arXiv subjects

D. Dal Fiume

Publications and source records attributed to D. Dal Fiume.

At least 37 records · Page 2Linked to original sources

arXiv2000

Cyclotron lines in X-ray pulsars as a probe of relativistic plasmas in superstrong magnetic fields

The systematic search for the presence of cyclotron lines in the spectra of accreting X-ray pulsars is being carried on with the BeppoSAX satellite since the beginning of the mission. These highly successful observations allowed the detection of cyclotron lines in many of the accreting X-ray pulsars observed. Some correlations between the different measured parameters were found. We present these correlations and discuss them in the framework of the current theoretical scenario for the X-ray emission from these sources.

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arXiv2000

ASCA and BeppoSAX observations of the peculiar X-ray source 4U1700+24/HD154791

The X-ray source 4U1700+24/HD154791 is one of the few galactic sources whose counterpart is an evolved M star. In X-rays the source shows extreme erratic variability and a complex and variable spectrum. While this strongly suggests accretion onto a compact object, no clear diagnosis of binarity was done up to now. We report on ASCA and BeppoSAX X-ray broad band observations of this source and on ground optical observations from the Loiano 1.5 m telescope.

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arXiv2000

BeppoSAX spectrum of GRB971214: evidence of a substantial energy output during afterglow

We report the X/gamma-ray spectrum of GRB971214 and of its afterglow. The afterglow was measured few hours after the main event and for an elapsed time of more than two days. The measure of this GRB and afterglow is relevant due to its extreme, cosmological distance (z=3.42). The prompt event shows a hard photon spectrum, consistent with a broken power law with photon indices Gamma_X~0.1 below ~20 keV and Gamma_g~1.3 above 60 keV. The afterglow spectrum, measured with the MECS and LECS BeppoSAX telescopes, is consistent with a power law with spectral photon index Gamma=1.6. Within the statistical accuracy of our measure no spectral evolution is detected during the observation of the afterglow. When integrated during the time span covered by BeppoSAX observations, the power in the afterglow emission, even with very conservative assumptions, is at least comparable with the power in the main event. The IR-to-X rays broad band spectrum is also presented, collecting data from the literature and adding them to the BeppoSAX measure. It shows that the predictions from synchrotron emission models is qualitatively confirmed. The BeppoSAX measurement of the X and gamma ray spectrum of this GRB/afterglow is discussed in the framework of current theoretical models.

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arXiv2000

X-ray/optical observations of XTE J0421+560/CI Cam in quiescence

We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ~150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of 6.7x10^{-12} erg/cm2/s and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same two-temperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ~7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L_x/Lopt~10^{-3} the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.

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arXiv2000

BeppoSAX and Chandra Observations of SAXJ0103.2-7209=2E0101.5-7225: a new Persistent 345s X-ray Pulsar in the SMC

We report the results of a 1998 July BeppoSAX observation of a field in the SMC which led to the discovery of 345s pulsations in the X-ray flux of SAXJ0103.2-7209. The BeppoSAX X-ray spectrum is well fit by an absorbed power-law with photon index 1.0 plus a black body component with kT=0.1keV. The unabsorbed luminosity in the 2-10 keV energy range is 1.2x10^{36} erg/s. In a very recent Chandra observation the 345s pulsations are also detected. The available period measurements provide a constant period derivative of -1.7s/yr over the last three years making SAXJ0103.2-7209 one of the most rapidly spinning-up X-ray pulsars known. The BeppoSAX position is consistent with that of the Einstein source 2E0101.5-7225 and the ROSAT source RXJ0103.2-7209. This source was detected at a luminosity level of few 10^{35}-10^{36} erg/s in all datasets of past X-ray missions since 1979. The ROSAT HRI and Chandra positions are consistent with that of a m_V=14.8 Be spectral type star already proposed as the likely optical counterpart of 2E0101.5-7225. We conclude that SAXJ0103.2-7209 and 2E0101.5-7225 are the same source, a relatively young and persistent X-ray pulsar in the SMC.

