arXiv subjects

M. Cocchi

Publications and source records attributed to M. Cocchi.

At least 37 records · Page 2Linked to original sources

arXiv2003

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 (L_X<=2E37 erg/s 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 L_pers=2E37 erg/s 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 difficult 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.

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arXiv2003

Burst-properties as a function of mass accretion rate in GX 3+1

GX 3+1 is a low-mass X-ray binary that is persistently bright since its discovery in 1964. It was found to be an X-ray burster twenty years ago proving that the compact object in this system is a neutron star. The burst rate is so low that only 18 bursts were reported prior to 1996. The Wide Field Cameras on BeppoSAX have, through a dedicated monitoring program on the Galactic center region, increased the number of X-ray bursts from GX 3+1 by 61. Since GX 3+1 exhibits a slow (order of years) modulation in the persistent flux of about 50%, these observations opens up the unique possibility to study burst properties as a function of mass accretion rate for very low burst rates. This is the first time that bursts are detected from GX 3+1 in the high state. From the analysis we learn that all bursts are short with e-folding decay times smaller than 10 s. Therefore, all bursts are due to unstable helium burning. Furthermore, the burst rate drops sixfold in a fairly narrow range of 2-20 keV flux; we discuss possible origins for this.

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arXiv2002

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 (3.79+/-0.15)x10^{38} erg/s, 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.

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arXiv2002

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 observe 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 last ~20 s. We suggest that SAX J1324.5-6313, SAX J1818.7+1424, SAX J1828.$-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.

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arXiv2002

The nature of the X-ray transient SAX J1711.6-3808

SAX J1711.6-3808 is an X-ray transient in the Galactic bulge that was active from January through May of 2001 and whose maximum 1-200 keV luminosity was measured to be 5X10-9 erg/s/cm2 which is less than ~25% of the Eddington limit, if placed at a distance equal to that of the galactic center. We study the X-ray data that were taken of this moderately bright transient with instruments on BeppoSAX and RXTE. The spectrum shows two interesting features on top of a Comptonized continuum commonly observed in low-state X-ray binaries: a broad emission feature peaking at 7 keV and extending from 4 to 9 keV, and a soft excess with a color temperature below 1 keV which reveals itself only during one week of data. High time-resolution analysis of 412 ksec worth of data fails to show bursts, coherent or high-frequency quasi-periodic oscillations. Given the dynamic range of the flux measurements, this would be unusual if a neutron star were present. SAX J1711.6-3808 appears likely to contain a black hole. No quiescent optical counterpart could be identified in archival data within the 5"-radius XMM error circle, but the limits are not very constraining because of heavy extinction (Av=16).

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arXiv2002

Discovery of the neutron star nature of SLX 1737-282

SLX 1737-282 is a persistent moderately bright X-ray source 1.2 deg from the Galactic center. X-ray observations with the Wide Field Cameras on BeppoSAX have for the first time revealed the true nature of SLX 1737-282: a 15-min long thermonuclear flash was detected exposing the source as a neutron star in a binary system. The flash was Eddington-limited constraining the distance to between 5 and 8 kpc. We analyze BeppoSAX, ROSAT, and RXTE data on SLX 1737-282. The persistent 0.5-200 keV luminosity is close to or less than 1% of the Eddington limit which implies a rarely-seen circumstance for thermonuclear flash activity.

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arXiv2001

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 13 min 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^{42} 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 superburst 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.

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arXiv2001

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 ~3 degrees 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 (L_x < 10^35 erg/s) and extremely rare bursting activity. The characteristics of the detected burst are consistent with a type I event, identifying the source as a weakly magnetised neutron star in a low-mass X-ray binary system. Evidence for photospheric radius expansion due to Eddington-limited burst luminosity allows to estimate the distance to the source (~9 kpc).

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arXiv2001

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.

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arXiv2000

Wide band observations of the new X-ray burster SAX J1747.0-2853 during the March 1998 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.

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arXiv2000

Turmoil on the accretion disk of GRO J1655-40

During the 1996/1997 outburst of the X-ray transient GRO J1655-40 the Wide Field Cameras onboard BeppoSAX observed the system with nearly continuous coverage during some stages of this outburst. On two occasions we find clear evidence for frequent dipping behaviour, characterized by several absorption dips occurring within one binary orbit. However, during preceding and following binary orbits there is no sign of such dips at the same range of orbital phases. We also collected light curves of absorption dips as seen with other X-ray instruments. Although the 65 dips that were collected occur preferably between orbital phases 0.68 and 0.92, there is evidence that the morphology of the material causing the dips is changing over just one binary orbit. The short duration (~minutes) and phasing of the dips are discussed in the framework of models where the stream from the companion star encounters a hot outer rim on the accretion disk.

