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J. J. M. in 't Zand

Publications and source records attributed to J. J. M. in 't Zand.

At least 109 records · Page 6Linked to original sources
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

GRB 990704: the most X-ray rich BeppoSAX gamma-ray burst

We present the X- and $γ$-ray detection of GRB 990704 and the discovery and study of its X-ray afterglow, 1SAX J1219.5-0350. Two pointed BeppoSAX observations with the narrow field instruments were performed on this source, separated in time by one week. The decay of the X-ray flux within the first observation appears unusually slow, being best-fit by a power law with negative index 0.83$\pm$0.16. Such a slow decay is consistent with the non-detection in our second observation, but its back-extrapolation to the time of the GRB largely underestimates the detected GRB X-ray prompt emission. In addition, the GRB prompt event shows, among the BeppoSAX-WFC detected sample, unprecedentedly high ratios of X- and gamma-ray peak fluxes (F$_{2-10 keV}$/F$_{40-700 keV}$$\sim$0.6, and F$_{2-26 keV}$/F$_{40-700 keV}$$\sim$1.6) and fluences (S$_{2-10 keV}$/S$_{40-700 keV}$$\sim$1.5 and S$_{2-26 keV}$/S$_{40-700 keV}$$\sim$2.8), making it, among the BeppoSAX arcminute-localized GRBs, the closest to the recently discovered class of Fast X-ray Transients.

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arXiv2001

BeppoSAX observation of the eclipsing dipping X-ray binary X1658-298

Results of a 2000 August 12-13 BeppoSAX observation of the 7.1 hr eclipsing, dipping, bursting, transient, low-mass X-ray binary (LMXRB) X1658-298 are presented. The spectrum outside of eclipses, dips and bursts can be modeled by the combination of a soft disk-blackbody and a harder Comptonized component with a small amount (1.3 10E21 atom/cm2) of low-energy absorption. In contrast, an RXTE observation 18 months earlier during the same outburst, measured an absorption of 5.0 10E22 atom/cm2. Such a change is consistent with a thinning of the accretion disk as the outburst progresses. Structured residuals from the best-fit spectral model are present which are tentatively identified with Ne-K/Fe-L and Fe-K shell emission. The spectral changes during dips are complex and may be modeled by a strong (~3 10E23 atom/cm2) increase in absorption of the Comptonized component only, together with reductions in normalizations of both spectral components. This behavior is in contrast to the ``complex continuum'' model for X-ray dip sources, where the softer blackbody component rapidly suffers strong absorption. It is however, similar to that found during recent XMM-Newton observations of the eclipsing, dipping, LMXRB EXO0748-676.

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arXiv2001

BeppoSAX measurements of the bright gamma-ray burst 010222

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the gamma-ray burst GRB010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40-700 keV fluence of (9.3+/-0.3)E-5 erg cm-2 is only surpassed by GRB990123. In terms of the isotropic 20-2000 keV energy output of 7.8E53 erg, it ranks third of all GRBs with measured distances. Since this burst is so bright, the data provide complete and valuable coverage up to 65 hr after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux history shows clear signs of a break which is consistent with a break seen in the optical, and provides supporting evidence for the achromatic nature of the break. An explanation for the break in the context of a collimated expansion is not straightforward. Rather, a model is favored whereby the fireball is braked to the non-relativistic regime quickly (within a fraction of day) by a dense 1E6 cm-3 circumburst medium. This implies that, after a mild beaming correction, GRB010222 may be the most energetic burst observed thus far. The X-ray decay index after the break is 1.33+/-0.04, the spectral index 0.97+/-0.05. The decay is, with unprecedented accuracy, identical to that observed in the optical.

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arXiv2001

A Sample of Radius-Expansion Bursters Observed by the BeppoSAX-WFCs

Since 1996 a monitoring campaign of the Galactic Bulge region has been performed by the BeppoSAX Wide Field Cameras in the 2-28 keV band. This allowed accurate and unbiased studies of classes of objects. In particular the type-I X-ray bursters, whose distribution is concentrated in the monitored sky region, have been investigated. High luminosity radius-expansion bursts have been observed from nine sources, allowing to estimate important physical parameters such as distance, photospheric radius, averaged luminosity and accretion rate. An overview of these results is presented.

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arXiv2001

Discovery of a 75 day orbit in XTE J1543-568

Dedicated monitoring of the transient X-ray pulsar XTE J1543-568 during the first year after its discovery has revealed the unambiguous detection of a binary orbit. The orbital period is 75.56+/-0.25 d, and the projected semi-major axis 353+/-8 lt-sec. The mass function and position in the pulse period versus orbital period diagram are consistent with XTE J1543-568 being a Be X-ray binary. The eccentricity of less than 0.03 (2 sigma) is among the lowest for the 12 Be X-ray binaries whose orbits have now been measured. This confirms the suspicion that small kick velocities of neutron stars during supernovae are more common than expected. The distance is estimated to be larger than 10 kpc, and the luminosity at least 1E37 erg/s.

