J. J. González

General formulation for the calibration and characterization of narrow-gap etalons: the OSIRIS/GTC tunable filters case

Tunable filters (TFs) are a powerful way of implementing narrow-band imaging mode over wide wavelength ranges, without the need of purchasing a large number of narrow-band filters covering all strong emission or absorption lines at any redshift. However, one of its main features is a wavelength variation across the field of view, sometimes termed the phase effect. Inthiswork,ananomalousphaseeffectisreportedandcharacterizedfortheOpticalSystemfor Imaging and low Resolution Integrated Spectroscopy (OSIRIS) instrument at the 10.4 m Gran Telescopio Canarias. The transmitted wavelength across the field of view of the instrument depends, not only on the distance to the optical centre, but on wavelength. This effect is calibrated for the red TF of OSIRIS by measuring both normal-incidence light at laboratory and spectral lamps at the telescope at non-normal incidence. This effect can be explained by taking into account the inner coatings of the etalon. In a high spectral resolution etalon, the gap between plates is much larger than the thickness of the inner reflective coatings. In the case of a TF, like that in OSIRIS, the coatings thickness could be of the order of the cavity, which changes drastically the effective gap of the etalon. We show that by including thick and dispersive coatings into the interference equations, the observed anomalous phase effect can be perfectly reproduced. In fact, we find that, for the OSIRIS red TF, a two-coatings model fits the data with an rms of 0.5 A at all wavelengths and incidence angles. This is a general physical model that can be applied to other TF instruments.

A detailed study of the optical attenuation of gamma-ray bursts in the Swift era

We present optical and near-infrared (NIR) photometry of 24 gamma-ray bursts (GRBs) detected by the Swift satellite and rapidly observed by the Reionization and Transients Infrared/Optical (RATIR) camera. We compare the optical flux at a fiducial time of 11 hours after the high-energy trigger to that in the X-ray regime to quantify optical darkness. 50 per cent (12/24) of all bursts in our sample and 55 per cent (12/22) of long GRBs are optically dark, which is statistically consistently with previous studies. Fitting RATIR optical and NIR spectral energy distributions (SEDs) of 16 GRBs, most (6/7) optically dark GRBs either occur at high-redshift (z > 4.5) or have a high dust content in their host galaxies (AV > 0.3). Performing K-S tests, we compare the RATIR sample to those previously presented in the literature, finding our distributions of redshift, optical darkness, host dust extinction and X-ray derived column density to be consistent. The one reported discrepancy is with host galaxy dust content in the BAT6 sample, which appears inconsistent with our sample and other previous literature. Comparing X-ray derived host galaxy hydrogen column densities to host galaxy dust extinction, we find that GRBs tend to occur in host galaxies with a higher metal-to-dust ratio than our own Galaxy, more akin to the Large and Small Magellanic Clouds. Finally, to mitigate time evolution of optical darkness, we measure βOX,rest at a fixed rest frame time, trest = 1.5 hours and fixed rest frame energies in the X-ray and optical regimes. Choosing to evaluate optical flux at λrest = 0.25 µm, we remove high-redshift as a source of optical darkness, demonstrating that optical darkness must result from either high-redshift, dust content in the host galaxy along the GRB sight line, or a combination of the two.

