Showing papers by "Royal Institute and Observatory of the Spanish Navy published in 2003"

Very High Column Density and Small Reddening toward GRB 020124 at z = 3.20*

Abstract: We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hr after the gamma-ray burst. The burst occurred in a very faint (R 29.5) damped Lyα absorber (DLA) at a redshift of z = 3.198 ± 0.004. The derived column density of neutral hydrogen is log(N) = 21.7 ± 0.2, and the rest-frame reddening is constrained to be E(B-V) < 0.065, i.e., AV < 0.20 for standard extinction laws with RV ≈ 3. The resulting dust-to-gas ratio is less than 11% of that found in the Milky Way but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. A gray extinction law (large RV), produced through preferential destruction of small dust grains by the gamma-ray burst, could increase the derived AV and dust-to-gas ratio. The dimness of the afterglow is, however, fully accounted for by the high redshift: if GRB 020124 had been at z = 1, it would have been approximately 1.8 mag brighter—in the range of typical bright afterglows.

Optical and near-infrared observations of the GRB020405 afterglow ?

Abstract: We report on photometric, spectroscopic and polarimetric monitoring of the optical and near-infrared (NIR) afterglow of GRB020405. Ground-based optical observations, performed with 8 different telescopes, started about 1 day after the high-energy prompt event and spanned a period of ∼10 days; the addition of archival HST data extended the coverage up to ∼150 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of Balmer and oxygen emission lines in the optical spectrum of the host galaxy indicates that the GRB is located at redshift z = 0.691. Fe II and Mg II absorption systems are detected at z = 0.691 and at z = 0.472 in the afterglow optical spectrum. The latter system is likely caused by absorbing clouds in the galaxy complex located ∼2" southwest of the GRB020405 host. Hence, for the first time, the galaxy responsible for an intervening absorption line system in the spectrum of a GRB afterglow is spectroscopically identified. Optical and NIR photometry of the afterglow indicates that, between 1 and 10 days after the GRB, the decay in all bands is consistent with a single power law of index a = 1.54 ′0.06. The late-epoch VLT J-band and HST optical points lie above the extrapolation of this power law, so that a plateau (or "bump") is apparent in the VRIJ light curves at 10-20 days after the GRB. The light curves at epochs later than day ∼20 after the GRB are consistent with a power-law decay with index α' = 1.85 ′ 0.15. While other authors have proposed to reproduce the bump with the template of the supernova (SN) 1998bw, considered the prototypical "hypernova", we suggest that it can also be modeled with a SN having the same temporal profile as the other proposed hypernova SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A single polarimetric R-band measurement shows that the afterglow is polarized, with P = 1.5 ′ 0.4% and polarization angle 0 = 172° ′8°. Broad-band optical-NIR spectral flux distributions show, in the first days after the GRB, a change of slope across the J band which we interpret as due to the presence of the electron cooling frequency v c . The analysis of the multiwavelength spectrum within the standard fireball model suggests that a population of relativistic electrons with index p ∼ 2.7 produces the optical-NIR emission via synchrotron radiation in an adiabatically expanding blastwave, with negligible host galaxy extinction, and the X-rays via Inverse Compton scattering off lower-frequency afterglow photons.

Optical Photometry of GRB 021004: The First Month

Abstract: We present UBVRCIC photometry of the optical afterglow of the gamma-ray burst GRB 021004 taken at the Nordic Optical Telescope between approximately 8 hours and 30 days after the burst. These data are combined with an analysis of the 87 ks Chandra X-ray observations of GRB 021004 taken at a mean epoch of 33 hr after the burst to investigate the nature of this GRB. We find an intrinsic spectral slope at optical wavelengths of ?UH = 0.39 ? 0.12 and an X-ray slope of ?X = 0.94 ? 0.03. There is no evidence for color evolution between 8.5 hr and 5.5 days after the burst. The optical decay becomes steeper approximately 5 days after the burst. This appears to be a gradual break due to the onset of sideways expansion in a collimated outflow. Our data suggest that the extragalactic extinction along the line of sight to the burst is between AV ? 0.3 and 0.5 and has an extinction law similar to that of the Small Magellanic Cloud. The optical and X-ray data are consistent with a relativistic fireball with the shocked electrons being in the slow cooling regime and having an electron index of p = 1.9 ? 0.1. The burst occurred in an ambient medium that is homogeneous on scales larger than approximately 1018 cm but inhomogeneous on smaller scales. The mean particle density is similar to what is seen for other bursts (0.1 cm-3 n 100 cm-3). Our results support the idea that the brightening seen at approximately 0.1 days was due to interaction with a clumpy ambient medium within 1017?1018 cm of the progenitor. The agreement between the predicted optical decay and that observed approximately 10 minutes after the burst suggests that the physical mechanism controlling the observed flux at t ? 10 minutes is the same as the one operating at t > 0.5 days.

