TITLE: GCN CIRCULAR NUMBER: 24592 SUBJECT: LIGO/Virgo S190426c: NICER X-ray Follow-Up of ZTF19aassfws DATE: 19/05/19 02:37:16 GMT FROM: Dheeraj R. Pasham at Mass. Inst. of Technology Dheeraj Pasham (MIT), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Stephen Eikenberry (UFL), Wynn C.G. Ho (Haverford) report on behalf of the NICER team: The transient ZTF19aassfws (Perley et al., GCN#24331) fell in the LIGO/Virgo error box of the trigger S190426c (LVC et al., GCN#24237), and Cenko et al. (GCN#24430) found that its distance is consistent with that inferred from the gravitational wave signal. NICER observed this target for 7 ks, approximately half of which was in especially low background/high sensitivity conditions. We do not detect any X-ray emission above the background from this region. Assuming an X-ray spectrum with a power-law index of 1.7 (similar to GW170817) at a redshift of 0.093 with an absorbing column of 8e20 cm**-2 (Milky Way column), we estimate an upper limit on the unabsorbed 0.35-11.5 keV X-ray flux of 1.3e-13 erg/s/cm**2. NICER can carry out prompt follow-up observations of transients and is planning to systematically follow up alerts from LIGO/Virgo and other X-ray bright extra-galactic transients in the future. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.