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arXiv2000

Broad-band BeppoSAX observation of the low-mass X-ray binary X1822-371

Results of a 1997 September 9-10 BeppoSAX observation of the 5.57 hr low-mass X-ray binary (LMXRB) X1822-371 are presented. The 0.3-40 keV spectrum is unusually complex and cannot be fit by any of the standard models applied to other LMXRB. At least two components are required. One component has a shape consistent with that expected from the Comptonization of an input soft (Wein) spectrum while the other, contributing ~40% of the 1-10 keV flux, is consistent with being a blackbody. In addition, there is a ``dip'' in the spectrum which can be modeled by a 1.33 +0.05 -0.11 keV absorption edge with an optical depth, tau, of 0.28 +/- 0.06. If the same model is fit to ASCA Solid-State Imaging Spectrometer spectra obtained in 1993 and 1996, then reasonable fits are also obtained, with a similar absorption feature required. The nature of this feature is highly uncertain; its energy corresponds to the K-edges of highly ionized Ne x and neutral Mg, or to an L-edge of moderately ionized Fe. Surprisingly, no strong (tau > 0.05) Fe-K or (tau > 0.18) O-K edges are visible. The folded lightcurve of X1822-371 is similar to previous observations, except that no strong softening is seen near the eclipse. An updated orbital ephemeris is provided.

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arXiv1999

Prompt and delayed emission properties of Gamma-Ray Bursts observed with BeppoSAX

We investigated the spectral evolution in the 2--700 keV energy band of Gamma-Ray Bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and localized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite before May 1998. Most of them have been followed-up with the Narrow Field Instruments aboard the same satellite. In the light of these results we discuss open issues on the GRB phenomenon. We find that the optically thin synchrotron shock model (SSM) provides an acceptable representation of most of the time-resolved GRB spectra extending down to 2 keV, except in the initial phases of several bursts and during the whole duration of the quite strong GRB970111, where a low-energy photon depletion with respect to the thin SSM spectrum is observed. We find that the X-ray afterglow starts at about 50% of the GRB duration, and that its fluence, as computed from the WFC light curve, is consistent with the decay law found from the afterglow NFI observations. We also investigate the hydrodynamical evolution of the GRB in our sample and their associated afterglow, when it was detected. We find that the photon index of the latest spectrum of the GRB prompt emission is correlated with the index of the afterglow fading law, when available, as expected on the basis of an external shock of a relativistic fireball.

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arXiv1999

Discovery of a faint 437 s X-ray pulsar 1SAX J1452.8-5949

A new pulsar, 1SAX J1452.8-5949, was discovered during a BeppoSAX galactic plane survey on 1999 July 20 at R.A.=14h 52m 49s, Dec=-59 49' 18'' (J2000) with a 90% confidence uncertainty radius of 50''. Coherent pulsations were detected with a barycentric period of a 437.4 +/- 1.4 s. The X-ray spectrum can be modeled by a power-law with a photon index of 1.4 +/- 0.6 and absorption consistent with the galactic value in the direction of the source ($1.9 \times 10^{22}$ atom cm$^{-2}$). An Fe K line with a equivalent width of $\approxgt$1.3 keV may be present in the spectrum. The unabsorbed 2-10 keV flux is $9 \times 10^{-13}$ erg cm$^{-2}$s$^{-1}$. The X-ray properties and lack of an obvious optical counterpart are consistent with a Be star companion at a distance of between approximately 6 and 12 kpc which implies a luminosity of (4-15)$\times 10^{33}$ erg s$^{-1}$.

