arXiv subjects

E. Pian

Publications and source records attributed to E. Pian.

At least 109 records · Page 6Linked to original sources

arXiv2009

Multiwavelength observations of a TeV-Flare from W Com

We report results from an intensive campaign of multiwavelength observations of the intermediate-frequency-peaked BL Lacertae object W Com (z=0.102) during a strong outburst of very high energy (VHE; E > 180 GeV) gamma-ray emission in June 2008. The initial detection of this VHE flare by VERITAS, an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes, was followed by observations in high-energy gamma rays (AGILE, E > 100 MeV), X-rays (Swift), optical and radio wavelengths. The VHE gamma-ray signal was detected by VERITAS on June 7-8 with a flux about three times brighter than during the discovery of VHE gamma-ray emission from W Com by VERITAS in March 2008. A detailed study of the spectral energy distribution of W Com during this flare, including theoretical work, will be presented.

astro-ph.HE↗

arXiv2009

The jet of the BL Lac object PKS 0521 -365 in the near-IR : MAD adaptive optics observations

BL Lac objects are low--power active nuclei exhibiting a variety of peculiar properties that are caused by the presence of a relativistic jet and orientation effects. We present here adaptive optics near-IR images at high spatial resolution of the nearby BL Lac object PKS 0521-365, which is known to display a prominent jet both at radio and optical frequencies. The observations were obtained in Ks--band using the ESO multi-conjugated adaptive optics demonstrator at the Very Large Telescope. This allowed us to obtain images with 0.1 arcsec effective resolution. We performed a detailed analysis of the jet and its related features from the near-IR images, and combined them with images previously obtained with HST in the R band and by a re-analysis of VLA radio maps. We find a remarkable similarity in the structure of the jet at radio, near-IR, and optical wavelengths. The broad--band emission of the jet knots is dominated by synchrotron radiation, while the nucleus also exhibits a significant inverse Compton component. We discovered the near-IR counterpart of the radio hotspot and found that the near-IR flux is consistent with being a synchrotron emission from radio to X-ray. The bright red object (red-tip), detached but well aligned with the jet, is well resolved in the near-IR and has a linear light profile. Since it has no radio counterpart, we propose that it is a background galaxy not associated with the jet. The new adaptive optics near-IR images and previous observations at other frequencies allow us to study the complex environment around the remarkable BL Lac object PKS 0521-365. These data exemplify the capabilities of multi conjugate adaptive optics observations of extragalactic extended sources.

astro-ph.CO↗

arXiv2009

Ultraviolet Spectroscopy of Supernovae: The First Two Years of Swift Observations

We present the entire sample of ultraviolet (UV) spectra of supernovae (SNe) obtained with the Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite during the first 2 years of observations (2005/2006). A total of 29 UV-grism and 22 V-grism spectra of 9 supernovae (SNe) have been collected, of which 6 are thermonuclear (type Ia) and 3 core collapse (type Ibc/II) SNe. All the spectra have been obtained during the photospheric phase. After a comparison of the spectra of our sample with those in the literature (SNe 1992A, 1990N and 1999em), we confirm some degree of diversity in the UV emission of Type Ia SNe and a greater homogeneity in the Type II Plateau SN sample. Signatures of interaction between the ejecta and the circumstellar environment have been found in the UV spectrum of SN 2006jc, the only SN Type Ib/c for which UVOT grism data are available. Currently, Swift UVOT is the best suited instrument for early SN studies in the UV due to its fast response and flexible scheduling capabilities. However, in order to increase the quality of the data and significantly improve our understanding of the UV properties of SNe and to fully maximize the scientific potential of UVOT grism observations, a larger investment in observing time and longer exposures are needed.

