# A tale of two transients: GW170104 and GRB170105A

## TL;DR

This study conducted multi-wavelength follow-up observations of the GW170104 gravitational wave event, found no X-ray counterpart, and identified an unrelated long GRB afterglow coincidentally located within the error circle.

## Contribution

The paper provides the first detailed multi-wavelength follow-up of GW170104 and clarifies the nature of the optical transient ATLAS17aeu as an unrelated long GRB afterglow.

## Key findings

- No hard X-ray counterpart detected for GW170104.
- ATLAS17aeu is an unrelated long GRB afterglow.
- The optical transient is not associated with GW170104.

## Abstract

We present multi-wavelength follow-up campaigns by the AstroSat-CZTI and GROWTH collaborations to search for an electromagnetic counterpart to the gravitational wave event GW170104. At the time of the GW170104 trigger, the AstroSat CZTI field-of-view covered 50.3\% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of $\approx 4.5 \times 10^{-7}~{\rm erg~cm}^{-2}{\rm~s}^{-1}$ for a 1\,s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB~170105A, with only a fortuitous spatial coincidence with GW170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models.