arXiv subjects

F. Frontera

Publications and source records attributed to F. Frontera.

At least 217 records · Page 12Linked to original sources

arXiv1999

A Giant Outburst from SGR 1900+14 Observed with the BeppoSAX Gamma Ray Burst Monitor

We report the detection by the Gamma Ray Burst Monitor onboard BeppoSAX of the strongest and longest ever detected outburst from SGR 1900+14. Oscillations are detectable with a period of ~5.16 s for the entire duration of the event ~300 s. The temporal analysis reveals also a remarkable periodic substructure: after about 35 s from the event onset each 5.16-s pulse shows a pattern of four subpulses and a dip, each separated by ~1 s. Significant spectral variation is detected during the event and for each individual oscillation. The first and most intense part of the outburst is quite hard, and similar to what previously detected from the `March 5th event'. A hard non-thermal spectral component persists for ~200 s. SGR 1900+14 was proposed to be a strongly magnetized neutron star (B>10^14 G) undergoing violent instabilities by internal magnetic/crustal stresses. However, the onset of an apparent 1-s periodicity within the 5.16-s pulsations and the observed spectral properties show a complex behaviour that is not satisfactorily modelled yet.

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arXiv1999

BeppoSAX observations of Mkn 3: Piercing through the torus of a Seyfert 2 galaxy

A new BeppoSAX broad-band (0.6-150 keV) spectrum of the Seyfert 2 galaxy Mkn 3 is presented. The spectrum provides a direct measurement of a large, neutral column of gas with Nh~10**24 cm**-2 in the source direction. The source, as bright as 3C 273 above 10 keV, has a steep (Gamma~1.8) spectrum without any evidence of a high-energy cutoff up to at least 150 keV. At lower energies, the data are best modeled with the addition of an unabsorbed reflection component. Combining these data with previous Ginga and ASCA observations, the Fe K$_α$ and reflection continuum indicate that the reprocessed emission is responding slower than the intrinsic continuum variations suggesting a size of the reprocessor >= 2 pc. Identifying such a reprocessor with a (close to edge-on) obscuring torus, the overall result fits well into unified models since, presumably, one can interpret the strong absorption as due to transmission through the rim of the torus and the unabsorbed (directly viewed) reflection component as due to reprocessing from the torus inner surface.

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arXiv1999

The X-ray afterglow of the Gamma-ray burst of May 8, 1997: spectral variability and possible evidence of an iron line

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. The line disappeared about 1 day after the burst. We have also analyzed the spectral variability during the outburst event that characterizes the X-ray afterglow of this GRB. The spectrum gets harder during the flare, turning to steep when the flux decreases. The variability, intensity and width of the line indicate that the emitting region should have a mass approximately greater than 0.5 solar masses (assuming the iron abundance similar to its solar value), a size of about 3 times 10^15 cm, be distributed anisotropically, and be moving with sub-relativistic speed. 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, 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.

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arXiv1999

BeppoSAX uncovers the hidden Seyfert 1 nucleus in the Seyfert 2 galaxy NGC 2110

The Seyfert 2 galaxy NGC 2110 has been observed with BeppoSAX between 0.5 and 150 keV. The high energy instrument onboard, PDS, has succeeded in measuring for the first time the spectrum of this source in the 13-150 keV range. The PDS spectrum, having a photon index Gamma~1.86 is fully compatible with that expected from a Seyfert 1 nucleus. In the framework of unified models, the harder (Gamma~1.67) 2-10 keV spectrum is well explained assuming the presence of a complex partial + total absorber (Nh~30x10^22 cm^-2 x25% + Nh~4x10^22 cm^-2 x100%). The high column density of this complex absorber is consistent both with the FeK_alpha line strength and with the detection of an absorption edge at E~7.1 keV in the power-law spectrum.

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arXiv1998

BeppoSAX observations of an orbital cycle of the X-ray binary pulsar GX301-2

We present preliminary results on our campaign of observations of the X-ray binary pulsar GX301-2. BeppoSAX observed this source six times in January/February 1998: at the periastron and apoastron, and at other four, intermediate, orbital phases. We present preliminary results on the GX301-2 spectral and temporal behaviour as a function of orbital phase.

