M. Cocchi

Photospheric radius expansion X-ray bursts as standard candles

We examined the maximum bolometric peak luminosities during type I X-ray bursts from the persistent or transient luminous X-ray sources in globular clusters. We show that for about two thirds of the sources the maximum peak luminosities during photospheric radius expansion X-ray bursts extend to a critical value of 3:790:1510 38 erg s 1 , assuming the total X-ray burst emission is entirely due to black-body radiation and the recorded maximum luminosity is the actual peak luminosity. This empirical critical luminosity is consistent with the Eddington luminosity limit for hydrogen poor material. Since the critical luminosity is more or less always reached during photospheric radius expansion X-ray bursts (except for one source), such bursts may be regarded as empirical standard candles. However, because significant deviations do occur, our standard candle is only accurate to within 15%. We re-evaluated the distances to the twelve globular clusters in which the X-ray bursters reside.

Six years of BeppoSAX Wide Field Cameras observations of nine galactic type I X-ray bursters

We present an overview of BeppoSAX Wide Field Cameras observations of the nine most frequent type I X-ray bursters in the Galactic center region. Six years of observations (from 1996 to 2002) have amounted to 7 Ms of Galactic center observations and the detection of 1823 bursts. The 3 most frequent bursters are GX 354-0 (423 bursts), KS 1731-260 (339) and GS 1826-24 (260). These numbers reflect an unique dataset. We show that all sources have the same global burst behavior as a function of luminosity. At the lowest luminosities ( LX < 2 10 37 erg s 1 ) bursts occur quasi-periodically and the burst rate increases linearly with accretion rate (clear in e.g. GS 1826-24 and KS 1731-260). At Lpers = 2 10 37 erg s 1 the burst rate drops by a factor of five. This corresponds to the transition from, on average, a hydrogen-rich to a pure helium environment in which the flashes originate that are responsible for the bursts. At higher luminosities the bursts recur irregularly; no bursts are observed at the highest luminosities. Our central finding is that most of the trends in bursting behavior are driven by the onset of stable hydrogen burning in the neutron star atmosphere. Furthermore, we notice three new observational fact which are dicult to explain with current burst theory: the presence of short pure-helium bursts at the lowest accretion regimes, the bimodal distribution of peak burst rates, and an accretion rate that is ten times higher than predicted at which the onset of stable hydrogen burning occurs. Finally, we note that our investigation is the first to signal quasi-periodic burst recurrence in KS 1731-260, and a clear proportionality between the frequency of the quasi-periodicity and the persistent flux in GS 1826-24 and KS 1731-260.

The First IBIS/ISGRI Soft Gamma-Ray Galactic Plane Survey Catalog

We report the first high-energy survey catalog obtained with the IBIS gamma-ray imager on board INTEGRAL. The analysis has been performed on the first-year Core Program ISGRI data comprising both Galactic Plane Scan and Galactic Centre Deep Exposure pointings for a total exposure time exceeding 5 Ms. This initial survey has revealed the presence of ~120 sources detected with the unprecedented sensitivity of ~1 mcrab in the energy range 20-100 keV. Each source is located to an accuracy between 1 and 3, depending on its brightness. The outstanding IBIS capability to locate soft γ-ray emitters has allowed us to identify most of the detected sources with already known Galactic X-ray binary systems, while 28 of the objects are of unknown nature.