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arXiv2000

"Clocked" Bursts From GS 1826-238

We report on the long term monitoring of the GINGA transient source GS 1826-238 performed with the BeppoSAX Wide Fiels Camera (WFC) instrument, during four different observing campaigns covering October 1996 - September 1999. WFC detection of type-I bursting activity from the source ruled out its proposed black hole candidacy and clearly suggested the compact object related to GS 1826-238 to be a weakly magnetized neutron star. The analysis of the arrival times of the observed 78 bursts lead to the discovery of a recurrence of ~5.75 hours with a spread of 38 minutes (FWHM) along more than 3 years monitoring data (Ubertini et al., 1999). We performed a more detailed analysis of the whole available data, and evidence of shortening of the recurrence time, together with a drastic narrowing in the spread (down to a few minutes) was observed on a one year time scale. Possible relation with the source X-ray persistent emission is discussed.

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arXiv2000

Bright X-ray bursts from 1E 1724-3045 in Terzan 2

During about 3 years wide field monitoring of the Galactic Center region by the WFC telescopes on board the BeppoSAX satellite, a total of 14 type-I X-ray bursts were detected from the burster 1E 1724-3045 located in the globular cluster Terzan 2. All the observed events showed evidence for photospheric radius expansion due to Eddington-limit burst luminosity, thus leading to an estimate of the source distance (~7.2 kpc). Preliminary results of the analysis of the bursts are presented.

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arXiv2000

Observations of Eddington-limited type-I X-ray bursts from 4U 1812-12

During more than 3 years (August 1996-October 1999) monitoring of a 40x40 degree sky region around the Galactic Centre by the Wide Field Cameras on board BeppoSAX, a total of 8 type-I bursts have been detected from a sky position consistent with that of 4U 1812-12, a likely neutron-star low-mass X-ray binary. We present the results of a detailed study of the bursts of 4U 1812-12, about 15 years after the last reported observations of X-ray bursts from this source (Murakami et al. 1983). Clear evidence for photospheric radius expansion due to Eddington-limited burst luminosity is present in most of the observed events, allowing an accurate estimate of the source distance (~4 kpc) and its burst parameters.

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arXiv2000

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.

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arXiv2000

BeppoSAX observations of the nearby low-mass X-ray binary and fast transient SAX J1819.3-2525

SAX J1819.3-2525 is a nearby X-ray transient which exhibited a fast and large X-ray outburst on Sep. 15, 1999 (Smith et al. 1999). The Wide Field Cameras and the Narrow Field Instruments (NFI) on board BeppoSAX observed SAX J1819.3-2525 at various stages of its activity before that, in the spring and fall of 1999. The fluxes range between 0.012 and 0.3 Crab units (2-10 keV). The NFI observation is unique because it is the longest semi-continuous observation of SAX J1819.3-2525 so far, and it offers a study of the spectrum at a relatively high resolution of 8% full width at half maximum at 6 keV. We discuss the observations with emphasis on the X-ray spectrum. A strong Fe-K emission line was detected in SAX J1819.3-2525 with an equivalent width between 0.3 and 1 keV. The line energy is up to 6.85 +/- 0.02 keV and suggests the presence of highly ionized iron. We identify this as fluorescent emission from a photo-ionized plasma. The continuum spectrum is typical for a low-mass X-ray binary in which emission from an accretion disk corona plays an important role. There is no sign of an eclipse or periodic signal due to the binary orbit in this exposure, despite the fact that the twin jets seen at radio wavelengths suggest a high inclination angle.

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arXiv1999

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.

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arXiv1999

Broad-band X-ray measurements of GS 1826-238

The broad band X-ray spectrum of the low-mass X-ray binary (LMXB) GS1826-238 was measured with the narrow-field instruments on BeppoSAX on April 6 and 7, 1997. The spectrum is consistent with the Comptonization of a 0.6 keV thermal spectrum by a hot cloud of temperature equivalent kT=20 keV. During the observation two type I X-ray bursts were detected. From the bursts an upper limit to the distance could be derived of 8 kpc. Combined with an elsewhere determined lower limit of 4 kpc this implies a persistent X-ray luminosity between 3.5 X 10^36 and 1.4 X 10^37 erg/s which is fairly typical for a LMXB X-ray burster. The accurate determination of the energetics of the two bursts and the persistent emission confirm results with the Wide Field Cameras on BeppoSAX in a narrower bandpass (Ubertini et al. 1999). Comparison with independent X-ray measurements taken at other times indicates that GS 1826-238 since its turn-on in 1988 is a rather stable accretor, which is in line with the strong regularity of type I X-ray bursts.

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