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arXiv1998

The optical/IR counterpart of the 3 July 1998 gamma-ray burst and its evolution

We imaged the X-ray error box of GRB 980703, beginning 22.5 hours after the gamma--ray event, in both the optical R and near-infrared H bands. A fading optical/IR object was detected within the X-ray error box, coincident with the variable radio source reported by Frail et al. (1998a), who also detected the optical transient independently of us. Further imagery revealed the GRB host galaxy, with R = 22.49 +/- 0.04 and H = 20.5 +/- 0.25, the brightest so far detected. When excluding its contribution to the total flux, both the R and H-band light curves are well-fit by a power-law decay with index alpha~1.4. Our data suggest an intrinsic column density in the host galaxy of ~ 3.5 x 10^21 cm^-2 which indicates the existence of a dense and rich-gas medium in which the GRB occurred, thus supporting the hypernova model scenarios.

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arXiv1998

The extraordinary X-ray spectrum of XTE J0421+560

We report results of two BeppoSAX observations of the transient X-ray source XTE J0421+560 during the outburst that started in March 1998. The source exhibits radio jets and coincides with the binary system CI Cam. The 0.1--50 keV spectrum is unlike those of other X-ray transients, and cannot be fit with any simple model. The spectra can be represented by an absorbed two component bremsstrahlung model with narrow Gaussian emission features identified with O, Ne/Fe-L, Si, S, Ca and Fe-K. During the second observation (TOO2) the energies of the O and the Ne/Fe-L features decreased smoothly by ~9% over an interval of 30 hrs. No significant energy shift of the other lines is detected with e.g, a 90% confidence upper limit to any Fe-K line shift of 3.5%. No moving lines were detected during the first observation (TOO1) with e.g., an upper limit of <1.4% to any shift of the Fe-K line. The low-energy absorption decreased by a factor >=1.8 from ~6 10E21 atom cm/2 between TOO1 and TOO2. We propose that the time variable emission lines arise in precessing relativistic jets, while the stationary lines originate in circumstellar material.

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arXiv1998

The Host Galaxy of the Gamma--Ray Burst 971214

We report on Hubble Space Telescope (HST) observations of the host galaxy of GRB 971214, about four months after the burst. The redshift of the proposed host galaxy at z=3.418, combined with optical and radio observations of the burst afterglow, implies the extremely large isotropic energy release from the burst in gamma-rays of approximately 3x10^53 ergs, some two orders of magnitude higher than the previously commonly assumed numbers. The positional offset between the optical transient observed at the Keck telescope and the centroid of the proposed host galaxy in the HST image is 0.14 -+0.07 arcsec. We find no evidence in our deep HST image for a chance foreground galaxy superposed along the line of sight to the proposed host at z=3.418. The morphology and photometric properties of this galaxy, such as the total flux, morphology, radial surface profile and scale length, are typical as compared to other, spectroscopically confirmed z>3 galaxies.

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arXiv1998

The 1998 outburst of the X-ray transient 4U 1630-47 observed with BeppoSAX

We report results of five pointed BeppoSAX observations of the black hole candidate 4U 1630-47 during part of its 1998 outburst when the 2-10 keV source luminosity decreased from 2.1x10^38 erg/s (for an assumed distance of 10 kpc) by a factor of ~7. The 2-200 keV spectrum can be modeled by a soft multi-temperature disk blackbody and a hard power-law. During four of the observations, there is evidence for deviations from a pure power-law shape >~20 keV which may be due to reflection. As the outburst decayed a number of spectral trends are evident: (1) the amount of photoelectric absorption decreased, (2) the spectrum became harder (i.e. the power-law index became smaller), (3) the temperature of the disk blackbody decreased and (4) the inferred inner disk radius increased by a factor of ~4. The change in the accretion disk inner radius during the outburst is in contrast to results from other soft X-ray transients. We find that the disk blackbody temperature depends on the inner disk radius roughly as T=r^(-3/4). For a standard Shakura-Sunyaev disk model this implies that the inferred mass accretion rate at the inner disk radius did not change appreciably during the part of the outburst observed by BeppoSAX.

