GROND coverage of the main peak of gamma-ray burst 130925A

Abstract

Aims. Prompt or early optical emission in gamma-ray bursts (GRBs) is notoriously difficult to measure, and observations of the dozen cases show a large variety of properties. Yet, such early emission promises to help us achieve a better understanding of the GRB emission process(es). Methods. We performed dedicated observations of the ultra-long duration (T90 about 7000 s) Swift GRB 130925A in the optical/nearinfrared with the 7-channel Gamma-Ray burst Optical and Near-infrared Detector (GROND) at the 2.2 m MPG/ESO telescope. Results. We detect an optical/near-infrared flare with an amplitude of nearly 2 mag which is delayed with respect to the keV−MeV prompt emission by about 300−400 s. The decay time of this flare is shorter than the duration of the flare (500 s) or its delay. Conclusions. While we cannot offer a straightforward explanation, we discuss the implications of the flare properties and suggest ways toward understanding it.

The sharpness of gamma-ray burst prompt emission spectra

“…A possibly similar inference can be made on the related phenomena of prompt optical emission showing a similar temporal profile as the gamma-ray emission (e.g., Elliott et al 2014;Greiner et al 2014) or very early X-ray flares (e.g., Pe'er et al 2006; see also Hu et al 2014 for a recent large Swift sample study): if the prompt emission is not dominated by synchrotron emission, this is likely the case for this longer wavelength emission as well (see, e.g., Starling et al 2012;Peng et al 2014).…”

Super-solar metallicity at the position of the ultra-long GRB 130925A

“…There was no evidence of further fading in the JHK filters between these two epochs, indicating that we were detecting the host galaxy at these times with a negligible contribution from the afterglow. The flux in the optical bands plateaued at 2 days after the GRB trigger, with GROND observations taken 2, 4 and 11 days after the trigger yielding consistent magnitudes (Greiner et al 2014). We modelled the host galaxy SED using the spectral fitting package LePHARE (Arnouts et al 1999;Ilbert et al 2006), which is a population-synthesis-based fitting procedure.…”

“…No optical afterglow redward of the R-band was detected, with the UltraViolet and Optical Telescope observations (UVOT; Roming et al 2005) taken just 157 s after the GRB trigger, resulting in a v-band 3σ upper limit of v > 20.0 (Holland & Lien 2013). Rapid-response observations with the simultaneous 7 channel imager, GROND, began just 7 min after the GRB trigger (Sudilovsky et al 2013a), and a decaying optical source was detected within the X-ray afterglow error circle in the i , z , J, H and K bands (Sudilovsky et al 2013a;Greiner et al 2014). A detailed analysis of the XRT and GROND afterglow observations are given in Evans et al (2014) and Greiner et al (2014), respectively.…”

“…Rapid-response observations with the simultaneous 7 channel imager, GROND, began just 7 min after the GRB trigger (Sudilovsky et al 2013a), and a decaying optical source was detected within the X-ray afterglow error circle in the i , z , J, H and K bands (Sudilovsky et al 2013a;Greiner et al 2014). A detailed analysis of the XRT and GROND afterglow observations are given in Evans et al (2014) and Greiner et al (2014), respectively.…”

“…With the typically rapid dissemination of sub-arcsecond GRB positions provided by the GRB-dedicated Swift mission (Gehrels et al 2004), launched in 2004, and the commissioning of (semi-)robotic telescopes equipped with near-infrared (NIR) facilities, such as the GRB Optical and Near-infrared Detector (GROND; Greiner et al 2008), the Peters automated IR Imaging Telescope (PAIRITEL; Bloom et al 2006) and the Reionization And Transients IR camera (RATIR; Butler et al 2012), the detection of heavily dust-extinguished GRBs and dust-rich host galaxies has become more common. GRB 130925A was at a redshift of z = 0.347 (Sudilovsky et al 2013b) and had a host galaxy visual extinction of A V = 5.0±0.7 mag as measured from the afterglow spectral energy distribution (SED; Greiner et al 2014), which is one of the largest host galaxy visual extinctions measured along the line of sight to any GRB, and certainly the largest along the line of sight to an ultra-long duration GRB.…”