arXiv subjects

L. Piro

Publications and source records attributed to L. Piro.

At least 109 records · Page 6Linked to original sources

arXiv2003

XMM-Newton Observations of the Field of Gamma-Ray Burst 980425

The error box of GRB980425 has been observed by XMM-Newton in March 2002, with the aim of measuring the late epoch X-ray emission of the supernova 1998bw and of clarifying its supposed association with the GRB itself. We present here the preliminary results obtained with the EPIC PN camera. Our observations confirm the association between SN 1998bw and GRB980425. The EPIC PN measurement of the SN 1998bw flux is significantly below the extrapolation of the power-law temporal trend fitted to the BeppoSAX points and implies a faster temporal decay. We propose different physical interpretations of the SN X-ray light curve, according to whether it is produced by one or more radiation components.

astro-ph↗

arXiv2003

Discovery of GRB 020405 and its Late Red Bump

We present the discovery of GRB 020405 made with the Inter-Planetary Network (IPN). With a duration of 60 s, the burst appears to be a typical long duration event. We observed the 75-square acrminute IPN error region with the Mount Stromlo Observatory's 50-inch robotic telescope and discovered a transient source which subsequently decayed and was also associated with a variable radio source. We identify this source as the afterglow of GRB 020405. Subsequent observations by other groups found varying polarized flux and established a redshift of 0.690 to the host galaxy. Motivated by the low redshift we triggered observations with WFPC2 on-board the Hubble Space Telescope (HST). Modeling the early ground-based data with a jet model, we find a clear red excess over the decaying optical lightcurves that is present between day 10 and day 141 (the last HST epoch). This `bump' has the spectral and temporal features expected of an underlying supernova (SN). In particular, the red color of the putative SN is similar to that of the SN associated with GRB 011121, at late time. Restricting the sample of GRBs to those with z<0.7, a total of five bursts, red bumps at late times are found in GRB 970228, GRB 011121, and GRB 020405. It is possible that the simplest idea, namely that all long duration GRBs have underlying SNe with a modest dispersion in their properties (especially peak luminosity), is sufficient to explain the non detections.

astro-ph↗

arXiv2003

The dark burst 010214 with BeppoSAX: possible variable absorption and jet X-ray emission

We report on the prompt and afterglow emission observations of the dark burst GRB010214 with BeppoSAX. The prompt emission shows possible evidence of variable absorption from N_H = 3.0^{+5.1}_{-2.0} x 10^23 cm-2 in the first 6 s of the event to a value consistent with the Galactic column density (N_H^(G) = 2.66 x 10^20 cm-2) in the GRB direction. An X-ray afterglow emission in the 2-10 keV energy band was detected with BeppoSAX, but an analogue search at lower wavelengths (optical, IR and radio) was unsuccessful. The X-ray afterglow spectrum is consistent with a power-law with Galactic absorption. The light curve shows a complex decay, if the tail of the prompt emission is assumed as the onset of the afterglow: if the origin of the afterglow is coincident with the GRB onset, a bump before ~ 3 x 10^4 s is inferred, while if the afterglow is assumed to start later, a steepening of the power-law light curve at t ~ 3 x 10^4 s is deduced. We discuss these results in the light of the current models of afterglows and the possible origin of the GRB darkness. Finally, we tentatively derive an estimate of the burst redshift.

astro-ph↗

arXiv2002

A comparative study of the X-ray afterglow properties of optically bright and dark GRBs

We have examined the complete set of X-ray afterglow observations of dark and optically bright GRBs performed by BeppoSAX until February 2001. X-ray afterglows are detected in 90% of the cases. We do not find significant differences in the X-ray spectral shape, in particular no higher X-ray absorption in GRBs without optical transient (dark GRBs) compared to GRBs with optical transient (OTGRBs). Rather, we find that the 1.6-10 keV flux of OTGRBs is on average about 5 times larger than that of the dark GRBs. A K-S test shows that this difference is significant at 99.8% probability. Under the assumption that dark and OTGRB have similar spectra, this could suggest that the first are uncaught in the optical band because they are just faint sources. In order to test this hypothesis, we have determined the optical-to-X ray flux ratios of the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which corresponds to an average optical-to-x spectral index 0.794\pm 0.054. We find that, while 75% of dark GRBs have flux ratio upper limits still consistent with those of OT GRBs, the remaining 25% are 4 - 10 times weaker in optical than in X-rays. The significance of this result is equal to or higher than 2.6 sigma. If this sub-population of dark GRBs were constituted by objects assimilable to OTGRBs, they should have shown optical fluxes higher than upper limits actually found. We discuss the possible causes of their behaviour, including a possible occurrence in high density clouds or origin at very high redshift and a connection with ancient, Population III stars.

astro-ph↗

arXiv2002

The Faint Optical Afterglow and Host Galaxy of GRB 020124: Implications for the Nature of Dark Gamma-Ray Bursts

