Swift and NICER high-cadence observations of the fast, blue optical transient AT2020neh: No X-rays but UV declining rapidly
ATel #13870; Dheeraj Pasham (MIT), Keith Gendreau (NASA/GSFC), Michael Loewenstein (UMCP/NASA/GSFC/CRESST2), and Ron Remillard (MIT)
on 16 Jul 2020; 21:32 UT
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)
Subjects: Ultra-Violet, Transient, Tidal Disruption Event
The transient AT2020neh/ZTF20abgwfek was discovered on 19 June 2020 by the Zwicky Transient Facility (ZTF). Based on its spatial coincidence with a low mass galaxy at a redshift of 0.062 and a blue optical spectrum, it has been classified as a candidate tidal disruption event by perhaps an intermediate-mass black hole (10^5 solar). Because it rose by roughly 3 magnitudes within 11 days it is also classified as a fast, blue optical transient.
Swift (XRT and UVOT) observed AT2020neh on a daily cadence between MJD 59031 and 59045. A stacked Swift/XRT image gives a total exposure of 11.3 ks and no X-ray point source is detected at the optical position. The X-ray count rate upper limit is 1e-3 cps. Assuming a power-law index of 1.7 this corresponds to a 0.3-10 keV unabsorbed flux limit of 4.3e-14 erg/s/cm^2 or a luminosity of 3.7e41 erg/s. NICER also conducted seven observations for a total of 23.8 ks between MJD 59027 and 59038, and detected X-rays above instrumental background concentrated around OVII and OVIII emission line energies at a rate of 0.15 c/s (corresponding to a 0.5-2 keV flux of 1.3e-13 erg/cm2). However, this is consistent with an origin in Galactic foreground emission.
The optical and the UV fluxes are declining at varying rates with the bluest flux reducing the fastest (0.1 mag per day). For convenience we include decline rate (magnitude per day) versus central wavelength of the UVOT filter.
Wavelength (Angstrom) decline rate (mag/day) |
5405 0.04 |
4332 0.04 |
3503 0.05 |
2592 0.09 |
2230 0.11 |
2033 0.11 |
|
The nature of this source is still unclear and follow-up observations are encouraged.
NICER carries out prompt follow-up observations of transients and is planning to systematically follow up alerts from LIGO and other X-ray-bright extragalactic 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.