The Remarkable Spin-down and Ultra-fast Outflows of the Highly-Pulsed Supersoft Source of Nova Hercules 2021

Abstract

Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eye brightness and has been detected from radio to γ-rays. An extremely fast optical decline of 2 magnitudes in 1.2 days and strong Ne lines imply a high-mass white dwarf. The optical pre-outburst detection of a 501.42 s oscillation suggests a magnetic white dwarf. This is the first time that an oscillation of this magnitude has been detected in a classical nova prior to outburst. We report X-ray outburst observations from Swift and Chandra which uniquely show: (1) a very strong modulation of supersoft X-rays at a different period from reported optical periods; (2) strong pulse profile variations and the possible presence of period variations of the order of 0.1-0.3 s; and (3) rich grating spectra that vary with modulation phase and show P Cygni-type emission lines with two dominant blue-shifted absorption components at ∼ 3000 and 9000 km s −1 indicating expansion velocities up to 11000 km s −1 . X-ray oscillations most likely arise from inhomogeneous photospheric emission related to the magnetic field. Period differences between reported pre-and post-outburst optical observations, if not due to other period drift mechanisms, suggest a large ejected mass for such a fast nova, in the range 2 × 10 −5 -2 × 10 −4 M . A difference between the period found in the Chandra data and a reported contemporaneous post-outburst optical period, as well as the presence of period drifts, could be due to weakly non-rigid photospheric rotation.

NICER monitoring of supersoft X-ray sources

“…A modulation with a periodicity of 501.4277±0.0002 s was discovered in archival Zwicky Transient Facility images by Mroz et al (2021); Drake et al (2021) and it was followed by the discovery that a very close period is measurable in the supersoft X-ray source that appeared shortly after the outburst. Maccarone et al (2021) reported a modulation with a 503.9 s period in a Chandra observation done for 10 ks on 2021 July 10 with the HRC-S camera, and in Pei et al (2021a) we reported a modulation with a 501.8±0.7 s period in the initial NICER data.…”

“…Maccarone et al (2021) reported a modulation with a 503.9 s period in a Chandra observation done for 10 ks on 2021 July 10 with the HRC-S camera, and in Pei et al (2021a) we reported a modulation with a 501.8±0.7 s period in the initial NICER data. A second Chandra observation by Drake et al (2021) revealed a period of 501.72±0.11 s in the 0-order light curve of an exposure of 30 ks done with the Chandra HRC-S camera and Low Energy Transmission Grating. The period measured at optical wavelengths before the outburst is not consistent, within the error estimate, with the one measured with Chandra by Drake et al (2021), or even with a new, recent optical period of 501.516±0.018 s measured by Patterson et al (2021).…”

“…A second Chandra observation by Drake et al (2021) revealed a period of 501.72±0.11 s in the 0-order light curve of an exposure of 30 ks done with the Chandra HRC-S camera and Low Energy Transmission Grating. The period measured at optical wavelengths before the outburst is not consistent, within the error estimate, with the one measured with Chandra by Drake et al (2021), or even with a new, recent optical period of 501.516±0.018 s measured by Patterson et al (2021). Assuming the period is the one of the WD rotation, this is an indication of spin down due to ejected mass in the range 0.2-2 ×10 −4 M .…”

“…V1674 Her was monitored with NICER on 45 different dates between 2021/07/10 and 2021/08/31, with daily frequency whenever it was possible. The X-ray light curve was previously followed with the Swift XRT, and although there always was a detectable X-ray source from the third day after optical maximum, initially there was a faint, non-supersoft X-ray source, most likely due to shocks in the ejected matter (see Drake et al 2021).…”

High energy neutrinos from fast winds in Novae

“…The surface magnetic fields of the intermediate polars are expected to be comparable. For example, recent observations of the Nova Herculis 2021 show modulation of the soft X-ray emission which is interpreted as due to the presence of a strong magnetic field (Drake et al 2021). The initial structure of the magnetic field of the WD is expected to be of the dipole type.…”