arXiv subjects

Elena Pian

Publications and source records attributed to Elena Pian.

89 records · Page 5Linked to original sources

arXiv2002

X-ray Variability of Blazars

Critical progress in our understanding of high energy emission from AGN has been determined in the last 10 years by X-ray monitoring campaigns with many space missions, notably ROSAT, ASCA, RXTE, BeppoSAX, and XMM, often in conjunction with observations at other frequencies. The emphasis of the present review is on recent findings about X-ray variability of blazars. Among AGN, these exhibit the largest amplitude variations of the X-ray emission, often well correlated with variations at higher energies (GeV and TeV radiation). The accurate sampling of the X-ray spectra over more than three decades in energy, made possible by the wide energy range of BeppoSAX, has also shown strong spectral variability in blazar active states, suggesting extreme electron energies and leading to the identification of a class of `extreme synchrotron' sources.

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arXiv2001

REM Optical Slitless Spectrograph (ROSS): an instrument for prompt low resolution spectroscopy of Gamma Ray Bursts

Since the discovery of Gamma-Ray Bursts (GRBs), attempts have been made to detect correlated optical transient emission from these objects. In January 1999, the ROTSE I robotic telescope detected a bright optical flash simultaneous with a GRB, thanks to the prompt dissemination to the ground of the high energy event coordinates. To date, that single observation remains unique as no other prompt flashes have been seen for other bursts observed with comparably short response times. This suggests that in general GRB prompt optical emission may be considerably dimmer than observed for the GRB990123 event. To exploit the better angular localization accuracy of the flying (HETE-2) or soon to fly (INTEGRAL, AGILE, SWIFT, GLAST) missions for high energy astrophysics, a new generation of robotic telescopes is being developed. These will have response times as short as a few seconds and will be sensitive to signals as faint as m_v ~ 20, thus increasing the chance of detecting even weak prompt emission. Results from these experiments should provide important new data about the dynamics and local medium composition of GRBs. In this paper we describe one of the new instruments, ROSS, to be mounted on the robotic facility REM, designed to perform low resolution spectroscopy on the prompt optical flares associated with GRBs.

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arXiv2001

BeppoSAX confirmation of beamed afterglow emission from GRB990510

We compare the prompt X-ray (2-10 keV) emission of GRB990510 measured by the BeppoSAX Wide Field Cameras (WFC) during the burst to the X-ray afterglow detected by the BeppoSAX Narrow Field Instruments. A single power-law model for the afterglow, f(t) ~ t^{-1.42}, is ruled out. Provided the initial time of the afterglow is assumed to coincide with the last short pulse in the X-ray prompt event (i.e., 72 seconds after the GRB trigger time), the X-ray emission from \~80 to 10^5 seconds after the GRB trigger is well described by an external shock expanding in a decelerating jet, in which synchrotron radiation takes place. This model, represented by a double power-law of indices alpha_1 ~ 1 and alpha_2 ~ 2 before and after a jet collimation break time of ~0.5 days after GRB, respectively, is consistent with the second and third upper limits measured by the WFC, but not with the first. This may be related to inhomogeneities in the circumburst medium. Our finding indicates that the temporal behavior of the GRB990510 X-ray afterglow is similar to that at optical wavelengths, and thus strengthens the interpretation of the multiwavelength afterglow as synchrotron emission in a jet with decreasing Lorentz factor. GRB990510 is thus the only burst in which evidence of a spreading jet has been found in X-rays.

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arXiv2001

The metamorphosis of SN1998bw

We present and discuss the photometric and spectroscopic evolution of the peculiar SN1998bw, associated with GRB980425, through an analysis of optical and near IR data collected at ESO-La Silla. The spectroscopic data, spanning the period from day -9 to day +376 (relative to B maximum), have shown that this SN was unprecedented, although somewhat similar to SN1997ef. Maximum expansion velocities as high as 3x10^4 km/s to some extent mask its resemblance to other Type Ic SNe. At intermediate phases, between photospheric and fully nebular, the expansion velocities (~10^4 km/s) remained exceptionally high compared to those of other recorded core-collapse SNe at a similar phase. The mild linear polarization detected at early epochs suggests the presence of asymmetry in the emitting material. The degree of asymmetry, however, cannot be decoded from these measurements alone. The HeI 1.083 mu and 2.058 mu lines are identified and He is suggested to lie in an outer region of the envelope. The temporal behavior of the fluxes and profiles of emission lines of MgI]4571A, [OI]6300,6364A and a feature ascribed to Fe are traced to stimulate future modeling work.

