Adalberto Piccioni

FAST OPTICAL VARIABILITY OF A NAKED-EYE BURST—MANIFESTATION OF THE PERIODIC ACTIVITY OF AN INTERNAL ENGINE

We imaged the position of the naked-eye burst, GRB080319B, before, during, and after its gamma-ray activity with sub-second temporal resolution using the TORTORA wide-field camera. The burst optical prompt emission, which reached 5.3 mag, has been detected, and its periodic optical variability has been discovered in the form of four equidistant flashes with a duration of several seconds. We also detected a strong correlation (r ≈ 0.82) between optical and gamma-ray light curves with a 2 s delay of the optical emission with respect to the gamma-ray emission. The revealed temporal structure of the optical light curve in comparison with the gamma-ray light curve can be interpreted in the framework of the model of shell collisions in the ejecta containing a significant neutron component. All observed emission features reflect the non-stationary behavior of the burst internal engine—supposedly, a hyperaccreting solar-mass black hole formed in the collapse of a massive stellar core.

The X-ray transient XTE J1859 + 226 in outburst and quiescence

We present optical photometry and spectroscopy of the X-ray transient XTE J1859+226, obtained during outburst and its subsequent decay to quiescence. Both the X-ray and optical properties are very similar to those of well-studied black hole soft X-ray transients. We have detected three mini-outbursts, when XTE J1859+226 was approaching quiescence, as has been previously detected in the soft X-ray transients GRO J0422+32 and GRS 1009-45. By 2000 August 24 the system had reached quiescence with R = 22.48 ′ 0.07. The estimated distance to the source is ∼11 kpc. Photometry taken during quiescence shows a sinusoidal modulation with a peak to peak amplitude of about 0.4 mag. A period analysis suggests that periods from 0.28 to 0.47 d are equally possible at the 68 per cent confidence level. The amplitude of the quiescent light curve and the relatively low ratio of X-ray to optical flux indicate that the binary inclination should be high. The measured colours during the outburst allow us to obtain the basic properties of the disc, which agrees well with irradiated disc model predictions.

X{ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her ?

We report on ASCA and BeppoSAX X{ray broad band observations of the galactic low-luminosity X{ray source 4U 1700+24 performed on 1995 and 1998, respectively, and on (quasi-)simultaneous ground observations of its optical counterpart, V934 Her, from the Loiano 1.5-meter telescope. In order to better understand the nature of the source we also analyze public archival ROSAT and RXTE data as well as the RXTE ASM light curve of 4U 1700+24; we also re{analyze a 1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; this allows us to determine a revised distance to the object of400 pc. While these spectra do not show either any spectral change between the two epochs or any peculiar feature apart from those observed in normal red giants, the spectroscopic measurements carried out in X{rays reveal a complex and long-term variable spectrum, with a clear soft excess. The X{ray spectral properties of the source are best described by a thermal Comptonization spectrum plus a soft energy (<1 keV) excess, which can be modeled in the form of a blackbody emission with kTBB 1k eV; the latter component is not detected at the lowest source flux levels. The ratio between the two components varies substantially with the source flux. The X{ray emission from the object appears to become harder as its luminosity increases: indeed, the RXTE data acquired during an outburst occurred in October-November 1997 display a hard tail, detected up to 100 keV and modeled with a comptonizing cloud which is hotter and less opaque than that seen in the low intensity state. Apart from erratic shot-noise variability on timescales of tens to thousands of seconds, no signicant properties (such as pulsations or QPOs) are found from the timing analysis of the X{ray light curves extracted from the observations presented here. With the new distance determination, the 2{10 keV X{ray luminosity range spanned in the considered observations lies between210 32 and110 34 erg s 1 .A ll this information, combined with the ndings by other authors, allows us to suggest that the scenario which best describes the object consists of a wide binary system in which a neutron star accretes matter from the wind of a M-type giant star. Implications of such a model are discussed.

From TORTORA to MegaTORTORA—Results and Prospects of Search for Fast Optical Transients

To study short stochastic optical flares of different objects (GRBs, SNs, etc.) of unknown localizations as well as NEOs it is necessary to monitor large regions of sky with high-time resolution. We developed a system consisting of widefield camera with field of view of 400–600 sq.deg. which uses TV-CCD with 0.13 s temporal resolution to record and classify optical transients, and a fast robotic telescope aimed to perform their spectroscopic and photometric investigation just after detection. Such two-telescope complex, combining wide-field camera TORTORA and robotic telescope REM, operated from May 2006 at La Silla ESO observatory. Some results of its operation, including first high time resolution study of optical transient accompanying GRB and discovery of its fine time structure, are presented. Also, prospects for improving the efficiency of such observations are given, and a project of a next generation wide field monitoring system, the MegaTORTORA, is described.