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arXiv1998

The absorption properties of gamma-ray burst afterglows measured by BeppoSAX

We present an analysis of the X-ray absorption properties of 6 gamma-ray burst (GRB) afterglows measured with BeppoSAX. Between 8 hrs and 20 hrs after the initial GRB trigger, individual spectra can be described by a power-law with a photon index of ~2 and absorption marginally consistent with the galactic value. Taken collectively, the data are inconsistent with zero absorption at the >99.999 confidence level. The data are only marginally consistent with a distribution of column densities varying as the total galactic absorption in the direction of each of the bursts (chi^2 =9.6 for 6 degrees of freedom). The data are consistent with cosmological models in which GRB occur within host galaxies. By simultaneously fitting a power-law spectral model with absorption fixed at the galactic value and additional, redshifted, absorption to all 6 afterglow spectra, the best-fit average absorption within the host objects is found to be (1.01 ^0.28_0.51) 10^22 atom cm^-2. This value is compatible with the host galaxy column densities inferred from optical data for GRB970508, GRB971214 and GRB980329, supporting the hypothesis that GRB occur within heavily absorbed star forming regions of their host galaxies.

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arXiv1998

BeppoSAX Observations of Bright Seyfert 2 Galaxies: Measuring the Intrinsic Continuum Emission

We report broad band (0.1-200 keV) X-ray observations, made by BeppoSAX, of a sample of bright Seyfert 2 galaxies: NGC7172, NGC2110, NGC4507, Mkn 3 and NGC7674. These spectra provide a better understanding of the effects of X-ray reprocessing by cold material in the source and allow to put tighter constraints on the various spectral parameters involved. In particular, the data are used to determine, with less ambiguities than in the past, the shape of the intrinsic continuum emission by means of the high energy data. Within the small sample both Compton thin and Compton thick sources are found according to the expectations of the unified theory.

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arXiv1998

Multifrequency observations of XTE J0421+560/CI Cam in outburst

We report on two X-ray observations of the transient source XTE J0421+560 performed by BeppoSAX, and on a series of observations performed by the 0.7m Teramo-Normale Telescope. Outburst peak occurrence time and duration depend on photon energy: the outburst peak is achieved first in the X-ray band, then in the optical and finally in the radio. An exponential decay law fits well the X-ray data except in the TOO2 0.5-1.0 keV band, where erratic time variability is detected. During TOO1 the e-folding time scale decreases with energy up to ~ 20 keV, when it achieves a saturation; during TOO2 it decreases up to ~ 2 keV and then increases. This change is correlated with a spectral change, characterized by the onset of a soft (< 2 keV) component in TOO2 (Orr et al. 1998). This component might originate from the relativistic jets, while the hard component is more likely associated to processes occurring in the circumstellar matter and/or near the compact object. Optical observations show that the object appears intrinsically red even during the outburst. The nature of the compact object is discussed.

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arXiv1998

An optical study of the GRB 970111 field beginning 19 hours after the Gamma-Ray Burst

We present the results of the monitoring of the GRB 970111 field that started 19 hours after the event. This observation represents the fastest ground-based follow-up performed for GRB 970111 in all wavelengths. As soon as the detection of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998) we reanalyzed the optical data collected for the GRB 970111 field. Although we detect small magnitude variability in some objects, no convincing optical counterpart is found inside the WFC error box. Any change in brightness 19 hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8. The bluest object found in the field is coincident with 1SAXJ1528.8+1937. Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with redshift z=0.657, which we propose as the optical counterpart of the X-ray source. Further observations allowed to perform multicolour photometry for objects in the GRB 970111 error box. The colour-colour diagrams do not show any object with unusual colours. We applied a photometric classification method to the objects inside the GRB error box, that can distinguish stars from galaxies and estimate redshifts. We were able to estimate photometric redshifts in the range 0.2 < z < 1.4 for several galaxies in this field and we did not find any conspicuous unusual object. We note that GRB 970111 and GRB 980329 could belong to the same class of GRBs, which may be related to nearby sources (z ~1) in which high intrinsic absorption leads to faint optical afterglows.

