J/ApJ/905/98 Keck/LRIS optical spectrum of ZTF20aajnksq (Ho+, 2020)
ZTF20aajnksq (AT 2020blt): a fast optical transient at z∼2.9 with no detected
gamma-ray burst counterpart.
Ho A.Y.Q., Perley D.A., Beniamini P., Cenko S.B., Kulkarni S.R.,
Andreoni I., Singer L.P., De K., Kasliwal M.M., Fremling C., Bellm E.C.,
Dekany R., Delacroix A., Duev D.A., Goldstein D.A., Golkhou V.Z.,
Goobar A., Graham M.J., Hale D., Kupfer T., Laher R.R., Masci F.J.,
Miller A.A., Neill J.D., Riddle R., Rusholme B., Shupe D.L., Smith R.,
Sollerman J., van Roestel J.
<Astrophys. J., 905, 98 (2020)>
=2020ApJ...905...98H 2020ApJ...905...98H
ADC_Keywords: Spectra, optical
Keywords: Gamma-ray bursts ; Radio transient sources ; Transient sources ;
X-ray transient sources ; Sky surveys ; Surveys
Abstract:
We present ZTF20aajnksq (AT 2020blt), a fast-fading (Δr=2.3mag
in Δt=1.3d) red (g-r∼0.6mag) and luminous (M1626Å=-25.9mag)
optical transient at z=2.9 discovered by the Zwicky Transient Facility
(ZTF). AT 2020blt shares several features in common with afterglows to
long-duration gamma-ray bursts (GRBs): (1) an optical light curve
well-described by a broken power law with a break at tj=1d (observer
frame); (2) a luminous (L0.3-10KeV=1046erg/s) X-ray counterpart;
and (3) luminous (L10GHz=4x1031erg/s/Hz) radio emission. However,
no GRB was detected in the 0.74d between the last ZTF nondetection
(r>21.36mag) and the first ZTF detection (r=19.60mag), with an upper
limit on the isotropic-equivalent gamma-ray energy release of
Eγ,iso<7x1052erg. AT 2020blt is thus the third
afterglow-like transient discovered without a detected GRB counterpart
(after PTF11agg and ZTF19abvizsw) and the second (after ZTF19abvizsw)
with a redshift measurement. We conclude that the properties of
AT2020blt are consistent with a classical (initial Lorentz factor
Γ0≳100) on-axis GRB that was missed by high-energy
satellites. Furthermore, by estimating the rate of transients with
light curves similar to that of AT 2020blt in ZTF high-cadence data,
we agree with previous results that there is no evidence for an
afterglow-like phenomenon that is significantly more common than
classical GRBs, such as dirty fireballs. We conclude by discussing the
status and future of fast-transient searches in wide-field
high-cadence optical surveys.
Description:
To measure the color of AT 2020blt, we triggered target-of-opportunity
(ToO) programs on the IO:O imager of the Liverpool Telescope (LT) and
with the Spectral Energy Distribution Machine (SEDM) on the automated
60-inch telescope at the Palomar Observatory (P60).
To monitor the light curve, we triggered a ToO program with the
Wafer-Scale Imager for Prime (WaSP) on the 200-inch Hale telescope at
the Palomar Observatory (P200) and obtained 2x180s exposures in each
of g-, r-, and i-bands.
For a final photometry measurement, we triggered a ToO observation
with the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-North
8-meter telescope on Maunakea. See Section 2.
For optical spectroscopy, we triggered ToO observations using the Low
Resolution Imaging Spectrometer (LRIS) on the Keck I 10-m telescope.
See Section 2.3.
We triggered ToO observations with the X-Ray Telescope (XRT) on board
the Neil Gehrels Swift observatory. We obtained two epochs of 4ks
exposures. See Section 2.4.
On February 3 we triggered our ToO program on the Karl G. Jansky Very
Large Array (VLA) for fast-rising and luminous transients.
See Section 2.5.
We searched the Fermi GBM Burst Catalog, the Fermi-GBM Subthreshold
Trigger list (with reliability flag not equal to 2), the Swift GRB
Archive, and the Gamma-Ray Coordinates Network archives for an
associated GRB between the last ZTF nondetection (January 27.54) and
the first ZTF detection (January 28.28). There were no GRBs coincident
with the position and time of AT 2020blt. See Section 2.6.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
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12 47 04.87 +45 12 02.3 AT2020blt = ZTF20aajnksq
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
fig2.dat 78 5132 Keck/LRIS spectrum of AT2020blt at dt=2.44d
(MJD=58878.644055)
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See also:
J/ApJ/682/1205 : Fast transients in the Fornax cluster (Rau+, 2008)
J/ApJ/693/1484 : Early optical afterglow catalog (Cenko+, 2009)
J/ApJ/696/870 : Catalina Real-time Transient Survey (CRTS) (Drake+, 2009)
J/ApJ/701/824 : Afterglows of short and long-duration GRBs (Nysewander+, 2009)
J/ApJ/720/1513 : The afterglows of Swift-era GRBs. I. (Kann+, 2010)
J/ApJ/746/156 : Radio afterglow observations of GRBs (Chandra+, 2012)
J/ApJS/207/15 : M dwarf flare spectra (Kowalski+, 2013)
J/ApJ/781/37 : Multi-band photometry of GRB 130427A (Perley+, 2014)
J/A+A/593/A68 : PTF12os & iPTF13bvn spectra and light curves (Fremling+, 2016)
J/ApJ/833/88 : GRB 160509A VLA monitoring campain results (Laskar+, 2016)
J/ApJ/859/134 : A VLA study of high-z GRBs. II. GRB140304A (Laskar+, 2018)
J/ApJ/886/152 : ZTF early observations of Type Ia SNe. I. LCs (Yao+, 2019)
Byte-by-byte Description of file: fig2.dat
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Bytes Format Units Label Explanations
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1- 10 F10.4 0.1nm lambda [3060.9/10304.1] Observed Wavelength, air,
in Angstrom units
12- 23 E12.5 cW/m2/nm Flux [-1.9e-17/2.3e-17]? Flux,
in erg/s/cm2/Angstrom
25- 35 E11.5 cW/m2/nm Sky [6.5e-18/7.3e-16] Sky flux,
in erg/s/cm2/Angstrom
37- 47 E11.5 cW/m2/nm e_Flux [5.2e-19/2.9e-17] Uncertainty in Flux
49- 52 I4 --- Xpix [172/4093] LRIS CCD X Pixel
54- 60 F7.3 --- Ypix [69.9/116] LRIS CCD Y Pixel
62- 72 E11.5 --- Response [3.3e+14/5.2e+16] Response
74- 78 F5.3 --- Flag [0/1] Flag
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 27-Jul-2022