TITLE:   GCN CIRCULAR
NUMBER:  29953
SUBJECT: ZTF21aayokph (AT2021lfa): X-ray Detection with Swift/XRT
DATE:    21/05/06 20:31:59 GMT
FROM:    Anna Ho at UC Berkeley  <annayqho@berkeley.edu>

Anna Ho (UC Berkeley), Yuhan Yao (Caltech), and Daniel Perley (LJMU)

We observed the cosmological fast transient ZTF21aayokph/AT2021lfa (Yao,
Perley, and Ho, GCN Circ. 29938) with Swift/XRT beginning at 2021-05-05
05:32 UT, or 1.0d after the first ZTF detection. In a 5ks exposure, we
detect X-ray emission with a count rate of 0.0084 +/- 0.0015 ct/s. Using
WebPIMMS [1] we estimate [2] that this corresponds to an unabsorbed 0.3-10
keV flux density of 2.9E-13 ergs/cm2/s. At z=1.063 (Yao, Ho, and Perley,
GCN Circ. 29945) this is a luminosity of 1.9E45 erg/s, typical of an X-ray
afterglow of a long-duration GRB at this epoch [3].

We thank the Swift staff for rapidly approving and scheduling our
observations.

[1] https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl
[2] Using a hydrogen column density of 2.16E20/cm2 (Willingale, R., et al.
2013, MNRAS, 431, 394) and a power-law index of 2
[3] Evans, P.~A., et al. 2007, AAP, 469, 379