We present ground-based optical observations of GRB 020124 starting 1.6 hours after the burst, as well as subsequent Very Large Array (VLA) and Hubble Space Telescope (HST) observations. The optical afterglow of GRB 020124 is one of the faintest afterglows detected to date, and it exhibits a relatively rapid decay, $F_ν\propto t^{-1.60\pm 0.04}$, followed by further steepening. In addition, a weak radio source was found coincident with the optical afterglow. The HST observations reveal that a positionally coincident host galaxy must be the faintest host to date, R>29.5 mag. The afterglow observations can be explained by several models requiring little or no extinction within the host galaxy, A_V~0-0.9 mag. These observations have significant implications for the statistics of the so-called dark bursts (bursts for which no optical afterglow is detected), which are usually attributed to dust extinction within the host galaxy. The faintness and relatively rapid decay of the afterglow of GRB 020124, combined with the low inferred extinction indicate that some dark bursts are intrinsically dim and not dust obscured. Thus, the diversity in the underlying properties of optical afterglows must be observationally determined before substantive inferences can be drawn from the statistics of dark bursts.

astro-ph↗

arXiv2002

GRB 010921: Strong Limits on an Underlying Supernova from HST

GRB 010921 was the first HETE-2 GRB to be localized via its afterglow emission. The low-redshift of the host galaxy, z=0.451, prompted us to undertake intensive multi-color observations with the Hubble Space Telescope with the goal of searching for an underlying supernova component. We do not detect any coincident supernova to a limit 1.34 mag fainter than SN 1998bw at 99.7% confidence, making this one of the most sensitive searches for an underlying SN. Analysis of the afterglow data allow us to infer that the GRB was situated behind a net extinction (Milky Way and the host galaxy) of A_V ~ 1.8 mag in the observer frame. Thus, had it not been for such heavy extinction our data would have allowed us to probe for an underlying SN with brightness approaching those of more typical Type Ib/c supernovae.

astro-ph↗

arXiv2002

The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB

The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within ~1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a Giant Molecular Cloud. We therefore conclude that either the gas is local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

astro-ph↗

arXiv2002

X-ray spectral diagnostics of the immediate environment of GRB 991216

The recent report of iron line features in the afterglow of the gamma-ray burst (GRB) 991216 has important implications for the properties of the radiating material and hence the nature of the immediate burst environment. We argue that the putative strong Fe emission line can be attributed to the reflected emission from Thomson-thick matter which is illuminated by a power-law continuum. The ionization parameter of the material (i.e., the flux to density ratio) is around 10^3, resulting in a Fe K line from He-like iron. A supersolar abundance of iron is not required by the data. Interestingly, the ionizing continuum must be harder than the observed one. We interpret this as due to the fact that the observed continuum is dominated by the standard blast wave emission, while the line is produced by reprocessing material located at much smaller radii.

astro-ph↗

arXiv2002

Detection of a supernova signature associated with GRB 011121

Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection of an intermediate-time flux excess which is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red ``bump,'' which exhibits a spectral roll-over at ~7200 Angstrom, is well described by a redshifted Type Ic supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts.

astro-ph↗

arXiv2002

The complex iron line in NGC 7469 observed by BeppoSAX

In this letter we present analysis of BeppoSAX data from a long look at the Seyfert 1 galaxy NGC 7469. The presence of a soft excess below 0.8 keV is confirmed by our analysis and no warm absorber component is required to fit the spectrum. A complex iron emission line and the Compton reflection hump are clearly detected. The profile of the line is too broad to associate this feature with distant matter. In addition, the observed soft excess and the energy of the iron line E_{Fe}=6.8 keV strongly support a scenario in which the hard X-rays are reprocessed by a photoionized accretion disc. This hypothesis was tested fitting the BeppoSAX spectrum with the ionized disc reflection model of Ross & Fabian. A second narrow line component, in addition to that produced in the disc, is also required to fit the observed iron line profile. A high energy cut-off around 150 keV is clearly detected in the spectrum.

astro-ph↗

arXiv2002

Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts

We present the main results of a study of spectral and energetics properties of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the redshift estimates and the good-quality BeppoSAX time-integrated spectra we deduce the main properties of GRBs in their cosmological rest frames. All spectra in our sample are satisfactorily represented by the Band model with no significant soft X-ray excesses or spectral absorptions. We find a positive correlation between the estimated total (isotropic) energies in the 1-10000 keV energy range (E_rad) and redshifts z. Interestingly, more luminous GRBs are characterized also by larger peak energies E_p of their EF(E) spectra. Furthermore, more distant GRBs appear to be systematically harder in the X-ray band compared to GRBs with lower redshifts. We discuss how selection and data truncation effects could bias our results and give possible explanations for the correlations that we found.