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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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arXiv2001

The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature

We report on the prompt X- and gamma-ray observations of GRB990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera No. 2. Due to Sun constraints, we could not perform a follow-up observation with the BeppoSAX Narrow Field Instruments. The light curve of the prompt emission shows two pulses and a total duration of about 40s in X-rays. In gamma-rays the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy Ep of the E F(E) spectrum: Ep is above our energy passband during the first pulse and goes down to ~10 keV during the second pulse. Another peculiarity is noted in this event for the first time: the possible evidence of a 2s duration emission feature during the tail of the first pulse. The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kTbb ~1.3 keV. We discuss the possible origin of the feature. The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball, when it becomes optically thin.

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arXiv2000

Discovery of a redshifted Iron K-line in the X-ray afterglow of GRB 000214

We report the detection (3 sigma significance level) of a strong iron emission line in the X-ray spectrum of the afterglow of GRB 000214 (``Valentine's Day Burst'') observed by BeppoSAX. An emission line feature was observed with a centroid energy of 4.7+/-0.2 keV which, if interpreted as K-alpha emission from hydrogen-like iron, corresponds to a redshift of z=0.47. The observed intensity (EW ~2 keV) and duration (tens of hours) of the line give information on the distance, from the burst region, of the emitting material (R>3x10^15 cm) and its mass (M>1.4 solar mass). These results are not easily reconciled with the binary merger and hypernova models for gamma ray bursts.

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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

X-ray afterglow detection of the short gamma-ray burst 991014

GRB 991014 is one of the shortest gamma-ray bursts detected so far with the Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. The duration is 9.6 sec in 2-28 keV and 3.2 sec in 40 to 700 keV (as measured between the times when 5 and 95% of the burst photons have been accumulated). We refine the InterPlanetary Network annulus of the burst, present the detection of the X-ray afterglow of GRB 991014 within this refined annulus, and discuss X-ray and gamma-ray observations of the prompt and afterglow emission. Except for the briefness of the prompt event, no other unusual aspects were found in the prompt and afterglow observations as compared to such measurements in previous gamma-ray bursts.

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arXiv2000

The transient X-ray source SAX J2239.3+6116 and its optical counterpart

We have discovered with the Wide Field Cameras on board BeppoSAX the weak transient X-ray source SAXJ2239.3+6116 whose position coincides with that of 4U2238+60/3A2237+608 and is close to that of the fast transient AT2238+60 and the unidentified EGRET source 3EG2227+6122. The data suggest that the source exhibits outbursts that last for a few weeks and peak to a flux of 4E-10 erg/s/cm2 (2-10 keV) at maximum. During the peak the X-ray spectrum is hard with a photon index of -1.1+/-0.1. Follow-up observations with the Narrow-Field Instruments on the same platform revealed a quiescent emission level that is 1E+3 times less. Searches through the data archive of the All-Sky Monitor on RXTE result in the recognition of five outbursts in total from this source during 1996-1999, with a regular interval time of 262 days. Optical observations with the KPNO 2.1 m telescope provide a likely optical counterpart. It is a B0 V to B2 III star with broadened emission lines at an approximate distance of 4.4 kpc. The distance implies a 2-10 keV luminosity in the range from 1E+33 to 1E+36 erg/s. The evidence suggests that SAXJ2239.3+6116 is a Be X-ray binary with an orbital period of 262 days.

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arXiv2000

Discovery of 1RXS J171824.2-402934 as an X-ray burster

We report the first-time detection of a type-I X-ray burst from a position that is consistent with that of 1RXS J171824.2-402934. The detection was made on Sep 23, 1996, with the Wide Field Cameras on board the BeppoSAX satellite. The burst had a peak intensity of 1.3 Crab units in 2 to 28 keV and is relatively long (at least 3.5 min). Analysis of the burst gives clear evidence for photospheric radius expansion. Assuming the peak luminosity to be close to the Eddington limit, standard burst parameters and taking into account gravitational redshift effects, the distance to the source is approximately 6.5 kpc. No other bursts from this source have been observed during the rest of the WFC observations. The detection of a type-I burst implies that 1RXS J171824.2-402934in all likelihood is a low-mass X-ray binary where the compact object is a neutron star.

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arXiv2000

X-ray and optical follow-up observations of the August 1998 X-ray transient in NGC6440

The BeppoSAX Wide Field Cameras detected a transient in NGC6440 on 1998 Aug 22. ROSAT did not detect this source on 1998 Sep 8, indicating that the X-ray flux had decreased by a factor 400 at least, and/or that the X-ray spectrum had become appreciably softer. Analysis of archival ROSAT HRI data reveals two sources associated with NGC6440; one of these may be the transient in quiescence. We have also obtained B and R images of NGC6440 on 1998 Aug 26 and again on 1999 July 15, when the transient had returned to quiescence. Subtraction of these images reveals one object in the core which was brighter in B, but not in R, during the X-ray outburst. We estimate B=22.7 and (B-R)o<0 on 1998 Aug 26, which makes it a viable candidate counterpart.

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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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