arXiv subjects

E. Pian

Publications and source records attributed to E. Pian.

231 records · Page 13Linked to original sources

arXiv1999

Progress in understanding Blazars from BeppoSAX observations

Results obtained with BeppoSAX observations of blazars within various collaborative programs are presented. The spectral similarity "paradigm", whereby the spectral energy distributions of blazars follow a sequence, leading to a unified view of the whole population, is briefly illustrated. We concentrate on recent observations of flares and associated spectral variability for three objects at the "blue" end of the spectral sequence, namely PKS 2155-304, Mkn 421 and Mkn 501. The results are discussed in terms of a general analytic synchrotron self-Compton interpretation of the overall spectrum. The physical parameters of the quasi-stationary emission region can be derived with some confidence, while the variability mechanism(s) must be complex.

astro-ph↗

arXiv1998

Fast X-ray Variability of PKS 2155-304: A Cross Correlation Analysis

PKS 2155-304 is one of the prototypes of BL Lac objects. Its spectral energy distribution is known from the radio to the TeV band and has two peaks, one in the soft X-rays and the other in the TeV region. These may be interpreted within the synchrotron self-Compton model, which attributes the first peak to synchrotron radiation of electrons, which also produce the second peak via Compton scattering of the same synchrotron photons. The emission is relativistically boosted in the observer direction. A way of testing the model and characterizing its geometrical and kinematical properties is through the study of correlated variability in different bands. Here we report on the search and accurate study of lags in the X-ray light curves obtained with the BeppoSAX satellite during the performance verification phase in November 1996 and compare the results with the findings of a similar analysis applied to the November 1997 BeppoSAX and 1994 ASCA data.

astro-ph↗

arXiv1998

BeppoSAX Observations of PKS 2155-304 during an active Gamma-ray State

PKS 2155-304 was observed with BeppoSAX in November 1997 for 64 ksec (total elapsed time 33.5 hours) and, for the first time, simultaneously in gamma-rays with EGRET on board the Compton Gamma Ray Observatory and with the ground based TeV telescope CANGAROO, during a phase of high brightness in the X-ray band. The LECS and MECS light curves show a pronounced flare (with an excursion of a factor 3.5 between min and max), with evidence of spectral hardening at maximum intensity. The source is weakly detected by the PDS in the 12-100 keV band with no significant evidence of variability. The broad band X-ray data from Beppo SAX are compared with the gamma-ray results and discussed in the framework of homogenous synchrotron self Compton models.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv1998

Ultraviolet Isotropic Emission from the Blazar 3C 279

Archival IUE SWP and HST FOS spectra show the presence of a relatively strong, broad Lyman alpha emission line superposed onto the UV spectral continuum of the blazar 3C 279. As opposed to a factor ~50 variation of the continuum flux during eight years, the emission line did not exhibit significant intensity changes. Simultaneous IUE SWP and LWP spectra of 3C 279 in low emission state are fitted by power-laws of index alpha_nu ~1 (f_nu ~ nu^{-alpha_nu}), significantly flatter than measured during higher states. Our observations suggest that the Lyman alpha line is not powered by the beamed, anisotropic synchrotron radiation which produces the observed continuum in 3C 279, but rather by an unbeamed component characterized by slower and lower amplitude variability. The latter may account for the UV continuum observed during the very low state of January 1993.

astro-ph↗

arXiv1998

High Energy Properties of X-ray Sources observed with BeppoSAX

We report on highlight results on celestial sources observed in the high energy band (>20 keV) with BeppoSAX. In particular we review the spectral properties of sources that belong to different classes of objects, i.e. stellar coronae (Algol), supernova remnants (Cas A), low mass X-ray binaries (Cygnus X-2 and the X-ray burster GS1826-238), black hole candidates (Cygnus X-1) and Active Galactic Nuclei (Mkn 3). We detect, for the first time, the broad-band spectrum of a stellar corona up to 100 keV; for Cas A we report upper limits to the ^44Ti line intensities that are lower than those available to date; for Cyg X-2 we report the evidence of a high energy component; we report a clear detection of a broad Fe K line feature from Cygnus X-1 in soft state and during its transition to hard state; Mkn 3 is one of several Seyfert 2 galaxies detected with BeppoSAX at high energies, for which Compton scattering process is important.

astro-ph↗

arXiv1997

BeppoSAX observations of PKS 0528+134

We report on the BeppoSAX observations of the gamma-ray blazar PKS 0528+134 performed in Feb and Mar 1997, during a multiwavelength campaign involving EGRET and ground based telescopes. The source was in a faint and hard state, with energy spectral index alpha=0.48+-0.04 between 0.1 and 10 keV, and [2-10] keV flux of 2.7E-12 erg/cm2/s. No significant variability was observed. The source was detected in the 20-120 keV band by the PDS, with a flux lying slightly above the extrapolation from lower X-ray energies. Comparing this low state with previous higher states of the source, there is an indication that the X-ray spectrum hardens and the gamma-ray spectrum steepens when the source is fainter.