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

We have discovered three certain (SAX J1324.5 6313, 2S 1711 339 and SAX J1828.5 1037) and two likely (SAX J1818.7+1424 and SAX J2224.9+5421) new thermonuclear X-ray burst sources with the BeppoSAX Wide Field Cameras, and observed a second burst ever from a sixth one (2S 0918 549). Four of them (excluding 2S 1711 339 and 2S 0918 549) are newly detected X-ray sources from which we observed single bursts, but no persistent emission. We ob- serve the first 11 bursts ever from 2S 1711 339; persistent flux was detected during the first ten bursts, but not around the last burst. A single burst was recently detected from 2S 0918 549 by Jonker et al. (2001); we observe a second burst showing radius expansion, from which a distance of 4.2 kpc is derived. According to theory, bursts from very low flux levels should last>100 s. Such is indeed the case for the last burst from 2S 1711 339, the single burst from SAX J1828.5 1037 and the two bursts from 2S 0918 549, but not for the bursts from SAX J1324.5 6313, SAX J1818.7+1424 and SAX J2224.9+5421. The bursts from the latter sources all last20 s. We suggest that SAX J1324.5 6313, SAX J1818.7+1424, SAX J1828.5 1037 and SAX J2224.9+5421 are members of the recently proposed class of bursters with distinctively low persistent flux levels, and show that the galactic distribution of this class is compatible with that of the standard low-mass X-ray binaries.

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.

A half-a-day long thermonuclear X-ray burst from KS 1731-260

We report on an approximately twelve hour long X-ray flare from the low-mass X-ray binary KS 1731-260. The flare has a rise time of less than 13min and declines exponentially with a decay time of 2.7 hours. The flare emission is well described by black-body radiation with peak temperature of 2.4 keV. The total energy release from the event is 10 4 2 erg (for an assumed distance of 7 kpc). The flare has all the characteristics of thermo-nuclear X-ray bursts (so-called type I X-ray bursts), except for its very long duration and therefore large energy release (factor of 1500-4000 longer and 250-425 more energy than normal type I X-ray bursts from this source). The flare is preceded by a short and weak X-ray burst, possibly of type I. Days to weeks before the flare, type I X-ray bursts were seen at a rate of ∼3 per day. However, after the flare type I X-ray bursting ceased for at least a month, suggesting that the X-ray flare affected the type I bursting behaviour. The persistent emission is not significantly different during the non-bursting period. We compare the characteristics of this event with similar long X-ray flares, so-called superbursts, seen in other sources (4U 1735-44, 4U 1820-30, 4U 1636-53, Ser X-1, GX 3+1). The event seen from KS 1731-260 is the longest reported so far. We discuss two possible mechanisms that might cause these superbursts, unstable carbon burning (as proposed recently) and electron capture by protons with subsequent capture of the resulting neutrons by heavy nuclei.

Bursts from GS 1826–238: A Clocked Thermonuclear Flashes Generator

The transient X-ray source GS 1826-238 was monitored during five different observing periods between 1996 August and 1998 October with the BeppoSAX Wide Field Camera instrument in the framework of a deep observation of the Galactic center region. During this time, a first detection of X-ray bursts from this source occurred, ruling out its previously suggested black hole candidacy and strongly suggesting the compact object to be a weakly magnetized neutron star. During the 2.5 yr of monitoring, corresponding to ~2.0 Ms of observing time, 70 bursts were detected from the source. Here we report a quasi-periodicity of 5.76 hr in the burst occurrence time that is present during all observations. This is the first example of quasi-periodic bursting over a period of years. It is in line with the history of a rather constant flux since the turn-on in 1988 and points to a very stable accretion.

Discovery of the X-ray burster SAX J1752.3-3138

During a 50 ks monitoring observation of the Galactic bulge performed in September 1999 by the Wide Field Cameras on board the BeppoSAX satellite, an X-ray burst was detected from a sky position ~

Discovery of the neutron star nature of SLX 1737-282

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BeppoSAX-WFC monitoring of the Galactic Center region

off the Galactic centre. No previously known X-ray sources are located within the position error circle of the observed burst. The new burster, SAX J1752.3-3138, did not show any persistent emission during the whole observation. No other bursting events, as well as steady emission, were reported so far by other instruments or detected in the WFC archive, which covers ~6 Ms and ~4 Ms for burst and persistent luminosity detection, respectively, starting from August 1996. Unless the source is a very weak transient, this could indicate SAX J1752.3-3138 is an atypical burster, a member of a possibly new class of sources characterised by very low steady luminosities and accretion rates (