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arXiv2000

Multiband optical polarimetry of the BL Lac object PKS 2155--304. Intranight and long term variability

The polarized and total flux of the BL Lac object PKS 2155-304 were monitored intensively and simultaneously in the optical UBVRI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during 4 campaigns in June, August, November 1998 and August 1999. The effective observation time amounted to ~47 hours. PKS 2155-304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (PA) mainly between 70 and 120 deg. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source, and has allowed us to detect amplitude variations of the linear polarization percentage from 6 to 7.5% in time scales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths, however the polarized spectrum does not vary significantly with time. The most remarkable variability event occurred on 1998 June 18, when the degree of linear polarization decreased by more than a factor 2 in one day in all bands, while the PA rotated by 90 deg. This is consistent with the presence of two emission components, of different polarization degree and position angle. Intranight variability of P and PA can be interpreted with small amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did not show light variations correlated with those of the linearly polarized flux.

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arXiv1999

X-ray Emission of Supernova 1998bw in the Error Box of GRB980425

The spatial and temporal coincidence of a GRB and a supernova explosion (1998bw) on 25 April 1998 has raised conjectures on the physical connection between the two phenomena, and in general on the association of GRBs with supernovae, at least with the most powerful among them (hypernovae or collapsars). In fact, multiwavelength observations of SN 1998bw have revealed unusual characteristics: extremely high energy output at radio and optical wavelengths, and relativistic expansion of the outgoing shock. The X-ray emission of SN 1998bw, monitored by BeppoSAX starting ~10 hours after the GRB detection, was remarkably prompt (within one day of supernova detonation), but exhibited spectral and temporal properties similar to those of other supernovae detected in X-rays.

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arXiv1999

HST and Palomar Imaging of GRB 990123: Implications for the Nature of Gamma-Ray Bursts and their Hosts

We report on HST and Palomar optical images of the field of GRB 990123, obtained on 8 and 9 February 1999. We find that the optical transient (OT) associated with GRB 990123 is located on an irregular galaxy, with magnitude V=24.20 +/- 0.15. The strong metal absorption lines seen in the spectrum of the OT, along with the low probability of a chance superposition, lead us to conclude that this galaxy is the host of the GRB. The OT is projected within the ~1'' visible stellar field of the host, nearer the edge than the center. We cannot, on this basis, rule out the galactic nucleus as the site of the GRB, since the unusual morphology of the host may be the result of an ongoing galactic merger, but our demonstration that this host galaxy has extremely blue optical to infrared colors more strongly supports an association between GRBs and star formation. We find that the OT magnitude on 1999 Feb 9.05, V = 25.45 +/- 0.15, is about 1.5 mag fainter than expected from extrapolation of the decay rate found in earlier observations. A detailed analysis of the OT light curve suggests that its fading has gone through three distinct phases: an early rapid decline (f_{nu} \propto t^{-1.6} for t < 0.1 days), a slower intermediate decline power-law decay (f_{nu} \propto t^{-1.1} for 0.1 < t < 2 days), and then a more rapid decay (at least as steep as (f_ν \propto t^{-1.8} for t > 2 days). The break to steeper slope at late times may provide evidence that the optical emission from this GRB was highly beamed.

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arXiv1999

BeppoSAX Observations of the galactic source GS1826-238 in a hard X-ray high state

The BeppoSAX Narrow Field Instruments observed the galactic source GS1826-238 in October 1997, following a hard X-ray outburst with a peak flux of about 100 mCrab detected by BATSE. Two short X-ray bursts (about 150 s) were detected up to 60 keV, with larger amplitude and duration at lower energies(up to a factor 20 times the persistent emission). This confirms the proposed identification of the source as a weakly magnetized neutron star in a LMXRB system. For both persistent and burst states, the spectrum in the 0.4-100 keV energy range is well fitted by an absorbed blackbody, plus a flat power-law with an exponential cutoff at 50 keV.

