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Title: iPTF Archival Search for Fast Optical Transients

There has been speculation about a class of relativistic explosions with an initial Lorentz factor Γ init smaller than that of classical gamma-ray bursts (GRBs). These "dirty fireballs" would lack prompt GRB emission but could be pursued via their optical afterglow, appearing as transients that fade overnight. We report a search for such transients (that fade by 5-σ in magnitude overnight) in four years of archival photometric data from the intermediate Palomar Transient Factory (iPTF). Our search criteria yielded 50 candidates. Of these, two were afterglows to GRBs that had been found in dedicated follow-up observations to triggers from the Fermi GRB Monitor. Another (iPTF14yb) was a GRB afterglow discovered serendipitously. Eight were spurious artifacts of reference image subtraction, and one was an asteroid. The remaining 38 candidates have red stellar counterparts in external catalogs. The photometric and spectroscopic properties of the counterparts identify these transients as strong flares from M dwarfs of spectral type M3-M7 at distances of d ≈ 0.15-2.1 kpc; three counterparts were already spectroscopically classified as late-type M stars. With iPTF14yb as the only confirmed relativistic outflow discovered independently of a high-energy trigger, we constrain the all-sky rate of transients that peak at m = 18more » and fade by Δm = 2 mag in Δt = 3 hr to be 680 yr -1, with a 68% confidence interval of 1119-2236 yr -1. This implies that the rate of visible dirty fireballs is at most comparable to that of the known population of long-duration GRBs.« less
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  1. California Inst. of Technology (CalTech), Pasadena, CA (United States). Cahill Center for Astrophysics
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. Univ. of California, Merced, CA (United States)
  4. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States). Astrophysics Science Division; Univ. of Maryland, College Park, MD (United States). Joint Space-Science Inst.
  5. Liverpool John Moores Univ., Liverpool (United Kingdom). Astrophysics Research Inst.
  6. Univ. of Washington, Seattle, WA (United States). Dept. of Astronomy
  7. Univ. of Wisconsin, Milwaukee, WI (United States). Dept. of Physics
  8. Weizmann Inst. of Science, Rehovot (Israel). Benoziyo Center for Astrophysics
  9. Inst. of Astrophysics of Andalusia (IAA-CSIC), Granada (Spain)
  10. California Inst. of Technology (CalTech), Pasadena, CA (United States). Infrared Processing and Analysis Center
  11. Stockholm Univ. (Sweden). Oskar Klein Centre and Dept. of Astronomy
  12. California Inst. of Technology (CalTech), Pasadena, CA (United States). LIGO Lab.
Publication Date:
Grant/Contract Number:
AC02-05CH11231; DGE1144469; AYA 2014-58381-P; IJCI-2015-261; 1545949
Accepted Manuscript
Journal Name:
The Astrophysical Journal. Letters
Additional Journal Information:
Journal Volume: 854; Journal Issue: 1; Journal ID: ISSN 2041-8213
Institute of Physics (IOP)
Research Org:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR) (SC-21); National Science Foundation (NSF); Ministry of Science, Innovation and Universities (Spain)
Country of Publication:
United States
OSTI Identifier: