Recent near-infrared power-spectra and panchromatic Extragalactic Background Light (EBL) measurements provide upper limits on the integrated near-infrared surface brightness (SB > ∼ 31mag arcsec −2 at 2µm) that may come from Population III (Pop III) stars and possible accretion disks around resulting stellar-mass black holes (BHs) in the epoch of First Light, broadly taken from z 7-17. Physical parameters for zero metallicity Pop III stars at z > ∼ 7 can be estimated from MESA stellar evolution models through helium-depletion, and for BH accretion disks from quasar microlensing results and multicolor accretion models. Second-generation non-zero metallicity stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions in their AGB stage. The near-infrared SB constraints can be used to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope (JWST) and the next generation 25-39 m ground-based telescopes may detect for both Pop III stars and stellar mass BH accretion disks. Because Pop III stars and stellar mass BH accretion disks have sizes of a few×10 −11 arcsec at z > ∼ 7, typical caustic magnifications can be µ 10 4 -10 5 , with rise times of hours and decline times of < ∼ 1 year for cluster transverse velocities of v T < ∼ 1000 km s −1 . Microlensing by intracluster medium objects can modify transit magnifications, and lengthen visibility times. Depending on BH masses, accretion-disk radii and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Pop III stars. To observe Pop III caustic transits directly may require monitoring 3-30 lensing clusters to AB < ∼ 29 mag over a decade or more. Such a program must be started with JWST at the start of Cycle 1, and -depending on the role of microlensing in the Intra Cluster Light (ICL) -should be continued for decades with the next generation 25-39 m ground-based telescopes, where both JWST and the ground-based facilities each will play a unique and strongly complementary role.

Context. The reionization of the Universe is one of the most important topics of present day astrophysical research. The most plausible candidates for the reionization process are star-forming galaxies, which according to the predictions of the majority of the theoretical and semi-analytical models should dominate the H i ionizing background at z 3. Aims. We aim at measuring the Lyman continuum escape fraction, which is one of the key parameters to compute the contribution of star-forming galaxies to the UV background. It provides the ratio between the photons produced at λ ≤ 912 Å rest-frame and those which are able to reach the CGM/IGM, not being absorbed by the neutral hydrogen or by the dust of the galaxy's ISM. Methods. We have used ultra-deep U-band imaging (U = 30.2mag at 1σ) by LBC/LBT in the CANDELS/GOODS-North field, as well as deep imaging in COSMOS and EGS fields, in order to estimate the Lyman continuum escape fraction of 69 star-forming galaxies with secure spectroscopic redshifts at 3.27 ≤ z ≤ 3.40 to faint magnitude limits (L = 0.2L * , or equivalently M 1500 ∼ −19). The narrow redshift range implies that the LBC U-band filter samples exclusively the λ ≤ 912 Å rest-frame wavelengths. Results. We have measured through stacks a stringent upper limit (<1.7% at 1σ) for the relative escape fraction of H i ionizing photons from bright galaxies (L > L * ), while for the faint population (L = 0.2L * ) the limit to the escape fraction is 10%. We have computed the contribution of star-forming galaxies to the observed UV background at z ∼ 3 and we have found that it is not enough to keep the Universe ionized at these redshifts, unless their escape fraction increases significantly (≥ 10%) at low luminosities (M 1500 ≥ −19). Conclusions. We compare our results on the Lyman continuum escape fraction of high-z galaxies with recent estimates in the literature and discuss future prospects to shed light on the end of the Dark Ages. In the future, strong gravitational lensing will be fundamental to measure the Lyman continuum escape fraction down to faint magnitudes (M 1500 ∼ −16) which are inaccessible with the present instrumentation on blank fields. These results will be important in order to quantify the role of faint galaxies to the reionization budget.

The eclipsing low-mass X-ray binary 4U 1822-371 is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectroscopy of 4U 1822-371 with the Advanced Camera for Surveys/Solar Blind Channel on the Hubble Space Telescope and new V -and J-band photometry with the 1.3-m SMARTS telescope at Cerro Tololo Inter-American Observatory. We use the new data to construct its UV/optical spectral energy distribution and its orbital light curve in the UV, V , and J bands. We derive an improved ephemeris for the optical eclipses and confirm that the orbital period is changing rapidly, indicating extremely high rates of mass flow in the system; and we show that the accretion disk in the system has a strong wind with projected velocities -2 -up to 4000 km s −1 . We show that the disk has a vertically-extended, opticallythick component at optical wavelengths. This component extends almost to the edge of the disk and has a height equal to ∼0.5 of the disk radius. As it has a low brightness temperature, we identify it as the optically-thick base of a disk wind, not as the optical counterpart of the ADC. Like previous models of 4U 1822-371, ours needs a tall obscuring wall near the edge of the accretion disk, but we interpret the wall as a layer of cooler material at the base of the disk wind, not as a tall, luminous disk rim.

We present two epochs of coordinated X-ray-optical timing observations of the black hole candidate Swift J1753.5−0127 during its 2005 outburst. The first epoch in July occurred at outburst peak. Two consecutive nights of observations using the McDonald Observatory Argos camera with the Rossi X-ray Timing Explorer show a consistent correlation with an immediate response and an extended tail lasting ∼5 s. The properties of the variability and the correlation are consistent with thermal reprocessing in an accretion disc. The shortness of the lag suggests a short orbital period consistent with that recently claimed. The second epoch in August used the Very Large Telescope Focal Reducer/low dispersion Spectrograph 2 (FORS2) High Time resolution (HIT) mode again in conjunction with RXTE. Again a repeatable correlation is seen between two independent subsets of the data. In this case, though, the cross-correlation function has an unusual structure comprising a dip followed by a double peak. We suggest that this may be equivalent to the dip plus single-peak structure seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to synchrotron emission; a similar structure was seen during later activity of Swift J1753.5−0127 by Durant et al. (2008).