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arXiv1998

Science with IUE and the Growth of an Astrophysical Group in Milan: from X-Ray Binaries to AGN

The contribution of an astrophysics group based in Milan to the science with the IUE satellite during its almost 20 years lifetime has focussed on high energy sources, of both galactic (LMXRB, HMXRB, and black hole candidates) and extragalactic (AGN) nature. The results of this long term research and in particular of the latest multiwavelength campaigns conducted simultaneously with IUE are reviewed here.

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arXiv1998

Extreme Synchrotron Blazars in Outburst: the Potentiality of INTEGRAL Imaging and Spectroscopy

Only ~10 blazars have been detected in soft gamma-rays by OSSE and the PDS, often in flaring state. We investigate the impact of the imaging and spectral capabilities of the IBIS instrument on board INTEGRAL on the study of blazars. The objectives are: 1) to remove ambiguities in the identification of some blazars as soft gamma-ray emitters; 2) to determine the spectral shape of "extreme synchrotron" blazars, namely sources with high F_X/F_radio ratios, for which the synchrotron component peaks at far UV-soft X-ray frequencies. We expect that in these sources the synchrotron peak shifts to higher energies during outbursts, making them strong soft gamma-ray emitters, and therefore possible targets for INTEGRAL wide band X- and gamma-ray spectroscopy.

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arXiv1998

BeppoSAX Observations of the BL Lac Object Mkn 501 in an Intermediate State

The BL Lac object Mkn 501 was observed with the BeppoSAX satellite at three epochs in April-May 1998, simultaneously with the Whipple and HEGRA Cherenkov telescopes. The X-ray spectrum is well detected up to 70 keV and it exhibits, at all epochs, a continuous curvature, which is here modeled with three power-laws of increasingly steeper index at larger energies. In the nu*f_nu representation the spectrum exhibits a peak at ~20 keV, which is interpreted as the maximum of the synchrotron emission. This implies that the synchrotron peak energy has lowered by an order of magnitude with respect to the powerful X-ray outburst observed in April 1997. The simultaneous TeV flux was comparable to the lowest levels observed for Mkn 501, possibly suggesting that the peak of the inverse Compton radiation had also shifted toward lower energies.

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arXiv1998

HST/STIS Observations of the Optical Counterpart to GRB 970228

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made on 4~September~1997 using the STIS CCD on the Hubble Space Telescope. The unresolved counterpart is detected at V=28 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. No proper motion is detected, in contradiction of an earlier claim. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude 25.7 +/- 0.25. Comparison with WFPC2 data taken one month after the initial burst and NTT data taken on March 13 shows no evidence for fading of the extended emission. After adjusting for the probable Galactic extinction in the direction of GRB 970228 of A_v=0.7, we find that the observed nebula is consistent with the sizes of galaxies of comparable magnitude found in the Hubble Deep Field and other deep HST images, and that only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore conclude that the extended source observed about GRB 970228 is most likely a galaxy at moderate redshift, and is almost certainly the host of the gamma-ray burst.

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arXiv1997

The Gamma Ray Bursts GRB970228 and GRB970508: What Have We Learnt?

We examine what we regard as key observational results on GRB 970228 and GRB 970508 and show that the accumulated evidence strongly suggests that gamma-ray bursts (GRBs) are cosmological fireballs. We further show that the observations suggest that GRBs are not associated with the nuclear activity of active galactic nuclei, and that late-type galaxies are more prolific producers of GRBs. We suggest that GRBs can be used to trace the cosmic history of the star-formation rate. Finally, we show that the GRB locations with respect to the star-forming regions in their host galaxies and the total burst energies can be used to distinguish between different theoretical models for GRBs.