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arXiv1999

BeppoSAX spectroscopy of the Hercules X-1 short-on state

We present results of a 5.7 day duration BeppoSAX observation of the short-on state of Her X-1 and a short observation during the decline of the preceding main-on state. The 0.1-10 keV spectra can be fit with a power-law and blackbody model together with Fe emission features at 1.0 keV and 6.5 keV. During the later stages of the short-on state there are long intervals when the absorption is $\approxgt$$5 \times 10^{22}$ atom cm$^{-2}$. These intervals become longer and occur ~0.3 day earlier in each orbital cycle as the short-on state progresses. During the intervals of high absorption the 0.1 keV blackbody is still clearly detected. This may indicate the presence of separate scattered and absorbed spectral components, although other explanations such as partial covering or a partially ionized absorber cannot be excluded. During the rest of the short-on state the ratio of flux in the blackbody compared to the power-law is consistent with that in the main-on state. This supports the view that much of the 35 day modulation is caused by an energy independent process, such as electron scattering. The discovery of strong absorption late in the short-on state is consistent with the predictions of the warped disk model (Petterson 1977, see also Schandl & Meyer 1994) where the end of the short-on state is caused by the accretion disk moving into the line of sight to the neutron star. The pulse phase difference between the blackbody and the power-law maxima is 250 +/- 20 degrees in both observations (separated by 0.43 of a 35 day cycle). This constant phase difference is consistent with the blackbody originating at the inner edge of a precessing accretion disk.

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arXiv1999

BeppoSAX Observations of GRB980425: Detection of the Prompt Event and Monitoring of the Error Box

We present BeppoSAX follow-up observations of GRB980425 obtained with the Narrow Field Instruments (NFI) in April, May, and November 1998. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova 1998bw, which exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of X-ray emission from a Type I supernova above 2 keV. The latter source exhibits only marginally significant variability. The X-ray spectra and variability of the supernova are compared with thermal and non-thermal models of supernova high energy emission. Based on the BeppoSAX data, it is not possible to firmly establish which of the two detected sources is the GRB X-ray counterpart, although probability considerations favor the supernova.

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arXiv1999

BeppoSAX spectroscopy of the globular cluster X-ray source X1746-371 (NGC6441)

During a BeppoSAX observation of the X-ray source X1746-371 located in the globular cluster NGC6441 a type I X-ray burst, parts of 4 intensity dips, and energy dependent flaring were detected. The dips repeat every 5.8 +0.3 -0.9 hr and show no obvious energy dependence. If the dips are due to electron scattering this energy independence implies an abundance >130 times less than solar, confirming an earlier measurement. Since the overall cluster abundance is close to solar this low abundance is unexpected. Photoionization of the absorbing material, obscuration of an extended source, and variations in multiple components that combine to produce an apparent energy independence are all excluded. Thus, the nature of the dips remains uncertain. The best-fit model to the overall 0.3-30 keV spectrum is a disk-blackbody with a temperature of 2.82 +/- 0.04 keV, together with a cutoff power-law with a photon index of -0.32 +/- 0.80 and a cutoff energy of 0.90 +/- 0.26 keV. Absorption, consistent with the optical extinction to NGC6441 of (0.28 +/- 0.04) 10^22 atom/cm2 is required. All previous best-fit spectral models for this source are excluded at high confidence. The spectrum is dominated by the blackbody-like component, with the cutoff power-law only contributing an average of 12% of the 1-10 keV flux. During flaring intervals the contribution of this component decreases to \~6% with variations in the intensity of the blackbody-like component being responsible for most of the flaring activity.

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arXiv1999

X-ray observations during a Her X-1 anomalous low-state

Results of a 1999 July 8-10 BeppoSAX observation during an anomalous low-state of Her X-1 are presented. The standard on-state power-law and blackbody continuum model is excluded at high confidence unless partial covering is included. This gives a power-law photon index of 0.63 +/- 0.02 and implies that 0.28 +/- 0.03 of the flux undergoes additional absorption of (27 +/- 7) 10^22 atom/cm2. 11% of the observed 0.1-10 keV flux is from the 0.068 +/- 0.015 keV blackbody. 1.237747(2) s pulses with a semi-amplitude of 2.1 +/- 0.8% are detected at >99.5% confidence and confirmed by RXTE measurements. This implies that Her X-1 underwent substantial spin-down close to the start of the anomalous low-state. The spectral and temporal changes are similar to those recently reported from 4U1626-67. These may result from a strongly warped disk that produces a spin-down torque. The X-ray source is then mostly viewed through the inner regions of the accretion disk. A similar mechanism could be responsible for the Her X-1 anomalous low-states. Shadowing by such an unusually warped disk could produce observable effects in the optical and UV emission from the companion star.