astro-ph.CO↗

arXiv2009

Variability of low-luminosity AGNs: a simultaneous X-ray/UV look with Swift

We present the results of an investigation of the X-ray and UV properties of four LINERs observed with Swift, aimed at constructing good S/N and strictly simultaneous UV-X-ray SEDs. In the current paradigm, LINER emission is dominated by geometrically thick, radiatively inefficient radiation flows (RIAFs) as opposed to radiatively efficient, geometrically thin accretion disks thought to power higher luminosity AGNs (Seyferts and QSOs). However, some recent studies have found more similarities than differences between the SEDs of LINERs and those of more luminous AGNs, suggesting that LINERs are powered by the same mechanisms active in higher luminosity AGNs. Our new observations allow us to test this idea. In particular, XRT affords long and sensitive monitoring of the X-ray emission. We detect significant variability in M81 and, for the first time, in NGC 3998. The maximum amplitude variations over time scales of some hours are 30% in both M81 and NGC 3998. NGC 3998 exhibits a variation of the same amplitude on a time scale of 9 days. M81 varies significantly over 2 years, with a maximum change of a factor 2 in 6 months. The X-ray variability detected in 2 of our sources, and in particular in NGC 3998, puts into question the interpretation of their powering mechanism as an inefficient accretion flow, because one of the characteristics of this model is the lack of variability. The identification of NGC 3998 with a low power AGN appears more viable.

astro-ph.HE↗

arXiv2009

Detection of Gamma-ray Emission from the Eta-Carinae Region

We present the results of extensive observations by the gamma-ray AGILE satellite of the Galactic region hosting the Carina nebula and the remarkable colliding wind binary Eta Carinae (Eta Car) during the period 2007 July to 2009 January. We detect a gamma-ray source (1AGL J1043-5931) consistent with the position of Eta Car. If 1AGL J1043-5931 is associated with the Eta Car system our data provide the long sought first detection above 100 MeV of a colliding wind binary. The average gamma-ray flux above 100 MeV and integrated over the pre-periastron period 2007 July to 2008 October is F = (37 +/- 5) x 10-8 ph cm-2 s-1 corresponding to an average gamma-ray luminosity of L = 3.4 x 10^34 erg s-1 for a distance of 2.3 kpc. We also report a 2-day gamma-ray flaring episode of 1AGL J1043-5931 on 2008 Oct. 11-13 possibly related to a transient acceleration and radiation episode of the strongly variable shock in the system.

astro-ph.HE↗

arXiv2009

Type Ib Supernova 2008D associated with the Luminous X-ray Transient 080109: An Energetic Explosion of a Massive Helium Star

We present a theoretical model for supernova (SN) 2008D associated with the luminous X-ray transient 080109. The bolometric light curve and optical spectra of the SN are modelled based on the progenitor models and the explosion models obtained from hydrodynamic/nucleosynthetic calculations. We find that SN 2008D is a more energetic explosion than normal core-collapse supernovae, with an ejecta mass of Mej = 5.3 +- 1.0 Msun and a kinetic energy of E = 6.0 +- 2.5 x 10^{51} erg. The progenitor star of the SN has a 6-8 Msun He core with essentially no H envelope (< 5 x 10^{-4} Msun) prior to the explosion. The main-sequence mass of the progenitor is estimated to be Mms =20-25 Msun, with additional systematic uncertainties due to convection, mass loss, rotation, and binary effects. These properties are intermediate between those of normal SNe and hypernovae associated with gamma-ray bursts. The mass of the central remnant is estimated as 1.6 - 1.8 Msun, which is near the boundary between neutron star and black hole formation.

astro-ph↗

arXiv2009

The changing look of PKS 2149-306

Aims: We study the blazar nature of the high-redshift Flat-Spectrum Radio Quasar PKS 2149-306 (z = 2.345) by investigating its long-term behavior. Methods: We analyzed all publicly available optical-to-X-ray observations performed by XMM-Newton, Swift, and INTEGRAL. Conclusions: PKS 2149-306 is one of four blazars at z>2 that have been observed in the hard-X-ray regime with both the BAT and ISGRI instruments. Observations acquired almost 1 year apart in the 60-300 keV energy band in the object rest frame, exhibit no noticeable change in spectral slope associated with a flux variation of more than a factor of two. Swift data appear to show a roll-off below ~1 keV, which becomes increasingly evident during a ~3-day time-frame, that can be explained as the natural spectral break caused by the Inverse Compton onset. The broad-band spectra allow us to identify two different states. The SED modeling suggests that they can be interpreted by only a change in the bulk Lorentz factor of the jet.

astro-ph.HE↗

arXiv2008

Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events

We present new spectroscopic and photometric data of the type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early-time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core-collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova explosions which occur within a He-rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of Wolf-Rayet progenitors still embedded in the He-rich material lost by the star in recent mass-loss episodes, which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system. We also suggest that there are a number of arguments in favour of a type Ibn classification for the historical SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event with some resemblance to SN 1991bg.