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arXiv1997

BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lac Object Mkn 501

The BL Lac object Mkn 501, one of the only three extragalactic sources (with Mkn 421 and 1ES 2344+514) so far detected at TeV energies, was observed with the BeppoSAX satellite on 7, 11, and 16 April 1997 during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA and CAT Cherenkov telescopes. Over the whole 0.1-200 keV range the spectrum was exceptionally hard (alpha =< 1, with F_nu ~ nu^{-alpha}) indicating that the X-ray power output peaked at (or above) ~100 keV. This represents a shift of at least two orders of magnitude with respect to previous observations of Mkn 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing intensity and, at each epoch, it is softer at larger energies. The correlated variability from soft X-rays to the TeV band points to models in which the same population of relativistic electrons produces the X-ray continuum via synchrotron radiation and the TeV emission by inverse Compton scattering of the synchrotron photons or other seed photons. For the first time in any blazar the synchrotron power is observed to peak at hard X-ray energies. The large shift of the synchrotron peak frequency with respect to previous observations of Mkn 501 implies that intrinsic changes in the relativistic electron spectrum caused the increase in emitted power. Due to the very high electron energies, the inverse Compton process is limited by the Klein-Nishina regime. This implies a quasi-linear (as opposed to quadratic) relation of the variability amplitude in the TeV and hard X-ray ranges (for the SSC model) and an increase of the inverse Compton peak frequency smaller than that of the synchrotron peak frequency.

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arXiv1997

Hubble Space Telescope Imaging of the Optical Transient Associated with GRB970508

We report on Hubble Space Telescope (HST) observations of the optical transient (OT) discovered in the error box of the gamma-ray burst GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph (STIS) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The observations reveal a point-like source with R = 23.1 +- 0.2 and H = 20.6 +- 0.3, in agreement with the power-law temporal decay seen in ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508. We set very conservative upper limits of R ~ 24.5 and H ~ 22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ~25.5. In combination with Keck spectra that show Mg I absorption and [O II] emission at a redshift of z = 0.835, our observations suggest that the OT is located in a star-forming galaxy with total luminosity one order of magnitude lower than the knee of the galaxy luminosity function, L*. Such galaxies are now thought to harbor the majority of star formation at z ~ 1; therefore, these observations may provide support for a link between GRBs and star formation.

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arXiv1997

Monitoring Ly alpha Emission from the Blazar 3C 279

The blazar 3C 279 is well studied and shows frequent large continuum flares from radio to gamma-ray wavelengths. There have been a number of multi-wavelength observations of 3C 279, and hence there are many ultraviolet data for this object available in the UV archives. In this paper we present Ly alpha emission line measurements for 3C 279 using all the archival IUE SWP spectra from 1988 to 1996 and all archival HST/FOS G190H spectra from 1992 to 1996. Individual archival IUE spectra of 3C 279 show weak Ly alpha emission at ~1868 A (z = 0.536), which is easily seen in the co-added data. The Ly alpha emission is observed in all the HST/FOS spectra. The strength of Ly alpha is nearly constant (~5E-14 erg/cm2/s), while the 1750 A continuum varies by a factor of ~50, from ~0.6 to 31.6E-15 erg/cm2/s/A. The behavior of the Ly alpha emission line flux and continuum flux is similar to that of the only other well observed blazar, 3C 273, which shows constant line flux while the continuum varies by a factor of ~3. This near-constancy of emission line flux in the two best-studied blazars, suggests that the highly variable beamed continuum is not a significant source of photoionization for the gas. Some other source, such as thermal emission from an accretion disk, must be providing a significant fraction of the photoionizing flux in these objects. The large amplitude variability seen at gamma-ray energies must be due to changes in the energetic electrons in the jet rather than changes in the external photon field.

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arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. II. The IUE Campaign

PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the IUE satellite at ~1 hour time-resolution for ten nearly uninterrupted days in May 1994. The campaign, which was coordinated with EUVE, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short time scale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for two days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (~<2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that for the first time, based on the UV emission alone, the traditional Delta L/Delta t limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux variations had completely different characteristics.

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