astro-ph↗

arXiv2002

Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies

A sample of nine bright Seyfert 1 and NELG type galaxies, observed with BeppoSAX, is analyzed to assess on a truly broad band basis (0.1-200 keV) the issue of the spectral contributions of Compton reflection and iron line fluorescence from circumnuclear gas. The empirical description adopted for the direct continuum is the commonly used power law with an exponential cut-off. The most direct test of the theoretical predictions, namely that the equivalent width of the line, Walpha, and the strength R of the reflection relative to the direct continuum are closely related to each other, gives a substantially positive result, that is their mean ratio is very close to expectation, and only a modest spread in the iron abundance seems implied. The existence of a steep correlation between R and the slope Gamma of the power law is not confirmed. A weak evidence is found that the existence of a very shallow trend to increase on average with Gamma cannot be altogether excluded in both R and Walpha, but needs to be tested with a larger sample. The energy Ef in the exponential cut-off spans a range from about 80 to more than 300 keV. A possible correlation is found, with Ef increasing on average with Gamma: if ignored, for instance by keeping Ef at a fixed value in a sample study, it could be cause of artificial steepening in a correlation between R and Gamma.

astro-ph↗

arXiv2002

The BeppoSAX broad-band spectrum and variability of the Seyfert 1 NGC 3783

BeppoSAX observed the Seyfert 1 galaxy NGC 3783 for ~ 5 days from 1998 June 6 to 1998 June 10. The average flux during the observation was F(2-10 keV) ~ 6 X 10^{-11} erg cm^-2 s^-1. The long exposure provided a very high quality spectrum that exhibits all the typical features of Seyfert 1s. The high energy cut-off of the intrinsic continuum, at E_c=340+560/-107 keV, was detected for the first time in this object. During the observation the source showed modest flux variations (~ 20% in 2-10 keV) and spectral variability that can be completely explained by a change of the intrinsic slope ΔΓ~ 0.1. NGC 3783 is one of the few Seyfert 1s that exhibits a soft emission component in the BeppoSAX spectrum. This soft excess is well reproduced by a black body component with temperature kT ~ 0.2 keV. A warm absorber is also required to fit the soft X-ray spectrum. Both reprocessing features, the Compton reflection hump and the Fe emission line, were detected in our observation. In addition to a narrow iron line (as observed by Chandra) a broad component with sigma=0.72 +1.28/-0.27 keV and EW=115+/-76 eV, is required to fit the line. The observed Compton hump and the Fe line are well reproduced with the ionized (lg(xi)=2.73+0.04/-0.05) disc reflection model of Ross & Fabian. In this case the soft excess observed below 2 keV can be accounted for by the disc emissivity and no additional soft X-rays component is required to fit the spectrum.

astro-ph↗

arXiv2002

Study of X-ray reflection features in a sample of Sy 1s observed by BeppoSAX

The ionized disc reflection model of Ross & Fabian is applied to a sample of five Seyfert 1 galaxies observed by BeppoSAX. Good fits were obtained for all the objects in the sample. NGC 4051 and Mkn 335 (usually classified as NLSy 1s) show evidence of a highly ionized accretion disc (xi ~ 350 erg cm s^{-1} and xi \~ 800 erg cm s^{-1}). While NGC 3783 seems to be a ``pure Sy 1'' characterized by a highly ionized accretion disc (xi ~ 500 erg cm s^{-1}). All the sources are consistent with having a face-on disc. The accurate determination of the continuum emission allows us to investigate the spectral variability in the case of Mkn 509.

astro-ph↗

arXiv2001

The complex iron line of NGC 5506

The bright Narrow Emission Line Galaxy, NGC 5506, has been observed simultaneously by XMM-Newton and BeppoSAX. The iron line is complex, with at least two components: one narrow and corresponding to neutral iron, the second one broad and corresponding to ionized iron. The latter line is equally well fitted by a truly broad line or by a blend of He- and H-like narrow lines. The bulk of the Compton reflection continuum originates in neutral matter, and is therefore associated with the narrow line: they are most likely emitted in distant matter. The origin of the ionized line(s) is less certain, but the solution in terms of a blend of narrow lines from photoionized matter seems to be preferable to emission from an ionized, relativistic accretion disc.

astro-ph↗

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.

astro-ph↗

arXiv2001

BeppoSAX observations of Mrk 841 and Mrk335

We present and discuss BeppoSAX observations of Mrk841 and Mrk335, two Seyfert 1 galaxies in which previous observations have established the presence of soft excesses. We confirm the soft excess in both sources, even if for Mrk~841 a warm absorber provides a fit almost as good as the one with a true excess. As far as the hard X-ray continuum is concerned, a Comptonization model provides a fit as good as a power law and a physically sound solution for Mrk841. For Mrk335, the Comptonization model gives a result which is somewhat better on statistical ground, but rather problematic on physical ground. The most interesting results regard the reprocessing components. For Mrk841 we find a very large reflection continuum but an almost normal iron line equivalent width even if, within the errors, a solution in which both components are a factor ~2 larger than expected for an accretion disc is still marginally acceptable. If this is the case, an anisotropy of the primary emission seems the best explanation. On the contrary, in Mrk335 we find a very large iron line EW but a reflection component not accordingly large. In this case, the best solution seems to be in terms of reflection from an ionized disc.

astro-ph↗

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.

astro-ph↗