# Target of Opportunity Proposals in Period 94

This list of OPC approved Target of Opportunity Proposals (ToOs) is updated every observing period. The ESO ToO Policy is available from a separate document. Please note that when activating the observations, the ToO requests have to be fully documented, indicating the trigger of the requested observations. Observing time is only granted up to the indicated maximum of allocated time.

 094.D-0005 Greiner We propose multi-epoch LABOCA target of opportunity observations of two GRB afterglows, selected due to their steep GROND spectral energy distribution (SED), to measure the flux in the sub-mm. The 7-band ($grizJHK$) imager GROND at the ESO/MPI 2.2m telescope observes all GRBs visible from La Silla/Chile, and measures their 400-2400 nm SED at a time resolution between 4 min and 1 hr. Combined with the public X-ray afterglow data from Swift and our proprietary GROND and ATCA radio data, the multi-epoch SEDs from 5 GHz to 10 keV will allow us to test the theoretical predictions concerning the time-evolution of the SED in the fireball scenario. This allows us to determine the energetics of the burst, and the energy partition between electrons and the magnetic field. As long as ALMA does not offer ToOs on a timescale of a day, we propose APEX for two bright events to not miss any opportunity.

 094.A-0119 D'Elia The Inter-Stellar Medium (ISM) of distant galaxies and the intervening Inter-Galactic Medium (IGM) are difficult to study especially at high redshifts. Gamma-Ray Bursts (GRBs) provide a unique opportunity to disclose these environments. Multi-epoch high resolution spectroscopy will be used to map the host galaxy ISM and the IGM. Detailed information about gas geometry, kinematics and physical state, as well as metal abundances will be gathered studying ground and excited line transitions and, most importantly, their variations. This variability is crucial to {\bf constrain the distance between the GRB and the absorbing gas, which is a powerful tool to study the GRB/ISM interaction, provided that the ISM is finely resolved into velocity components}. We thus propose to use the unique ESO-RRM capabilities to perform VLT/UVES monitoring of bright (R$\ls16-19$) GRB afterglows, fully exploiting the UVES spectral resolution.

 094.A-0134 Fynbo The \textit{Swift} satellite offers a window of opportunity to exploit gamma-ray bursts (GRBs) to peer through the hearts of star forming galaxies through cosmic time. We here propose to secure the crucial ground-based spectroscopy of the GRB afterglows. We target all suitably observable GRBs (up to 15 bursts per semester). The goal of our program, started in GTO time in P84, is to produce a well-defined, homogeneous, statistically useful sample of GRBs. Currently, our sample consists of 72 spectra (with a redshift range from 0.059 to about 8), and about 15 robust metallicity measurements (over a redshift range from 1.7 to 5.9) and 3 secure detections of molecular absorption. Given the broad redshift range of GRBs in the sample ($0 < z < 8$), the sample is thus still limited by low-number statistics. Because redshifts are central to GRBs research and physics as a whole, and there is a very high community interest in the proposed observations, we forfeit any proprietary period.

 094.D-0144 Kann While recent years have seen large advances in the study of the early optical evolution of long gamma-ray bursts (GRBs), this time domain is still mostly unexplored for short GRBs. Their afterglows are rarely brighter than magnitude 20 even seconds after the GRB, which implies that robotic follow-up with small telescopes is not feasible. Here, we propose to observe one early short-GRB afterglow with VLT FORS2 in RRM mode, reaching deeper limits within minutes after the GRB than are possible with any other facility, even the 7-channel imager GROND on La Silla operated by this team. An optical detection in multiple colours will allow us to construct the spectral energy distribution at early times (allowing a direct comparison with X-ray data), derive a very precise afterglow localization to support a secure host identification, and study phenomena such as the initial Lorentz factor of the jet, possible short-term optical variability as well as any dust extinction.