astro-ph↗

arXiv1997

Hubble Space Telescope Imaging of the Field of GRB970508

We report on Hubble Space Telescope observations of the optical transient discovered in the error box of GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrometer. 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 previous ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508, although Mg I absorption and [O II] emission seen in Keck spectra at a redshift of z = 0.835 suggest the presence of a dense, star-forming medium. The HST observations 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. Subsequent photometry suggests a flattening of the light curve at later epochs. Assuming the OT decline follows a pure power-law and ascribing the flattening to the presence of an underlying component of constant flux, we find that this must have R = 25.4, consistent with the upper limits determined by HST. At z = 0.8, this would correspond to an absolute magnitude in the U band of ~-18, similar to that of the Large Magellanic Cloud. We propose a scenario in which the host galaxy of the GRB is of Magellanic type, possibly being a ``satellite'' of one of the bright galaxies located at few arcseconds from the optical transient.

astro-ph↗

arXiv1997

Multiwavelength Observations of a Dramatic High Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the gamma-ray sky, underwent a flare of a factor 10 amplitude in gamma-rays towards the end of a 3-week pointing by CGRO, in 1996 January-February. The flare peak represents the highest gamma-ray intensity ever recorded for this object. During the high state, extremely rapid gamma-ray variability was seen. Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSAT and IUE and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of ~3) well correlated with the gamma-ray flare without any apparent lag. The optical-UV light curves, which are not well sampled during the high energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near an historical maximum during the gamma-ray flare peak, with suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The gamma-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron-self Compton model would require that the largest synchrotron variability occurred in the mostly unobserved sub-mm/far-infrared region. Alternatively, a large variation in the external photon field could occur over a time scale of few days. This occurs naturally in the ``mirror'' model, wherein the flaring region in the jet photoionizes nearby broad-emission-line clouds, which in turn provide soft photons that are Comptonized to gamma-ray energies.

astro-ph↗

arXiv1997

Optical-IUE observations of the gamma-ray loud BL Lacertae object S5 0716+714: data and interpretation

We monitored the BL Lac object S5 0716+714 in the optical band during the period November 1994-April 1995, which includes the time of a gamma-ray observation by the Energetic Gamma Ray Experiment Telescope (EGRET) on February 14--28, 1995. The light curves in the R and B bands show fast fluctuations superimposed on longer timescale variations. The color index correlates with intensity during the rapid flares (the spectrum is flatter when the flux is higher), but it is rather insensitive to the long term trends. Over the 5 month observational period the light curve shows an overall brightening of about 1 mag followed by a fast decline. The EGRET pointing covers part of the very bright phase (V about 13.2) and the initial decline. An ultraviolet spectrum was also obtained with the International Ultraviolet Explorer (IUE) (1200-3000 A) during the EGRET observations. The variability of the optical emission of S5 0716+714 by itself sets important constraints on the magnetic field strength and on the physical processes responsible for it. Interpreting the whole electromagnetic spectrum with synchrotron self Compton models leads to the prediction of a bright gamma-ray state during the EGRET pointing. We discuss how the gamma-ray data could be used as a diagnostic of the proposed models.

astro-ph↗

arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. III. Probing the Inner Jet through Multiwavelength Correlations

In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ~10 d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with ground- based radio, IR, and optical telescopes. The light curves show a well-defined X-ray flare (x2 increase in 1/2 d, with similar decay) followed by a broader, lower amplitude EUV flare ~1 day later, and a broad, low-amplitude UV flare ~2 d later (amplitude ~35%, duration >2 d). In the preceding week there was at least one X-ray flare of comparable amplitude or perhaps ongoing stochastic X-ray variations. An extremely rapid change in UV flux was seen at the beginning of the IUE observation. Assuming the central X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous relativistic jet. We can rule out a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t<<hours) injection of high energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions (e.g., behind a shock front and/or in an inhomogeneous jet). The present light curves are in sharp contrast to November 1991, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hours. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes. (abbreviated abstract)

astro-ph↗

arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. I. The Ground-Based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superimposed on longer timescale variations. Rapid variations of ~0.01 mag/min, which, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wavebands (UBVRI) track each other well over the period of observation with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the ultraviolet flux.

astro-ph↗

arXiv1994

The Near-IR-Optical-UV Emission of BL Lacertae Objects

Near--infrared, optical and ultraviolet quasi--simultaneous observations of 11 BL Lacertae objects are reported. For all but one source the dereddened spectral flux distribution in the $8\cdot10^{13}-2\cdot10^{15}$ Hz frequency range can be described by a single power law f$_ν\propto ν^{-α}$ with average spectral index $<α>$ = 0.88 $\pm$ 0.42 (standard deviation) plus, where relevant, the contribution of the host galaxy. In most cases the non simultaneous soft X--ray fluxes obtained by the {\it Einstein Observatory} lie on or below the extrapolation of the power law. The results are compared with the average spectral properties of other samples of BL Lacs studied separately in the IR--optical and in the UV bands. The implications for existing models of the objects are shortly discussed.

astro-ph↗