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arXiv1998

BeppoSAX observations of GRB970402

GRB970402 is the fourth gamma-ray burst detected by BeppoSAX simultaneously in the Gamma Ray Burst Monitor (GRBM) and one of the two Wide Field Cameras (WFCs). A rapid pointing of the BeppoSAX Narrow Field Instruments (NFIs) 8 hours after the GRB led to the identification of an unknown weak X-ray source: 1SAX J1450.1-6920. Its position was approximately at the center of the 3 arcmin error circle derived from the WFC image. Both the Medium Energy (MECS, 2-10 keV) and Low Energy (LECS, 0.1-10 keV) concentrators detected the source. A follow-up observation performed 1.5 days later and lasting 54 ks showed that the source had faded almost to, but not below the detectability threshold. The flux decrease between the two observations was a factor ~= 2.5. 1SAX J1450.1-6920 was the second X-ray afterglow associated with a GRB. Searches promptly started at other wavelengths (optical, IR, radio) did not reveal any transient event within the 3 arcmin error circle.

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arXiv1998

A `Hypernova' model for SN 1998bw associated with gamma-ray burst of 25 April 1998

The discovery of the peculiar supernova (SN) 1998bw and its possible association with the gamma-ray burst (GRB) 980425$^{1,2,3}$ provide new clues to the understanding of the explosion mechanism of very massive stars and to the origin of some classes of gamma-ray bursts. Its spectra indicate that SN~1998bw is a type Ic supernova$^{3,4}$, but its peak luminosity is unusually high compared with typical type Ic supernovae$^3$. Here we report our findings that the optical spectra and the light curve of SN 1998bw can be well reproduced by an extremely energetic explosion of a massive carbon+oxygen (C+O) star. The kinetic energy is as large as $\sim 2-5 \times 10^{52}$ ergs, more than ten times the previously known energy of supernovae. For this reason, the explosion may be called a `hypernova'. Such a C+O star is the stripped core of a very massive star that has lost its H and He envelopes. The extremely large energy, suggesting the existence of a new mechanism of massive star explosion, can cause a relativistic shock that may be linked to the gamma-ray burst.

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arXiv1998

Gamma-Ray Burst 980329 and its X-Ray Afterglow

GRB 980329 is the brightest gamma-ray burst detected so far with the Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. With respect to its fluence (2.6 X 10**-5 erg/s/cm**2 in 50 to 300 keV) it would be in the top 4% of gamma-ray bursts in the 4B catalog (Meegan et al. 1998). The time-averaged burst spectrum from 2 to 20 and 70 to 650 keV can be well described by the empirical model of Band et al. (1993). The resulting photon index above the break energy is exceptionally hard at -1.32 +/- 0.03. An X-ray afterglow was detected with the narrow-field instruments aboard BeppoSAX 7 h after the event within the error box as determined with the Wide Field Cameras. Its peak flux is (1.4 +/- 0.2) X 10**-12 erg/s/cm**2 (2 to 10 keV). The afterglow decayed according to a power law function with an index of -1.35 +/- 0.03. GRB 980329 is characterized by being bright and hard, and lacking strong spectral evolution.

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arXiv1998

Discovery of the peculiar supernova 1998bw in the error box of GRB980425

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties.

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arXiv1998

The Discovery of the Radio Afterglow From the Optically Dim Gamma-Ray Burst of March 29, 1998

We report on the discovery of a variable radio source, VLA J070238.0+385044, associated with the proposed x-ray counterpart, 1SAX J0702.6+3850 of GRB 980329. The source was monitored from one day after the burst to one month later at centimeter wavelengths (1.4, 4.9, 8.3 and 15 GHz; Very Large Array -- VLA), and in the 3-mm band (90 GHz; Owen Valley Radio Observatory -- OVRO). Based on its variability, compactness and spectrum, we identify VLA J070238.0+385044 as the afterglow from GRB 980329. We interpret the rapid flux density variations as interstellar scintillation and the sharp turnover in the radio spectrum below 13 GHz as arising from synchrotron self-absorption. This suggests that the angular diameter of the fireball from GRB 980329 was of order a few microarcseconds in the first two weeks. The absence of a readily detectable afterglow in the optical, but clear detections in both the radio and infrared, can be understood as the result of extinction by dust. In this context it is interesting to note that half of all well-localized gamma-ray bursts have no optical counterpart. Based on our study of GRB 980329 we suggest that the progenitors of a sizable fraction of gamma-ray bursts are associated with regions of moderate to high gas density.

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