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The EVN detection of AT2018cow at 1.67GHz

ATel #12067; http://science. sciencemag. org/content/361/6401/482. full
on 28 Sep 2018; 10:49 UT
Credential Certification: Tao An (antao@shao.ac.cn)

Subjects: Radio, Gamma-Ray Burst, Supernovae, Transient, Tidal Disruption Event

AT2018cow is a recently detected transient (Smartt et al. ATel #11727), associated with a galaxy CGCG 137-068 at z = 0.014. Its physical nature is debatable, with possible scenarios including the type Ic supernova (e.g., Xu et al. ATel#11740, Izzo et al. ATel#11753), an off-axis gamma-ray burst afterglow (e.g., Rivera Sandoval & Maccarone ATel#11737), a tidal disruption of a white dwarf by an intermediate mass black hole, or a magnetar (e.g., Kuin et al. 2018 ; Perley et al. 2018 ; Prentice et al. 2018 ). To enable the distinction between these models, we conducted the very long baseline interferometry (VLBI) observations of AT2018cow using the European VLBI Network (EVN).

The first-epoch 2-h pilot observation was carried out in real-time e-VLBI mode at 1.67 GHz on 2018 September 18 (project code: RY007a) employing the high-precision phase-referencing technique. The quasar J1619+2247 is used as the phase-referencing calibrator. We detected an unresolved compact source with a signal-to-noise ratio of 22. The accurate position of the brightness peak is R.A.=16h16m00.2243s, Dec.=+22d16'04.893" (J2000; formal uncertainty ~1 mas), which is roughly in agreement with the position derived from previous 22-GHz VLBA+Effelsberg observations on 2018 July 31 (R.A. = 16h16m00.2242s, Dec.=+22d16'04.890: Bietenholz et al. ATel#11900). The total flux density is 0.9 mJy (uncertainty about 10 percent) at 1.67 GHz, more than double the reported 0.44 mJy from the latest Giant Metrewave Radio Telescope (GMRT) on 2018 Aug 12.71 UT in the 1390 MHz band (Nayana & Chandra ATel#11950), indicating that the source brightness is increasing. The increasing radio flux density suggests that the effect of synchrotron self-absorption at centimeter wavelengths is decreasing.

The follow-up disk-based experiment using the full EVN array, with better (u,v) coverage and image quality, will be conducted on 20-21 October 2018 at C band (5 GHz). We expect to reduce the data and report the result in a timely manner.

The participant stations are : Effelsberg, Jodrell Bank, Medicina, Onsala, Sardinia, Torun, Westerbork and three e-MERLIN stations (Cambridge, Defford, Knockin).

We thank the EVN PC for approving the trigger-of-opportunity observations, and the staff of the EVN and the observatories for carrying out the experiment. The European VLBI Network is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. We thank for the National Key R&D Programme of China under grant number 2018YFA0404602.