 094.A-0168 Greiner The discovery of GRBs 080913 / 090423 / 120923A, for which we measured the redshifts (6.7/8.2/8.5) with VLT, has demonstrated the great potential of GRBs as probes of the early Universe, and the key role ESO can play in their exploitation. The enormous luminosity of long-duration GRBs means they are detectable in principle with present technology to $z$$\sim$20. They act as beacons for absorption studies of the IGM and their host galaxy ISM back to the era of reionization, and, being associated with the deaths of massive stars, trace global star formation to these early times, even in very faint galaxies. We will identify candidate $z>6$ bursts based on photometric observations (e.g., with MPG-LaSilla-2.2m/GROND, WHT, NOT, Gemini, AAT) prior to triggering deep optical/nIR spectroscopy on VLT. This program has only a moderate probability to be triggered in a given period, but aims to ensure that rare and precious ultra-high-$z$ bursts are not missed.

 094.B-0282 Feltzing Bulges were once considered to be uniformly old populations assembled in the earliest stages of galactic evolution. However, bulges can have multiple components, classical spherical bulges which formed from the initial spherical collapse phase and bulges and bars that form through secular instabilities in the disk. In the Milky Way the bar is a significant presence. The study of bulge dwarf stars, observed when they are considerably brightened by microlensing, is having a large impact on this field. Apart from detailed chemical compositions, dwarf stars at the turn-off also offer the unique opportunity to derive ages for {\it individual} stars. Our observations of micro-lensed dwarf stars show an old as well as a population with an age of about 3 Gyr in the bulge in contradiction with photometric studies which point to an exclusively old bulge population. We wish to obtain further spectra of micro-lensed dwarf stars to investigate the reality of this age-spread.

 094.C-0352 Sicardy We propose to observe stellar occultations by transneptunian objects (TNOs) and Centaurs, using ToO time on HAWKI \& SOFI. Potential candidates during P94 are: Pluto system, Chariklo, Eris, Sedna, Varuna, 2003AZ84, Orcus, Makemake, and Quaoar (other candidates may be available as we update our predictions). Goals are (1) monitor the \bf rings of Chariklo \rm (that we discovered in June 2013), (2) monitor putative \bf Pluto's tenuous atmosphere \rm with a sensitivity of a few nanobars, (3) measure \bf sizes and shapes of TNOs at kilometric accuracy\rm, to derive albedos, densities and rotational states. Occultations involve rapid flux variations and usually \bf red stars\rm, so that the fast photometry modes of HAWKI \& SOFI are ideally suited instruments for our project. Observations would be triggered a couple of weeks beforehand, as predictions are refined. The requested time (3h on HAWKI and 4.5h on SOFI) will allow us to observe up to 4 events during P94.

 094.D-0358 Bufano The first light from a supernova (SN) explosion comes when the shock wave reaches the surface of the progenitor star (shock breakout, SBO). The cooling that follows a SBO combined with the ejecta expansion creates a short-lived optical transient that could be seen for a few hrs, before the main SN rises, thus few detections have been possible. Our high cadence search, taking observations every 2 hrs over a period of 5 days using the DECam instrument, has enabled the possibility of routinely detecting extremely young SNe and constraining this phase for the first time. The constraints that such observations can put on properties such as progenitor radius, and the presence of a binary companion (SN type Ia), can have profound implications for our understanding of stellar evolution. Here, we request FORS2 low resolution spectroscopy to get some of the earliest SN spectra ever obtained (hard-ToO), and additional later spectra (soft-ToO) to further characterise these events.

 094.D-0407 Malesani We seek to locate and characterize, for the first time, optical and NIR counterparts of the so-called fast radio bursts'' (FRBs), mysterious radio transients with ms duration. Their radio dispersion measures'' point to an extragalactic origin ($z \sim 0.5$--1), but this is debated because of the lack of counterparts at shorter wavelengths. We propose to 1) look for the NIR counterparts of FRBs (with VIRCAM, unique for its wide field and sensitivity), and 2) if detected, obtain spectra and light curves of the counterparts. Nothing is known about FRB optical emission: SEDs, time variability, quiescent counterparts could provide the first clues to their nature. A redshift measurement is our top goal, providing the key information: distance and energetics. FRBs may open an entirely new, fruitful field of astrophysical research. If cosmological, FRBs would also allow a novel way to probe the intergalactic medium, and they may even be associated with gravitational-wave events.