A multi-colour study of the dark GRB 000210 host galaxy and its environment

Abstract: We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark gamma-ray burst (GRB) of February 10, 2000. These observations represent the most exhaustive photometry given to date of any GRB host galaxy. A grid of spectral templates have been fitted to the Spectral Energy Distribution (SED) of the host. The derived photometric redshift is z = 0.842 +0.054 −0.042 , which is in excellent agreement with the spectroscopic redshift (z = 0.8463 ± 0.0002) proposed by Piro et al. (2002) based on a single emission line. Furthermore, we have determined the photometric redshift of all the galaxies in an area of 6 ' × 6 ' around the host galaxy, in order to check for their overdensity in the environment of the host. We find that the GRB 000210 host galaxy is a subluminous galaxy (L � 0.5 ± 0.2L ⋆ ), with no companions above our detection threshold of 0.18 ± 0.06L ⋆ . Based on the restframe ultraviolet flux a star formation rat e of 2.1 ± 0.2M⊙ yr −1 is estimated. The best fit to the SED is obtained for a starburst template with an age of 0.181 +0.037 −0.026 Gyr and a very low extinction (AV � 0). We discuss the implications of the inferred low value of AV and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow.

GRB 011121: A Collimated Outflow into Wind-blown Surroundings

Abstract: We report optical and near-infrared follow-up observations of GRB 011121 collected predominantly at ESO telescopes in Chile. We discover a break in the afterglow light curve after 1.3 days, which implies an initial jet opening angle of about 9 � . The jet origin of this break is supported by the fact that the spectral energy distribution is achromatic during the first 4 days. During later phases, GRB 011121 shows significant excess emission above the flux predicted by a power law, which we interpret as additional light from an underlying supernova. In particular, the spectral energy distribution of the optical transient approximately 2 weeks after the burst is clearly not of power-law type but can be presented by a blackbody with a temperature of � 6000 K. The deduced parameters for the decay slope and the spectral index favor a wind scenario, i.e., an outflow into a circumburst environment shaped by the stellar wind of a massive gamma-ray burst (GRB) progenitor. Because of its low redshift of z ¼ 0:36, GRB 011121 has been the best example for the GRB-supernova connection until GRB 030329 and provides compelling evidence for a circumburster wind region expected to exist if the progenitor was a massive star. Subject headings: gamma rays: bursts — supernovae: general — techniques: photometric On-line material: color figures

Frozen Orbits About the Moon

Abstract: Frozen orbits are of special interest to mission designers of artificial satellites. On average the eccentricity and argument of the perigee of such orbits remain stationary. Frozen orbits correspond to equilibria in an averaged form of the zonal problem and are almost periodic solutions of the full (nonaveraged) problem. In the zonal problem of a satellite around the moon, we numerically continue natural families of periodic orbits with the polar component of the angular momentum as the parameter. Three families of frozen orbits are discovered.

Variable polarization in the optical afterglow of GRB 021004

Abstract: We present polarimetric observations of the afterglow of gamma-ray burst (GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among the observations shows a 45 degree change in the position angle from 9 hours after the burst to 16 hours after the burst, and comparison with published data from later epochs even shows a 90 degree change between 9 and 89 hours after the burst. The degree of linear polarization shows a marginal change, but is also consistent with being constant in time. In the context of currently available models for changes in the polarization of GRBs, a homogeneous jet with an early break time of tb 1d ay provides a good explanation of our data. The break time is a factor 2 to 6 earlier than has been found from the analysis of the optical light curve. The change in the position angle of the polarization rules out a structured jet model for the GRB.

The blue host galaxy of the red GRB 000418

Abstract: We report on multi-band ( UB VRIZJ sKs) observations of the host galaxy of the April 18, 2000 gamma-ray burst. The Spectral Energy Distribution (SED) is analysed by fitting empirical and synthetic spectral templates. We find that: (i) the best SED fit is obtained with a starburst template, (ii) the photometric redshift is consistent with the spectroscopic redshift, (iii) the colours of the host are inconsistent with an old stellar population, and (iv) the global extinction is constrained to be in the range AV = 0:12-0:61 mag. The derived global extinction agrees with the one reported for the afterglow (AV = 0:4-0:9 mag), suggesting a homogeneous distribution of the interstellar medium (ISM) in the host galaxy. These findings are supplemented by morphological information from Hubble Space Telescope (HST) imaging: the surface brightness profile is smooth, symmetric and compact with no underlying structures (like dust lanes, spiral arms or disks). A natural scenario which accounts of all the above results is a nuclear starburst that harbours a young population of stars from which the GRB originated.

