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F. Verbunt

Publications and source records attributed to F. Verbunt.

At least 55 records · Page 3Linked to original sources
arXiv2002

Chandra follow-up of bursters with low persistent emission

We report on Chandra ACIS-S observations of five type I X-ray bursters with low persistent emission: SAX J1324.5-6313, SAX J1752.3-3128, SAX J1753.5-2349, SAX J1806.5-2215 and SAX J1818.7+1424. We designate candidate persistent sources for four X-ray bursters. All candidates are detected at a persistent luminosity level of 10^(32-33) erg/s, comparable to soft X-ray transients in quiescence. From the number of bursters with low persistent emission detected so far with the Wide Field Cameras, we estimate a total of such sources in our Galaxy between 30 and 4000.

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arXiv2002

High resolution spatial and timing observations of millisecond pulsar PSR J0218+4232 with Chandra

We report on high-resolution spatial and timing results for binary millisecond pulsar PSR J0218+4232 obtained with the Chandra HRC-I and HRC-S in imaging mode. We found no indications for extended emission at 1 arcsec scales. With the HRC-S observation we obtained a 0.08-10 keV pulse profile with high statistics which could be compared in absolute time with the pulse profiles at radio-frequencies and at high-energy gamma-rays. The two X-ray pulses at aligned with 2 of the 3 radio-pulses, and with the 2 gamma-ray pulses.

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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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arXiv2002

Optical Identification of Multiple Faint X-ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables

We report on the Chandra ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect 6 X-ray sources within the 10.5" core radius and 13 more within the 115" half-mass radius down to a limiting luminosity of Lx approx 10^{30} erg/s for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), 1-3 likely RS CVn or BY Dra systems, and 1 or 2 possible background objects. Of the 7 sources for which no optical identifications were made, we expect that ~2-4 are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.

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arXiv2001

Photometric variability in the old open cluster M 67. II. General Survey

We use differential CCD photometry to search for variability in BVI among 990 stars projected in and around the old open cluster M 67. In a previous paper we reported results for 22 cluster members that are optical counterparts to X-ray sources; this study focuses on the other stars in our observations. A variety of sampling rates were employed, allowing variability on time scales ranging from \sim 0.3 hours to \sim 20 days to be studied. Among the brightest sources studied, detection of variability as small as sigma approx 10 mmag is achieved (with > 3 sigma confidence); for the typical star observed, sensitivity to variability at levels sigma approx 20 mmag is achieved. The study is unbiased for stars with 12.5 < B < 18.5, 12.5 < V < 18.5, and 12 < I < 18 within a radius of about 10 arcmin from the cluster centre. In addition, stars with 10 < BVI < 12.5 were monitored in a few small regions in the cluster. We present photometry for all 990 sources studied, and report the variability characteristics of those stars found to be variable at a statistically significant level. Among the variables, we highlight several sources that merit future study, including stars located on the cluster binary sequence, stars on the giant branch, blue stragglers, and a newly discovered W UMa system.

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arXiv2001

A four-hours long burst from Serpens X-1

During a serendipitous observation of the BeppoSAX Wide Field Cameras, a very long Type I X-ray burst was observed from the low mass X-ray binary Serpens X-1. The burst lasted for approximately 4 hours and had an exponential decay time of 69+/-2 min (2-28 keV). The bolometric peak-luminosity is (1.6+/-0.2)x10^38 erg/s and the fluence (7.3+/-1.4)x10^41 erg. The first 'normal' Type I burst was observed 34 days after the superburst. This is in rough agreement with recent predictions for unstable carbon burning in a heavy element ocean.

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arXiv2001

Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star.

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arXiv2001

Population synthesis for double white dwarfs. II. Semi-detached systems: AM CVn stars

We study two models for AM CVn stars: white dwarfs accreting (i) from a helium white dwarf companion and (ii) from a helium-star donor. We show that in the first model possibly no accretion disk forms at the onset of mass transfer. The stability and the rate of mass transfer then depend on the tidal coupling between the accretor and the orbital motion. In the second model the formation of AM CVn stars may be prevented by detonation of the CO white dwarf accretor and the disruption of the system. With the most favourable conditions for the formation of AM CVn stars we find a current Galactic birth rate of 6.8 10^{-3} yr^{-1}. Unfavourable conditions give 1.1 10^{-3} yr^{-1}. The expected total number of the systems in the Galaxy is 9.4 10^{7} and 1.6 10^{7}, respectively. We model very simple selection effects to get some idea about the currently expected observable population and discuss the (quite good) agreement with the observed systems.

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arXiv2000

BeppoSAX study of the broad-band properties of luminous globular cluster X-ray sources

We have performed a detailed study of the broadband spectra of the luminous globular cluster X-ray sources using BeppoSAX. With the exception of X2127+119, located in NGC7078, all the other spectra are well represented by a two component model consisting of a disk-blackbody and Comptonized emission. The measured low-energy absorptions are in good agreement with those predicted from optical measurements of the host globular clusters. This implies that there is little intrinsic X-ray absorption within the binaries themselves, and that the above spectral model provides a good representation of the low-energy continua. The sources can be divided into two groups. In the first group, composed of 3 ultra-compact (orbital period <1hr) sources, the disk-blackbody temperatures and inner-radii appear physically realistic and the Comptonization seed photons temperatures and radii of the emission areas are consistent with the disk temperatures and inner radii. For all the other sources, the disk-blackbody parameters appear not to be physically realistic and the Comptonization parameters are unrelated to those of the disk-blackbody emission. If this is a spectral signature of ultra-compact binaries, this implies that no other ultra-compact binaries are present among those studied here. It is unclear why this difference between the two types of binaries should exist.