OTELO SURVEY: DEEP BVRI BROADBAND PHOTOMETRY OF THE GROTH STRIP. II. OPTICAL PROPERTIES OF X-RAY EMITTERS

The Groth field is one of the sky regions that will be targeted by the OSIRIS Tunable Filter Emission Line Object survey in the optical 820 nm and 920 nm atmospheric windows. In the present paper, public Chandra X-ray data with total exposure time of 200 ks are analyzed and combined with optical broadband data of the Groth field, in order to study a set of optical structural parameters of the X-ray emitters and its relation with X-ray properties. To this aim, we processed the raw, public X-ray data using the Chandra Interactive Analysis of Observations, and determined and analyzed different structural parameters, in order to produce a morphological classification of X-ray sources. We present the morphology of 340 X-ray emitters with optical counterpart detected. Objects have been classified by X-ray type using a diagnostic diagram relating X-ray-to-optical ratio (X/O) to hardness ratio. We did not find any clear correlation between X-ray and morphological types. We analyzed the angular clustering of X-ray sources with optical counterpart using two-point correlation functions. A significant positive angular clustering was obtained from a preliminary analysis of four subsamples of the X-ray sources catalog. The clustering signal of the optically extended counterparts is similar to that of strongly clustered populations like red and very red galaxies, suggesting that the environment plays an important role in active galactic nuclei phenomena. Finally, we combined optical structural parameters with other X-ray and optical properties, and we confirmed an anticorrelation between the X/O ratio and the Abraham concentration index, which might suggest that early-type galaxies have lower Eddington rates than those of late-type galaxies.

Studying the evolution of galaxies in compact groups over the past 3 Gyr – II. The importance of environment in the suppression of star formation

We present an in depth study on the evolution of galaxy properties in compact groups over the past 3 Gyr. We are using the largest multiwavelength sample to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift range of 0.01 < z < 0.23. To derive the physical properties of the galaxies, we rely on ultraviolet (UV)-to-infrared spectral energy distribution modelling, using cigale. Our results suggest that during the 3 Gyr period covered by our sample, the star formation activity of galaxies in our groups has been substantially reduced (3 to 10 times). Moreover, their star formation histories as well as their UV-optical and mid-infrared colours are significantly different from those of field and cluster galaxies, indicating that compact group galaxies spend more time transitioning through the green valley. The morphological transformation from late-type spirals to early-type galaxies occurs in the mid-infrared transition zone rather than in the UV-optical green valley. We find evidence of shocks in the emission line ratios and gas velocity dispersions of the late-type galaxies located below the star forming main sequence. Our results suggest that in addition to gas stripping, turbulence and shocks might play an important role in suppressing the star formation in compact group galaxies.

The central engine of GRB 130831A and the energy breakdown of a relativistic explosion

Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe, yet the nature and physical properties of their energy sources are far from understood. Very important clues, however, can be inferred by studying the afterglows of these events. We present optical and X-ray observations of GRB 130831A obtained by Swift, Chandra, Skynet, Reionization And Transients Infra-Red camera, Maidanak, International Scientific Optical-Observation Network, Nordic Optical Telescope, Liverpool Telescope and Gran Telescopio Canarias. This burst shows a steep drop in the X-ray light curve at asymptotically equal to 10(exp 5) s after the trigger, with a power-law decay index of alpha that is approximately 6. Such a rare behaviour cannot be explained by the standard forward shock (FS) model and indicates that the emission, up to the fast decay at 10(exp 5) s, must be of internal origin, produced by a dissipation process within an ultrarelativistic outflow. We propose that the source of such an outflow, which must produce the X-ray flux for an asymptotically equal to 1 d in the cosmological rest frame, is a newly born magnetar or black hole. After the drop, the faint X-ray afterglow continues with a much shallower decay. The optical emission, on the other hand, shows no break across the X-ray steep decrease, and the late-time decays of both the X-ray and optical are consistent. Using both the X-ray and optical data, we show that the emission after an asymptotically equal to 10(exp 5) scan be explained well by the FS model. We model our data to derive the kinetic energy of the ejecta and thus measure the efficiency of the central engine of a GRB with emission of internal origin visible for a long time. Furthermore, we break down the energy budget of this GRB into the prompt emission, the late internal dissipation, the kinetic energy of the relativistic ejecta,and compare it with the energy of the associated supernova, SN 2013 fu.