Non-linear filter, using the wavelet transform, applied to seismological records

Abstract: As any process in Nature, seismic recordsare affected by noise that the analystwould want to eliminate. One of the mostcommon techniques used to minimise thisnoise effect is the application of linearfilters, which reduce the bandwidth of thesignal. This method is based on the FourierTransform, and therefore any perturbationon the coefficients affects the entirerecord.We have developed a non-linear filter basedon the multiresolution analysis of theDiscrete Time Wavelet Transform (DTWT). Themain idea is to use the time-frequencylocalisation properties of the waveletdecomposition. Each coefficient isassociated to a window on thetime-frequency plane, so any perturbationwould only affect the time and frequencyrange of the correspondent window.The procedure we propose has three stages:periodic noise elimination, spikesreduction and, finally, the non-linearfiltering. The non-linear filter acts bythresholding the wavelet coefficients. Thethresholding estimator will depend on thesignal-noise ratio (SNR) in each of thefrequency bands associated to the waveletdecomposition.We have compared the proposed method to thecoherent structures method (Mallat, 1998)and to two 4th order linear filterbanks (Butterworth and Elliptic filters),applying all of them to a syntheticdatabase, and a real earthquake databaserecorded by the Short Period ROA Network.The proposed method improves the SNR in the87% of the tested events, being therelative rms error less than three, and themaximum amplitude relative error less than10% in the 90% of the synthetic database.

A magnetic anomaly study offshore the Canary Archipelago

Abstract: A scalar magnetic anomaly map of the regions offshore the Canary Archipelago is presented here. This map is based on measurements taken inside the Exclusive Economic Zone Project framework. This paper contains a description of the data set, whose accuracy and internal consistency are analysed. The magnetic anomalies are described and the main structural trends are highlighted. This analysis has served to detect two possible fractures with a NW—SE component: one separates Fuerteventura from Gran Canaria Island, and the other apparently runs through Fuerteventura. The latter finding agrees with a 1.7 km depth offset, between its northern and southern halves, detected by the algorithm based on the Euler Deconvolution. A similar approach has been performed, estimating the depth to the top of the most significant anomalies. This result shows that the most noteworthy magnetic anomaly source of the archipelago, which lies between Tenerife and Gran Canaria Island, seems to be located at an average depth of 4 km below sealevel. A spectral analysis was performed to estimate the depth extent of the deepest anomalies. It argues the presence of sub-crustal magnetic sources (underplating) in the archipelago. A standard Euler Deconvolution analysis was executed to analyse the spatial distribution of these mantle-like sources. Our results seem to support the existence of magmatic underplating under Gran Canaria, Tenerife, and Fuerteventura, and suggest this possibility for La Palma.

The BOOTES experiment in support of the Gran Telescopio Canarias (GTC) in the study of the high energy Universe

Abstract: A GTC target of opportunity program (ToO) for gamma ray burst (GRB) afterglows will be a very powerful tool for studying the primitive Universe. Photometry and spectroscopy of GRBs are very suitable for tracing the star formation rate of the Universe at high redshifts (prompt optical and near-infrared observations with the GTC could detect GRBs to z < 17; Castro-Tirado et al., this volume, p. 252; Gorosabel et al., this volume, p. 288). BOOTES (Burst Observer and Optical Transient Exploring System) can very ecien tly support a ToO at the GTC through its robotized, extremely fast response capabilities. In this paper, we discuss the ongoing eort to nalize a real time, online, automatic data analysis system for BOOTES. Such a system will be able to provide early, reliable, and accurate GRB positions (conrmed by satellite) to trigger a GTC ToO.

Last Results in the GRB Field

Abstract: Much has been advanced in the GRB field in the last 5 years. The central engines that power these extraordinary events are thought to be the collapse of massive stars rather than the merging of compact objects as previously also suggested. Short GRBs still remain a mystery as no counterparts have been detected so far.

Variable polarization in the optical afterglow of GRB 021004

Abstract: We present polarimetric observations of the afterglow of gamma-ray burst (GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among the observations shows a 45 degree change in the position angle from 9 hours after the burst to 16 hours after the burst, and comparison with published data from later epochs even shows a 90 degree change between 9 and 89 hours after the burst. The degree of linear polarization shows a marginal change, but is also consistent with being constant in time. In the context of currently available models for changes in the polarization of GRBs, a homogeneous jet with an early break time of t_b ~ 1 day provides a good explanation of our data. The break time is a factor 2 to 6 earlier than has been found from the analysis of the optical light curve. The change in the position angle of the polarization rules out a structured jet model for the GRB.

Observations with the ROA CCD Transit Circle in Argentina

Abstract: The Real Instituto y Observatorio de la Armada (ROA) meridian circle was built in 1948 by Grubb Parsons. It has an aperture of 18 cm and a focal length of 266 cm. In 1996 June was moved to the Estacion de Altura Carlos Ulrrico Cesco in the Republica Argentina in 1996. Until november 1999 the observations were carry out with a moving slit micrometer. In spring 2001 the result of these observations has been published, forming the first Hispano-Argentinian Meridian Catalogue (HAMC). In december 1999 was installed a SpectraSource CCD camera of 1552 × 1024 pixels of 9 µm. The CCD camera observe in drift scan mode. A survey of the south hemisphere is being observed from +3° to −60° of declination.