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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

The likely detection of pulsed high-energy gamma-ray emission from millisecond pulsar PSR J0218+4232

We report circumstantial evidence for the first detection of pulsed high-energy gamma-ray emission from a millisecond pulsar, PSR J0218+4232, using data collected with the Energetic Gamma Ray Experiment (EGRET) on board the Compton Gamma Ray Observatory (CGRO). The EGRET source 3EG J0222+4253 is shown to be spatially consistent with PSR J0218+4232 for the energy range 100 - 300 MeV. Above 1 GeV the nearby BL Lac 3C 66A is the evident counterpart, and between 300 MeV and 1 GeV both sources contribute to the gamma-ray excess. Folding the 100-1000 MeV photons with an accurate radio ephemeris of PSR J0218+4232 yields a double peaked pulse profile with a ~3.5 sigma modulation significance and with a peak separation of ~0.45 similar to the 0.1-10 keV pulse profile.

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arXiv2000

The orbital period of the recurrent X-ray transient in Terzan 6

Four or five new outbursts were detected of the bright X-ray transient GRS 1747-312 in the globular cluster Terzan 6 between 1996 and 1999, through monitoring campaigns with the Wide Field Cameras (WFCs) on BeppoSAX and the Proportional Counter Array (PCA) on RossiXTE. This is the first time that the source is seen to exhibit recurrent outbursts after the discovery in September 1990 with ART-P on Granat. Three target-of-opportunity observations in 1998 and 1999, with the narrow-field instruments on BeppoSAX and the PCA, revealed one sharp drop in the flux which we identify as an eclipse of the compact X-ray source by the companion star. A detailed analysis of WFC data identifies further eclipses and we measure the orbital period at 12.360+/-0.009 hr. This is consistent with an identification as a low-mass X-ray binary, as suggested already by the association with a globular cluster. The eclipse duration is 0.72+/-0.06 hr. This implies that the inclination angle is larger than 74 deg. The 0.1-200 keV unabsorbed peak luminosity is 7X10^36 erg/s. The nature of the compact object is unclear.

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arXiv2000

Pulse shape and spectrum of PSR J0218+4232 observed with BeppoSAX

We present the results of a BeppoSAX observation of PSR J0218+4232 which, for the first time, provides detailed information on the pulsar's temporal and spectral emission properties in the broad band 1-10 keV. We detected X-ray pulses with a pulsed fraction of 73 +/- 12 %. The pulse profile is characterized by two peaks phase separated by 0.47 +/-0.05. The pulsed spectrum is best described by a power-law of photon index 0.61 +/- 0.32 with an unabsorbed (2-10 keV) X-ray flux of 4.1E-13 erg/cm^2/s implying a luminosity of Lx = 1.3E32 Theta (d/5.7 kpc})^2 erg/s and an X-ray efficiency of 4.8E-4 Theta (d/5.7 kpc})^2 where Theta is the solid angle spanned by the emission beam.

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arXiv1999

Likely detection of pulsed high-energy gamma-rays from millisecond pulsar PSR J0218+4232

We report on the likely detection of pulsed high-energy gamma-rays from the binary millisecond pulsar PSR J0218+4232 in 100-1000 MeV data from CGRO EGRET. Imaging analysis demonstrates that the highly significant gamma-ray source 2EG J0220+4228 (~10 sigma) is for energies > 100 MeV positionally consistent with both PSR J0218+4232 and the BL Lac 3C66A. However, above 1 GeV 3C66A is the evident counterpart, whereas between 100 and 300 MeV PSR J0218+4232 is the most likely one. Timing analysis using one ephemeris valid for all EGRET observations yields in the 100-1000 MeV range a double-pulse profile at a ~3.5 sigma significance level. The phase separation is similar to the component separation of ~0.47 observed at X-rays. A comparison of the gamma-ray profile with the 610 MHz radio profile in absolute phase shows that the two gamma- ray pulses coincide with two of the three emission features in the complex radio profile.

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arXiv1999

The X-ray spectra of VW Hydri during the outburst cycle

We report six BeppoSAX X-ray observations of VW Hydri during and after the outburst of Sep 23 1998. The outburst flux is lower than the quiescent flux in the entire observed energy band (0.1-10 keV), in agreement with earlier observations. The X-ray spectra are fitted with two-temperature plasma and cooling flow spectral models. These fits show a clear spectral evolution in X-rays for the first time in VW Hydri: the hard X-ray turn-up after the outburst is reflected in the emission measure and the temperature. Moreover, during outburst the 1.5-10 keV flux decreases significantly. We argue that this is not consistent with the constant flux during a ROSAT outburst observation made eight years earlier. We conclude from this observation that there are significant differences between outburst X-ray lightcurves of VW Hydri.

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arXiv1999

Gravitational waves from double white dwarfs

Double white dwarfs could be important sources for space based gravitational wave detectors like OMEGA and LISA. We use population synthesis to predict the current population of double white dwarfs in the Galaxy and the gravitational waves produced by this population. We simulate a detailed power spectrum for an observation with an integration time of 10^6 s. At frequencies below ~3 mHz confusion limited noise dominates. At higher frequencies a few thousand double white dwarfs are resolved individually. Including compact binaries containing neutron stars and black holes in our calculations yields a further few hundred resolved binaries and some tens which can be detected above the double white dwarf noise at low frequencies. We find that binaries in which one white dwarf transfers matter to another white dwarf are rare, and thus unimportant for gravitational wave detectors. We discuss the uncertainties and compare our results with other authors.

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