 094.C-0422 Rojo The presence of condensable volatiles and a thick atmosphere make Titan unique among known extraterrestrial environments, providing a natural laboratory in which to study a hydrologic cycle on a body other than Earth. We propose to continue a monitoring campaign, started during ESO Period 93, that observes the short and long-term evolution of Titan's cloud systems with SINFONI. The combined analysis of morphology and cloud top altitude variation will constrain climate system models, diagnose cloud formation mechanisms, and supply context for interpreting surface features observed by Cassini. We will determine cloud locations to within $15^{\circ}$ latitude and $5$-$10$~km altitude. ToO triggers will allow measurements of wind speeds and short-term cloud evolution. The proposed campaign will provide the statistics required to investigate meteorological seasonal variation and, for the first time, observe cloud activity as Titan approaches northern summer solstice.

 094.D-0550 de Ugarte Postigo This proposal is aimed at \textbf{obtaining spectroscopy of an active magnetar} by using the unique properties of X-shooter. It comes at a moment when the rate of detections of these sources is greater than ever thanks to the wide suite of available high-energy satellites (\textit{Swift}, \textit{Fermi}, etc.). It is a short ToO proposal but with a very strong scientific potential if successful, as \textbf{there is still no optical/NIR spectrum of a magnetar in the literature}. Current models are not clear on the expectations of such observations and optical-nIR would be crucial for determining the powering mechanisms of magnetars. In the best case, time resolved spectroscopy would allow us to observe the evolution of the spectrum during flaring activity. The wide-wavelength-range coverage, intermediate resolution and high sensitivity makes \textbf{X-shooter the best suited instrument in the world for this project.}

 094.D-0652 Levan Gamma-ray bursts divide into two classes: the long-duration bursts which have been the subject of intensive study in recent years; and the rarer, fainter short-duration bursts (SGRBs) which have proved much more elusive. We have recently uncovered the first direct evidence for the origin of SGRBs in neutron star binary mergers, via an infrared kilonova detection. Here we propose to capitalise on this discovery, with the continuation of our long running SGRB programme at ESO. Using this we will a) measure redshifts for SGRBs, providing energies and space densities, directly impacting the expected rates of gravitational wave transient, and r-process element production b) track optical light curves, enabling the parameters of the explosion to be reconstructed and c) locate and track associated kilonovae, pinpointing the level of radioactive r-process production in neutron star mergers. Through this comprehensive study we will make the next, definitive steps in the study of SGRBs.

 094.D-0755 Amanullah We propose a continuation of our RRM/ToO X-shooter programme that will start in P93 to systematically obtain spectroscopic observations of all types of supernovae (SNe) within $24\,$h of explosion. This is achievable now thanks to the development of current SN surveys. The intermediate Palomar Transient Factory (iPTF) is regularly discovering these young SNe, with a rate of $\sim1$~per week. Prompt optical to near-IR spectroscopy of these transients will provide new insights on: (i) the progenitor scenario of SNe and the outer stellar envelope of massive stars, (ii) early interaction of the SN ejecta with circumstellar material, and (iii) shock-breakout science. The VLT offers both the ability of RRM/ToO and advanced instrumentation that is unique among 8~m-class telescopes.

 094.D-0775 Sternberg We propose a continuation of our project that was approved in 290.D-0510, 290.D-5023, 091.D-0352, 091.D-0780, 092.D- 0686, and 093.D-0675. The goal of the project is to uncover the nature of the elusive Type Ia supernova progenitors by studying time-variable features observed in high-resolution spectra of nearby Type Ia supernovae. These features are believed to be direct evidence of circumstellar material and can be used to test the different proposed Type Ia models. An additional new goal will be to observe very early targets. Early-time data can be used to validate the ionization-recombination explanation to the varying absorption features. Such observations may also reveal early-time interaction which has not been seen to date, possibly due to scarce early-time high-spectral-resolution data. Moreover, they can be used to probe the small scale structure of the SN host's ISM.