Radio transients from newborn black holes

Abstract

We consider radio emission from a newborn black hole (BH), which is accompanied by a mini-disk with a mass of M ⊙ . Such a disk can be formed from an outer edge of the progenitor's envelope, especially for metal-poor massive stars and/or massive stars in close binaries. The disk accretion rate is typically super-Eddington and an ultrafast outflow with a velocity of ∼ 0.1-0.3 c will be launched into the circumstellar medium. The outflow forms a collisionless shock, and electrons are accelerated and emit synchrotron emission in radio bands with a flux of ∼ 10 26−30 erg s −1 Hz −1 days to decades after the BH formation. The model predicts not only a fast UV/optical transient but also quasi-simultaneous inverse-Compton X-ray emission ∼ a few days after the BH formation, and the discovery of the radio counterpart with coordinated searches will enable us to identify this type of transients. The occurrence rate can be 0.1 − 10 % of the core-collapse supernova rate, which makes them a promising target of dedicated radio observations such as the Jansky VLA Sky Survey.

AT2018lqh: Black Hole Born from a Rotating Star?

“…Stellar evolution calculations suggest that a BSG collapsar can typically maintain significant rotation at the outermost layers of a few to 10 M so that to form an accretion disk (e.g., Kashiyama et al 2018), although angular momentum transport inside the star or binary interactions may complicate this picture (e.g. Fuller & Ma 2019).…”

“…We investigate the consistency of this model and radio observations at day 218. We use the formulations in Kashiyama et al (2018) to calculate the radio light curves, and adopt the parameters of the disk wind inferred from the fit to the optical emission. Parameters of the CSM and mi- crophysical parameters are taken to be same as those adopted in Kashiyama et al (2018) (see Table 1…”

Radio emission from accreting isolated black holes in our galaxy

“…where ζ e ≈ 0.4 (Park et al 2015) is the efficiency of the kinetic energy of protons being transferred to electrons. Hereafter we use the value ζ e = 0.4, as was done in other works that considered radio transients by mildly relativistic outflows (Murase et al 2016;Kimura et al 2017;Kashiyama et al 2018). We note that this estimate on γ min may be inaccurate for outflow velocities much lower than ∼ 0.1c.…”

VTC J095517.5+690813: A radio transient from an accretion-induced collapse of a white dwarf?

“…The pre-transient images provided by the Hubble Space Telescope (HST ) found two red giants at the location of VTC J095517.5+690813, which could be a chance ⋆ E-mail: takashi.moriya@nao.ac.jp 1 http://www.astronomy.ohio-state.edu/~assassin/ coincidence, and no luminous stars were present at the location. Therefore, the origin of VTC J095517.5+690813 is not likely related to deaths of massive stars during which optically-faint radio-bright transients may be generated (e.g., Kashiyama et al 2018). The exact nature of VTC J095517.5+690813 has not been clarified yet.…”