Photometry and spectroscopy of GRB 060526: a detailed study of the afterglow and host galaxy of a z = 3.2 gamma-ray burst

Aims. With this paper we want to investigate the highly variable afterglow light curve and environment of gamma-ray burst (GRB) 060526 at z = 3: 221. Methods. We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the ultraviolet to the near infrared. The data set contains 4 12 data points in total to which we add additional data from the literature. Furthermore, we present low-resolution high signal-to-noise spectra of the afterglow. The afterglow light curve is modeled with both an analytical model using broken power law fits and with a broad-band numerical model wh ich includes energy injections. The absorption lines detec ted in the spectra are used to derive column densities using a multi-ion single-component curve-of-growth analysis from which we derive the metallicity of the host of GRB 060526. Results. The temporal behaviour of the afterglow follows a double broken power law with breaks at t = 0: 090�0: 005 and t = 2: 401�0: 061 days. It shows deviations from the smooth set of power laws that can be modeled by additional energy injections from the central engine, although some significant microvariability remains. The broadband spect ral-energy distribution of the afterglow shows no significa nt extinction along the line of sight. The metallicity derived from S II and Fe II of [S/H] = ‐0.57� 0.25 and [Fe/H] = ‐1.09� 0.24 is relatively high for a galaxy at that redshift but comparable to the metallicity of other GRB hosts at similar redshifts. At the position of the afterglow, no host is detect ed to F775W(AB)= 28.5 mag with the HST, implying an absolute magnitude of the host M(1500 A)>‐18.3 mag which is fainter than most long-duration hosts, although the GRB may be associated with a faint galaxy at a distance of 11 kpc.

TORTORA discovery of Naked‐Eye Burst fast optical variability

Features characterizing gamma‐ray bursts in the different spectral bands may be a clue for the nature of their inner engine. Up to now, only several bursts have been observed in optical band during the gamma activity, and the only one—GRB080319B—was covered from rise till fall with high temporal resolution. Here we discuss these data, acquired with TORTORA fast wide‐field monitoring optical camera, as well as results of its analysis.The camera observed the position of Naked‐Eye Burst, GRB080318B, before, during and after the trigger. It detected the fast rise of optical emission, which reached the peak of V 5.3 at the eighteenth second, had a complex evolution till T+43s and monotonously faded then. The brightest part of the light curve contains two 15–20 s segments with different fluxes, each having two clearly‐seen peaks of 5–8 s duration; all four peaks look quasi‐periodic with separation of 9 s. There is no clear evidence of any sub‐second variability. However, there are signs of quasi‐periodic variab...

Ultraviolet and Optical Observations of HDE 245770/ A 0535+26 System during an X-Ray Flare

We present high and low dispersion UV spectra of HDE 245770,which is the optical counterpart of the recurrent transient X-ray pulsar A 0535+26,during a decay of an X-ray flare. UBV photometric measurements and medium dispersion optical spectra were simultaneously obtained.

Wide and Fast: Monitoring the Sky in Subsecond Domain with the FAVOR and TORTORA Cameras

In order to detect and investigate short stochastic optical flares from a number of variable astrophysical objects (GRBs, SNe, flare stars, CVs, X-Ray binaries) of unknown localizations as well as near-earth objects (NEOs), both natural and artificial, it is necessary to perform the systematic monitoring of large regions of the sky with high temporal resolution. Here we discuss the criteria for a system that is able to perform such a task and describe two cameras we created for wide-field monitoring with high temporal resolution—FAVOR and TORTORA. Also, we describe basic principles of real-time data processing for the high frame rates needed to achieve subsecond temporal resolution on a typical hardware.

Did GRB970228 and GRB970508 present similar optical properties

The analysis of the light curves of the optical counterparts of GRB970228 and GRB970508 points out remarkable similarities. The spectral distribution obtained from the color indices shows that both the transients became bluer during the increasing stage, and redder after the maximum. A main difference concerns the behaviour of the optical fading which is well fitted by a single power law in the case of GRB970508 but not in the case of GRB970228.

The optical light curve of GRB 970228 refined

Abstract We present the R and V light curves of the optical counterpart of GRB 970228. A critical analysis of all the available data is made in light of the results achieved in the recent GRB Symposium held in Huntsville and by considering the latest information from the HST images on the underlying nebulosity.