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We present an early-phase g-band light curve and visual-wavelength spectra of the normal Type Ia supernova (SN) 2013gy. The light curve is constructed by determining the appropriate S-corrections to transform KAIT natural-system B- and V-band photometry and Carnegie Supernova Project natural-system g-band photometry to the Pan-STARRS1 g-band natural photometric system. A Markov chain Monte Carlo calculation provides a best-fit single power-law function to the first ten epochs of photometry described by an exponent of 2.16+0.06−0.06 and a time of first light of MJD 56629.4+0.1−0.1, which is 1.93+0.12−0.13 days (i.e., < 48 h) before the discovery date (2013 December 4.84 UT) and −19.10+0.12−0.13 days before the time of B-band maximum (MJD 56648.5 ± 0.1). The estimate of the time of first light is consistent with the explosion time inferred from the evolution of the Si IIλ6355 Doppler velocity. Furthermore, discovery photometry and previous nondetection limits enable us to constrain the companion radius down to Rc ≤ 4 R⊙. In addition to our early-time constraints, we used a deep +235 day nebular-phase spectrum from Magellan/IMACS to place a stripped H-mass limit of < 0.018 M⊙. Combined, these limits effectively rule out H-rich nondegenerate companions.

We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu. For GRB 130215A/SN 2013ez, we also present optical spectroscopy at t − t0 = 16.1 d, which covers rest-frame 3000-6250 Å. Based on Fe ii λ5169 and Si ii λ6355, our spectrum indicates an unusually low expansion velocity of ~4000-6350 km s-1, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A/SN 2013fu, we used our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we took advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Mészáros (2001, ApJ, 552, L35), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P = 12 ms and a magnetic field of B = 1.1 × 1015 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs.

Context. We present our optical multiband (B and R) observations of the blazar 3C 454.3, which were acquired over 30 nights from August 2007 to December 2010. Aims. We analyse our data to study the optical emission variability properties of 3C 454.3. Methods. The observations were carried out using both the 1 m EOS Telescope (EST) and the 0.8 m Tsinghua-NAOC Telescope (TNT) simultaneously in most nights at the Xinglong Observatory of National Astronomical Observatories, Chinese Academy of Sciences (NAOC). Both telescopes are equipped with the same type of Princeton Instrument CCD and standard Johnson-Cousin UBVRI filters. To reach a similar signal-to-noise ratio (S/N) in different bands with the same exposure time, observations using the larger diameter EST were carried out with B-band filter, while those of the smaller diameter TNT were carried out with R-band filter. Any time differences between the two telescopes were corrected using GPS. The typical sampling rate was one data point per minute, which is the highest time-resolution so far. Results. A multi-peak outburst was observed during November‐December 2007, in addition to a later outburst during October 27‐November 1, 2010. The brightest magnitudes for 3C 454.3 during our observations were B = 13.77 mag and R = 12.69 mag at JD =2454436.0952 (December 1, 2007). One of the most striking events was detected on December 1, 2007, when a flux increase of about 0.67 mag occurred in 3.2 h. A colour analysis shows that the colour variability processes are apparently complex. A bluer-when-brighter trend, a redder-when-brighter trend (generally in faint states), and no significant colour variance were all detected during the outburst of November‐December 2007. Furthermore, the kind of “saturation” effect in the B − R trend is not visible in the bright state (R < 14) during August 12‐14, 2007. These trends may be due to the interactive contribution from the accretion disc and the jet components. Meanwhile, intra-night variability in B and R bands was found to be correlated, without any time delay exceeding one minute.