astro-ph↗

arXiv2008

Infrared to X-ray observations of PKS 2155-304 in a low state

AIMS: Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy gamma-rays, through the investigation of the prototypical blazar PKS 2155-304, which shows complex behaviour. METHODS: We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. RESULTS: We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Gamma_2 ~ 2.4 harder than Gamma_1 ~ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour.

astro-ph↗

arXiv2008

Swift and Chandra Detections of Supernova 2006jc: Evidence for Interaction of the Supernova Shock with a Circumstellar Shell

The peculiar Type Ib supernova (SN) 2006jc has been observed with the UV/Optical Telescope (UVOT) and X-Ray Telescope (XRT) on board the Swift observatory over a period of 19 to 183 days after the explosion. Signatures of interaction of the outgoing SN shock with dense circumstellar material (CSM) are detected, such as strong X-ray emission (L_{0.2-10} > E39 erg/s) and the presence of MgII 2800A line emission visible in the UV spectra. In combination with a Chandra observation obtained on day 40 after the explosion, the X-ray light curve is constructed, which shows a unique rise of the X-ray emission by a factor of ~5 over a period of ~4 months, followed by a rapid decline. We interpret the unique X-ray and UV properties as a result of the SN shock interacting with a shell of material that was deposited by an outburst of the SN progenitor two years prior to the explosion. Our results are consistent with the explosion of a Wolf-Rayet star that underwent an episodic mass ejection qualitatively similar to those of luminous blue variable stars prior to its explosion. This led to the formation of a dense (>E7 cm**-3) shell at a distance of ~E16 cm from the site of the explosion, which expands with the WR wind at a velocity of (1300+/-300) km/s.

astro-ph↗

arXiv2007

Field Camera Unit for WSO-UV: Phase A Study Report

World Space Observatory UltraViolet (WSO-UV) is a multipurpose space observatory, made by a 170 cm aperture telescope, capable of UV high-resolution spectroscopy, long slit low-resolution spectroscopy, and deep UV and optical imaging. With a nominal mission life time of 5 years, and a planned extension to 10 years, from a geosynchronous orbit with an inclination of 51.8 degrees, WSO-UV will provide observations of exceptional importance for the study of many unsolved astrophysical problems. WSO-UV is implemented in the framework of a collaboration between Russia (chair), China, Germany, Italy, Spain, and Ukraine. This book illustrates the results of the feasibility study for the Field Camera Unit (FCU), a multi-spectral radial instrument on the focal plane of WSO-UV. The book provides an overview of the key science topics that are drivers to the participation of the Italian astronomical community in the WSO-UV project. The science drivers here illustrated have been used to define the technical requirements for the conceptual and architectural design of the Field Camera Unit (FCU) focal plane instrument. In Chapter I we show that WSO-UV will give a significant contribution to solve the key astronomical problems individuated by the ASTRONET consortium, and which are driving the European Space Agency Cosmic Vision program. Chapter II elucidates the scientific requirements for WSO-UV FCU instrument, discussed in Chapter I, which are translated in a list of verifiable top level requirements usable to make the conceptual design of the FCU instrument. Chapter III is dedicated to the Field Camera Unit opto-mechanical design, its detectors and electronics subsystems. Finally, Chapter IV outlines the AIV and GSE plans and activities for the FCU instrument.

astro-ph↗

arXiv2007

REM near-IR and optical multiband observations of PKS2155-304 in 2005

Spectral variability is the main tool for constraining emission models of BL Lac objects. By means of systematic observations of the BL Lac prototype PKS 2155-304 in the infrared-optical band, we explore variability on the scales of months, days and hours. We made our observations with the robotic 60 cm telescope REM located at La Silla, Chile. VRIJHK filters were used. PKS 2155-304 was observed from May to December 2005. The wavelength interval explored, the total number of photometric points and the short integration time render our photometry substantially superior to previous ones for this source. On the basis of the intensity and colour we distinguish three different states of the source, each of duration of months, which include all those described in the literature. In particular, we report the highest state ever detected in the H band. The source varied by a factor of 4 in this band, much more than in the V band (a factor ~2). The source softened with increasing intensity, contrary to the general pattern observed in the UV-X-ray bands. On five nights of November we had nearly continuous monitoring for 2-3 hours. A variability episode with a time scale of ~24 h is well documented, a much more rapid flare with t=1-2 h, is also apparent, but is supported by relatively few points.

