{
  "bibcode": "2019GCN.24700....1K",
  "body": "D. A. Kann, L. Izzo (both HETH/IAA-CSIC), A. de Ugarte Postigo \n(HETH/IAA-CSIC, DARK/NBI), M. Blazek, C. C. Thoene, K. Bensch (all \nHETH/IAA-CSIC), and V. Casanova (IAA-CSIC) report:\n\nWe observed the afterglow position (Lipunov et al., GCN 24680) of the \nextremely bright GRB 190530A (GBM detection and localization: Fermi GBM \nteam, GCN 24676, Biltzinger et al., GCN 24677; Fermi-LAT localization: \nLongo et al., GCN 24679; Agile-MCAL/GRID detections: Lucarelli et al., \nGCN 24678, Verecchia et al., GCN 24683) a second time with the 1.5-m \ntelescope of the Sierra Nevada Observatory (OSN), Spain. The afterglow \nis now only marginally detected in single frames, so we stack 7 x 120 s \nimages.\n\nWe measure the following AB magnitude:\n\nRc = 20.60  +- 0.15 mag at 1.44509 days after the GRB.\n\nThe magnitude was derived against 1 nearby star from the SDSS catalog, \nusing the transformation equations of Lupton (2005), and transformed \nback into AB mag.\n\nThis magnitude is significantly fainter than expected from an \nextrapolation of the decay from the first day, indicating a break in the \nlight curve must have occurred. Using other observations (Lipunov et \nal., GCN 24680, GCN 24693; Kann et al., GCN 24684; Heintz et al., GCN \n24686; Izzo et al., GCN 24687; Xin et al., GCN 24688; Watson et al., GCN \n24690; Xin et al., GCN 24697) we find the afterglow is described by a \nbroken power-law with pre-break decay slope alpha_1 = 1.21 +/- 0.07, \npost-break decay slope alpha_2 = 3.75 +/- 0.40, and break time t_b = \n0.78 +/- 0.06 days.\n\nThe steeper slope mentioned by Watson et al. (GCN 24690) may indicate \nthe break was occurring during the COATLI observations. We note the \nmagnitude given by Belkin et al. (GCN 2469 is significantly too bright, \nthis may stem from their use of USNO stars as standard stars.\n\nThe post-break decay slope is extremely steep and generally not expected \nfrom the forward shock model and a typical electron energy distribution. \nFurther observations are highly encouraged.",
  "circularId": 24700,
  "createdOn": 1559344385000,
  "email": "kann@iaa.es",
  "subject": "GRB 190530A: Steep afterglow decay found by OSN",
  "submitter": "Alexander Kann at IAA-CSIC  <kann@iaa.es>",
  "eventId": "GRB 190530A"
}