a time when the persistent emission is below the detection limit, roughly 30 days after outburst maximum. This burst is three times longer than the first two bursts, and 50% brighter. It is the brightest burst within the ~1700 type-I bursts detected so far with the WFCs. A spectral analysis of the data reveals a distance to SAX J1808.4-3658 of ~2.5 kpc. This is an update from a previously reported value of 4 kpc. We present the evidence that we have for the presence of oscillations at the pulsar frequency during part of the newly found burst. Such an oscillation would lend support to the idea that the frequency of millisecond burst oscillations in other objects is very close to the neutron star rotation frequency.

The eclipsing bursting X-ray binary EXO 0748-676 revisited by XMM-Newton

The bright eclipsing and bursting low-mass X-ray binary EXO 0748-676 has been observed at several occasions by XMM-Newton during the initial calibration and performance verification (CAL/PV) phase. We present here the results obtained from observations with the EPIC cameras. Apart from several type-I X-ray bursts, the source shows a high degree of variability with the presence of soft flares. The wide energy coverage and high sensitivity of XMM-Newton allows for the first time a detailed description of the spectral variability. The source is found to be the superposition of a central (~2 10^8 cm) Comptonized emission, most probably a corona surrounding the inner edge of an accretion disk, associated with a more extended (~3 10^10 cm) thermal halo at a typical temperature of ~0.6 keV with an indication of non-solar abundances. Most of the variations of the source can be accounted for by a variable absorption affecting only the central comptonized component and reaching up to NH ~1.3 10^23 cm^{-2}. The characteristics of the surrounding halo are found compatible with an irradiated atmosphere of an accretion disc which intercepts the central emission due to the system high inclination.

Beppo-SAX observation of the burster GS 1826-238

One of the most intriguing features of the burster GS 1826-238 is the peculiar stability of its bursting behaviour over more than 3 years of observations by the Wide Field Cameras on board BeppoSAX. In fact, the source exhibits a type-I X-ray burst every ∼ 6 hours, and the burst wait times are constant within a few minutes for events spanning up to 6 months apart. In order to better study the physical parameters of GS 1826-238 in connection with its quasi-periodic bursting activity, a BeppoSAX-NFI observation was performed in Autumn 1999. The source was thus studied during consecutive bursts with wide band and sensitive instruments for the first time. Preliminary results of our analysis are presented. In particular, the possible observation of an occasional transition to pure helium burning regime is discussed.

Broadband Observations of the New X-Ray Burster SAX J1747.0−2853 during the 1998 March Outburst

We report on our discovery and follow-up observations of the X-ray source SAX J1747.0-2853 detected in outburst on 1998, March 10 with the BeppoSAX Wide Field Cameras in the energy range 2-28 keV. The source is located about half degree off the Galactic Nucleus. A total of 14 type-I X-ray bursts were detected in Spring 1998, thus identifying the object as a likely low-mass X-ray binary harboring a weakly magnetized neutron star. Evidence for photospheric radius expansion is present in at least one of the observed bursts, leading to an estimate of the source distance of about 9 kpc. We performed a follow-up target of opportunity observation with the BeppoSAX Narrow Field Instruments on March 23 for a total elapsed time of 72 ks. The source persistent luminosity was 2.6x10^36 erg/s in the 2-10 keV energy range. The wide band spectral data (1-200 keV) are consistent with a remarkable hard X-ray spectrum detected up to 150 keV, highly absorbed at low energies (Nh of the order of 10^23 cm^-2) and with clear evidence for an absorption edge near 7 keV. A soft thermal component is also observed, which can be described by single temperature blackbody emission at about 0.6 keV.

SAX J1810.8−2609: A New Hard X‐Ray Bursting Transient

The transient X-ray source SAX J1810.8-2609 was discovered on 1998, March 10 with the Wide Field Cameras on board the BeppoSAX satellite, while observing the Galactic Bulge in the 2-28 keV energy range. On March 11, a strong type-I X-ray burst was detected with evidence of photospheric radius expansion. A follow-up target of opportunity observation with the Narrow-Field Instruments (NFI) was performed on March 11 and 12, for a total elapsed time of 8.51 x 10^{4} s. The wide band spectral data (0.1-200 keV) obtained with the NFI show a remarkable hard X-ray spectrum detected up to ~200 keV, which can be described by a power law with photon spectral index Gamma=1.96\pm0.04, plus a soft component which is compatible with blackbody radiation of temperature kT~0.5 keV. The detection of the type-I X-ray burst is a strong indication that the compact object is a neutron star in a low mass X-ray binary system. Assuming standard burst parameters and attributing the photospheric radius expansion to near Eddington luminosity, we estimate a distance of ~5 kpc. The inferred 2-10 keV X-ray luminosity is ~ 9 x 10^{35} erg/s at the time of the discovery.

A New Bursting X-Ray Transient: SAX J1750.8−2900

We have analysed in detail the discovery measurements of the X-ray burster SAX J1750.8-2900 by the Wide Field Cameras on board BeppoSAX in spring 1997, at a position ~1.2 degrees off the Galactic Centre. The source was in outburst on March 13th when the first observation started and showed X-ray emission for ~ 2 weeks. A total of 9 bursts were detected, with peak intensities varying from ~ 0.4 to 1.0 Crab in the 2-10 keV range. Most bursts showed a fast rise time (~ 1s), an exponential decay profile with e-folding time of ~ 5s, spectral softening during decay, and a spectrum which is consistent with few keV blackbody radiation. These features identify them as type-I X-ray bursts of thermonuclear origin. The presence of type-I bursts and the source position close to the Galactic Centre favours the classification of this object as a neutron star low mass X-ray binary. X-ray emission from SAX J1750.8-2900 was not detected in the previous and subsequent Galactic bulge monitoring, and the source was never seen bursting again.

Statistics of the burst model at super-critical phase

We investigate the statistics of a model of type-I X-ray burst (Phys. Rev. E 51 (1995) 3045) in its super-critical phase. The time evolution of the burnable clusters, places where fire can pass through, is studied using simple statistical arguments. We offer a simple picture for the time evolution of the percentage of space covered by burnable clusters. A relation between the time-average and the peak percentage of space covered by burnable clusters is also derived.

Observation of type IX-ray bursts from Cir X-1

EXOSAT has recently observed three X-ray bursts from the direction of Circinus X-1 which have all the properties of type I bursts. These bursts most likely originate from Cir X-1 and we therefore identify Cir X-1 as a neutron star and not a black hole. The properties of the new bursts, such as the ratio of the energy released between bursts to that released during a burst, and the blackbody radius, differ from those of earlier bursts from Cir X-1 in a manner similar to that found in EXO0748–676.

Shell-Flashes on Accreting Neutron Stars and Mode Profiles of Type-I X-Ray Bursts

Thermal stabilities of hydrogen and helium shell-burnings on accreting neutron stars are studied semi-analytically, the progress of nuclear reactions during the flash is followed numerically, and the mechanism which makes different modes of Type-I X-ray bursts is discussed.