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An Initial NICER Observation of AT2018cow = ATLAS18qqn

ATel #11773; J. M. Miller, R. M. Ludlam, M. Reynolds (University of Michigan), E. Cackett (Wayne State University), P. Ray (NRL), T. Maccarone, L. Rivera (Texas Tech), K. Gendreau, Z. Arzoumanian (NASA/GSFC), on behalf of the NICER Science Team
on 24 Jun 2018; 02:51 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

Subjects: X-ray, Gamma-Ray Burst, Supernovae, Transient

Referred to by ATel #: 11775, 11776

NICER observed the new transient AT2018cow = ATLAS18qqn (ATEL #11727, Smartt et al. 2018) on 2018 June 22 at 21:02 UT, for a total of 950s.

After normalizing the spectrum against residuals found in direct fits to the Crab nebula (see Ludlam et al. 2018), and using a standard background region, the spectrum was grouped according to the "optimal" binning scheme of Kaastra & Bleeker (2016), and fits were made over the 0.5-10.0 keV band. A simple absorbed power-law model (ztbabs*zpow, assuming z=0.014) provides an acceptable fit to the time-averaged spectrum:

N_H = 1.4(2) E+21 cm^-2,
Gamma = 2.2(1),
Norm = 2.5(2) E-3,
chi^2 / dof = 94.5/93,
Flux = 8.4 E-12 erg/cm^2/s (0.5-10.0 keV).

The short light curve from the observation shows weak variability but no clear flux trend. No significant coherent pulsations were found in an acceleration search, after accounting for the number of trials in the search.

Additional NICER observations are being executed.

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.

References:
Kaastra, J. S., & Bleeker, J., 2016, A&A, 587,151
Ludlam, R. M., et al., 2018, ApJ, 858, L5
Smartt, S. J., et al., 2018, ATEL 11727