astro-ph↗

arXiv2007

The Broad-lined Type Ic SN 2003jd

The results of a world-wide coordinated observational campaign on the broad-lined Type Ic SN 2003jd are presented. In total, 74 photometric data points and 26 spectra were collected using 11 different telescopes. SN 2003jd is one of the most luminous SN Ic ever observed. A comparison with other Type Ic supernovae (SNe Ic) confirms that SN 2003jd represents an intermediate case between broad-line events (2002ap, 2006aj), and highly energetic SNe (1997ef, 1998bw, 2003dh, 2003lw), with an ejected mass of M_{ej} = 3.0 +/- 1 Mo and a kinetic energy of E_{k}(tot) = 7_{-2}^{+3} 10^{51} erg. SN 2003jd is similar to SN 1998bw in terms of overall luminosity, but it is closer to SNe 2006aj and 2002ap in terms of light-curve shape and spectral evolution. The comparison with other SNe Ic, suggests that the V-band light curves of SNe Ic can be partially homogenized by introducing a time stretch factor. Finally, due to the similarity of SN 2003jd to the SN 2006aj/XRF 060218 event, we discuss the possible connection of SN 2003jd with a GRB.

astro-ph↗

arXiv2007

The afterglow onset for GRB060418 and GRB060607A

Gamma-ray burst are thought to be produced by highly relativistic outflows. Although upper and lower limits for the outflow initial Lorentz factor $Γ_0$ are available, observational efforts to derive a direct determination of $Γ_0$ have so far failed or provided ambiguous results. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $Γ_0$ which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburst material share a comparable amount of energy. We now comment early-time observations of the near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM robotic telescope. For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz, $Γ_0\sim 400$.

astro-ph↗

arXiv2007

On the July 2007 flare of the blazar 3C 454.3

In July 2007, the blazar 3C 454.3 underwent a flare in the optical, reaching R~13 on July 19. Then the optical flux decreased by one magnitude, being R~14 when the source was detected by the gamma-ray satellite AGILE, that observed the source on July 24-30. At the same time, the Swift satellite performed a series of snapshots. We can construct the simultaneous spectral energy distribution using optical, UV, X-ray and gamma-ray data. These shows that an increased gamma-ray flux is accompanied by a weaker optical/X-ray flux with respect to the flare observed in the Spring 2005 by INTEGRAL and Swift. This confirms earlier suggestions about the behaviour of the jet of 3C 454.3.

astro-ph↗

arXiv2007

Blazar microvariability at hard X-rays

Blazars are known to display strong and erratic variability at almost all the wavelengths of electromagnetic spectrum. Presently, variability studies at high-energies (hard X-rays, gamma-rays) are hampered by low sensitivity of the instruments. Nevertheless, the latest generation of satellites (INTEGRAL, Swift) have given suggestions not yet fully confirmed of variability on intraday timescales. Some specific cases recently observed are presented and physical implications are discussed (e.g. NRAO 530). The contribution that SIMBOL-X could give in this research topic is also outlined.

astro-ph↗

arXiv2007

Constraints on an optical afterglow and on supernova light following the short burst GRB 050813

We report early follow-up observations of the error box of the short burst 050813 using the telescopes at Calar Alto and at Observatorio Sierra Nevada (OSN), followed by deep VLT/FORS2 I-band observations obtained under very good seeing conditions 5.7 and 11.7 days after the event. Neither a fading afterglow, nor a rising SN component was found, so the potential GRB host galaxy has not been identified based on a comparison of the two VLT images taken at different epoches. We discuss if any of the galaxies present in the original 10 arcsec XRT error circle could be the host. In any case, the optical afterglow of GRB 050813 was of very low luminosity. We conclude that all these properties are consistent with the binary compact merger hypothesis for the progenitor of GRB 050813.

astro-ph↗

arXiv2007

REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination

Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Currently, only lower limits were securely set to the initial fireball Lorentz factor Gamma_0. We aim to provide a direct measure of Gamma_0. The early-time afterglow light curve carries information about Gamma_0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Gamma_0 ~ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be R_dec ~ 10^17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.

astro-ph↗