Happy Birthday Swift: Ultra-long GRB 141121a and its Broadband Afterglow

We present our extensive observational campaign on the Swift-discovered GRB 141121A, almost 10 years after its launch. Our observations cover radio through X-rays and extend for more than 30 days after discovery. The prompt phase of GRB 141121A lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is E_(γ,iso) = 8.0 × 10^(52) erg. Due to the long prompt duration, GRB 141121A falls into the recently discovered class of ultra-long GRBs (UL-GRBs). Peculiar features of this burst are (1) a flat early-time optical light curve and (2) a radio-to-X-ray rebrightening around three days after the burst. The latter is followed by a steep optical-to-X-ray decay and a much shallower radio fading. We analyze GRB 141121A in the context of the standard forward–reverse shock (FS, RS) scenario and we disentangle the FS and RS contributions. Finally, we comment on the puzzling early-time (t ≾ 3 days) behavior of GRB 141121A, and suggest that its interpretation may require a two-component jet model. Overall, our analysis confirms that the class of UL-GRBs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful gamma-ray burst explosions, and future missions (like SVOM, XTiDE, or ISS-Lobster) will provide many more of such objects.

OTELO Survey: Optimal Emission-Line Flux Determination with OSIRIS/GTC

Emission-line galaxies are important targets for understanding the chemical evolution of galaxies in the universe. Deep, narrowband imaging surveys allow detection and study of the flux and the equivalent widths (EWs) of the emission line studied. The present work has been developed within the context of the OTELO project, an emission-line survey using the tunable filters (TF) of OSIRIS, the first-generation instrument on the Gran Telescopio Canarias (GTC) 10.4 m telescope located in La Palma, Spain, that will observe through selected atmospheric windows that are relatively free of sky emission lines. With a total survey area of 0.1 deg2 distributed in different fields, reaching a 5σ depth of 10-18 ergs cm-2 s-1 and detecting objects of EW < 0.3 A, OTELO will be the deepest emission-line survey to date. As part of the OTELO preparatory activities, the objective of this study is to determine the best combination of sampling and full width at half-maximum (FWHM) for the OSIRIS tunable filters for deblending Hα from [N II] lines by analyzing the flux errors obtained. We simulated the OTELO data by convolving a complete set of synthetic H II galaxies in EWs, with different widths of the OSIRIS TFs. We estimated relative flux errors of the recovered Hα and [N II] λ6583 lines. We found that for the red TF, a FWHM of 12 A and a sampling of 5 A is an optimal combination that allows deblending Hα from the [N II] λ6583 line with a flux error lower than 20%. This combination will allow estimating SFRs and metallicities using the Hα flux and the N2 method, respectively.

Identifying high-redshift gamma-ray bursts with RATIR

We present a template fitting algorithm for determining photometric redshifts, zphot, of candidate high-redshift gamma-ray bursts (GRBs). Using afterglow photometry, obtained by the Reionization And Transients InfraRed (RATIR) camera, this algorithm accounts for the intrinsic GRB afterglow spectrum, host dust extinction and the effect of neutral hydrogen (local and cosmological) along the line of sight. We present the results obtained by this algorithm and RATIR photometry of GRB 130606A, finding zphot = 6.01 +0.16 −0.08 for a model with negligible host dust extinction, consistent with spectroscopic measurements of the redshift of this GRB. Using simulated RATIR photometry, we find our algorithm provides precise measures of zphot in the ranges 4 4. Further testing highlights the required caution in cases of highly dust extincted host galaxies. These tests also show that our algorithm minimizes false negatives thus allowing us to rapidly identify all potential high-redshift events.

The OTELO survey. I. Presentation, data reduction, and multi-wavelength catalogue

The evolution of galaxies through cosmic time is studied observationally by means of extragalactic surveys. The OTELO survey aims to provide the deepest narrow-band survey to date in terms of minimum detectable flux and emission line equivalent width in order to detect the faintest extragalactic emission line systems. In this way, OTELO data will complements other broad-band, narrow-band, and spectroscopic surveys. The red tunable filter of the OSIRIS instrument on the 10.4 m Gran Telescopio Canarias (GTC) is used to scan a spectral window centred at

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