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arXiv1999

The broad-band spectrum of OAO1657-415 with BeppoSAX: In search of cyclotron lines

We report on a 30 ks observation of the high-mass X-ray binary pulsar OAO1657-415 performed by BeppoSAX on September 1998. The wide band spectrum is well fit by both a cutoff power law, or a power law modified by a high energy cutoff, plus a fluorescence Iron line at 6.5 keV. The two models are statistically equivalent. The inclusion of a cyclotron resonance feature at ~36 keV -- corresponding to a magnetic field strength 3.2 (1+z) x 10^12 G, where z is the gravitational redshift -- improves significantly the X^2 for the cutoff model but only marginally for the power law plus high energy cutoff model. The statistical significance of our data is not adequate to discriminate between the two models, and even the "Normalized Crab Ratio" technique, successfully used to pinpoint cyclotron features in the spectra of other X-ray pulsars, is not conclusive in answering the question whether the feature is real or it is an artifact due to an improper modeling of the continuum used to fit the data.

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arXiv1999

A BeppoSAX study of the pulsating transient X0115+63: the first X-ray spectrum with four cyclotron harmonic features

The recurrent hard pulsating X-ray transient §was observed with \B on March 19, when the source was at a 2--10 keV flux level of $\sim$310~mCrab. We report on the high energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74 keV, 24.16 keV, 35.74 keV, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1:(1.9):(2.8):(3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field, when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for {\it four harmonically spaced lines} in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.

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arXiv1999

BeppoSAX Detection and Follow-up of GRB980425

We present BeppoSAX GRBM and WFC light curves of GRB980425 and NFI follow-up data taken in 1998 April, May, and November. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova SN 1998bw, exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of hard X-ray emission from a Type I supernova. The latter source exhibits only marginally significant variability. Based on these data, it is not possible to select either source as a firm candidate for the GRB counterpart.

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arXiv1999

Integrating the BeppoSAX Gamma-Ray Burst Monitor into the 3rd Interplanetary Network

We have added the BeppoSAX Gamma-Ray Burst Monitor to the 3rd Interplanetary Network of burst detectors. We analyze 16 bursts whose positions are known to good accuracy from measurements at other wavelengths. We show that there is excellent agreement between the Ulysses/BeppoSAX triangulation annuli and the known positions of these events, and that these annuli can in many cases provide useful constraints on the positions of bursts detected by the BeppoSAX Wide Field Camera and Narrow Field Instruments.

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arXiv1999

Iron line signatures in X-ray afterglows of GRB by BeppoSAX

We report the possible detection (99.3% of statistical significance) of redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material ($>0.5 \Ms$), consistent with pre-explosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase. We did not find evidence of Fe line in two other GRB with known redshift (GB971214 and GB980613), but we note that the upper limits are of the same order of the intensity measured in GB97508 and that none of these afterglows shows rebursting activity.

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arXiv1999

The broad band spectral properties of binary X-ray pulsars

The X-ray telescopes on board BeppoSAX are an optimal set of instruments to observe bright galactic binary pulsars. These sources emit very hard and quite complex X-ray spectra that can be accurately measured with BeppoSAX between 0.1 and 200 keV. A prototype of this complexity, the source Her X-1, shows at least seven different components in its spectrum. A broad band measure is therefore of paramount importance to have a thorough insight into the physics of the emitting region. Moreover the detection of cyclotron features, when present, allows a direct and highly significant measure of the magnetic field intensity in the emission region. In this paper we briefly report the results obtained with BeppoSAX on this class of sources, with emphasis on the detection and on the measured properties of the cyclotron lines.

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