This week on HST

HST Programs: April 14 - April 20, 2014

Selected highlights

GO 13230: A supernova in the brightest gamma-ray burst

HST images of GRB-11121, linked with SN2001ke

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected since their discovery in the late 1960s (by military satellites - there were some initial worries that they might be of sinister intent). Analyses of their properties suggest that GRBs can be divided into two basic caregories: short, hard bursts, that are likely due to component mergers in neutron star or black hole binaries; and long, soft bursts, which originate in the collapse of very massive stars. Over the last few years, it has become clear that a subset of GRBs are associated with a particular class of supernova, type IC SNe, sometimes termed hypernovae. The first clear example of this type of object was probably GRB 030329 (identified in 2003); an optical counterpart to the GRB was identified by ground-based telescopes within ~90 minutes, and monitoring over the next few days by the ESO Very Large Telescope showed the gradual energence of a supernovae spectrum, as the GRB afterglow faded. Subsequently, a number of other examples have been discovered, although all have fairly low energies for GRBs. A lingering question is whether the supernova mechanism that is now generally accepted for those sources is a reasonable explanation for much higher energy events. the present program aims to tackle that question through HST observations of GRB 130427A, the brightest burst so far observed by Swift. The host galaxy lies at a relatively low redshift, z=0.34, allowing clear separation of the supernova and the afterglow through ACS and WFC3 observations, and a characterisation of the underlying stellar population.

GO 13282: A Search for Surviving Companions of Type Ia Supernovae in the Large Magellanic Cloud

Combined HST and Chnadra imaging of the supernova remnant SNR 0509-67.5 in the LMC

Type Ia supernovae are generally believed to be produced by the explosive deflagration of white dwarf star that exceeds the Chandrasekhar due to accretion from a binary companion, either a hydrogen-burning main-sequence/red giant star or another degenerate. In double degenerate systems, the white dwarfs merge and explode; in the single degenerate mode, the non-degenerate companion is likely to survive, albeit in a stripped, ablated form. Besides providing crucial information on stellar evolution and how stars enrich the interstellar medium, Type Ia supernovae have acquired global importance in recent years through their use as distance indicators. Indeed, these objects played a crucial role in identifying dark energy and the accelerating universe. In that context, it is important to obtain a better udnerstanding of the underlying mechanism and the distribution of intrinsic properties of these exploding stars.Relatively nearby supernovae that can be probed in detail are therefore crucial to the large mapping of the cosmic flow. The present program aims to probe whether the double degenerate or single degenerate mode of formation is more important by using HST imaging to search for survivors in a sample of nine supernova remnants in the LMC. If the double degenerate mechanism is the dominant mechanism, then few survivors of the progenitor explosion will be found.

GO 13330: Mapping the AGN Broad Line Region by Reverberation

Simulations of the appearance and velocity structure within an AGN disk (see Keith Horne's web page ).

Active galaxies (AGNs) are generally luminous systems, characterised by the presence of strong nuclear emission lines of numerous species including H, He I, He II, and Fe, Ca, O, C and S over a range of ionisations. These features originate from gas clouds in the nuclear regions, with the energy supplied through accretion onto a central massive black hole. The high-temperature, rapidly-rotating gas clouds nearest the central engine are responsible for producing broad emission lines (hence, the "Broad Line Region"). The structure of the BLR can be discerned using a technique known as reverberation mapping: variations in the accretion rate lead to fluctuations in luminosity; those variations lead, in turn, to variations in the photoionisation of the BLR, and corresponding changes in spectral line strengths and velocities; monitoring those changes, and correlating them with the photometric variability of the central source, measures the light travel time from nucleus to BLR gas, and hence maps the size of the BLR. The present prorgam will use the Cosmic Origins Spectrograph to undertake systematic monitoring of the nuclear regions of the Seyfert I galaxy, NGC 5548. The observations are spread over the next 6 months, with one orbit per day for 179 days.

GO 13503: Searching for Satellites of Ceres: Support for the Dawn Mission

HST images of the dwarf planet, Ceres

Ceres is one of the largest members of the main belt asteroids. Indeed, Ceres was the first such object discovered, by Giuseppe Piazzi in the coruse of his 1801 survey for a new planet that might fill out the Titius-Bode numerical sequence. Roughly spherical, with a diameter of 950 km, Ceres has been studied extensively from both ground and space. It is also targeted for study by NASA's DAWN mission, launched on September 27 2007. Dawn previously encountered the asteroid Vesta in August 2011, mapping the surface from a low altitude orbit which it maintained through September 5 2012. At that point the spacecraft used its xenon-ion engine to leave orbit and head for Ceres, where it is expected to arrive in March or April 2015. Dawn's observations of Vesta reveal a highly cratered surface, together with dark features that are believed to have originated in past impact events. Ceres is currently at opposition, and the present program aims to use deep WFC3 imaging to search for very low mass companions ("moons"), whose presence would be of significance in planning the details of the DAWN's final encounter.