We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ∼5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities.Unified Astronomy Thesaurus concepts: Low-mass x-ray binary stars (939); Stellar mass black holes (1611); High energy astrophysics (739); Black hole physics (159); X-ray transient sources (1852)

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (1.4 ± 0.1 mag/hr) and luminous (M g,peak = −20 mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity (L bol 3×10 44 erg s −1 ), the short rise time (t rise = 3 day in g-band), and the blue colors at peak (g −r ∼ −0.4) all resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature (T eff 40, 000K) arXiv:1904.11009v4 [astro-ph.HE] 1 Dec 2019 2 spectra of a stripped-envelope SN. A retrospective search revealed luminous (M g ∼ M r ≈ −14 mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release E γ,iso < 4.9 × 10 48 erg, a limit on X-ray emission L X < 10 40 erg s −1 , and a limit on radio emission νL ν 10 37 erg s −1 . Taken together, we find that the early (< 10 day) data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 M ) at large radii (3 × 10 14 cm) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time (> 10 day) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.

We present Neutron Star Interior Composition Explorer X-ray and Arcminute Microkelvin Imager Large Array radio observations of a rapid hard-to-soft state transition in the black hole X-ray transient MAXI J1820+070. During the transition from the hard state to the soft state a switch between two particular types of quasiperiodic oscillations (QPOs) was seen in the X-ray power density spectra, from type-C to type-B, along with a drop in the strength of the broadband X-ray variability and a brief flare in the 7-12 keV band. Soon after this switch (∼1.5-2.5 hr) a strong radio flare was observed that corresponded to the launch of superluminal ejecta. Although hints of a connection between QPO transitions and radio flares have been seen in other black hole X-ray transients, our observations constitute the strongest observational evidence to date for a link between the appearance of type-B QPOs and the launch of discrete jet ejections. are usually recognized: the hard state, which shows strong rapid (subsecond) X-ray variability and a spectrum dominated by Comptonized power-law emission, and the soft state, in which rapid X-ray variability is weak and where a thermal disk blackbody component dominates the spectrum. The transitions between these two spectral states, during which sources are often classified as being in an intermediate state, are accompanied by strong evolution of the X-ray variability properties. As a source starts the transition from the hard state to the soft state, strong (type-C) low-frequency quasi-periodic oscillations (QPOs) are usually present in the power density spectra (see Wijnands et al. 1999;Remillard et al. 2002;Casella et al. 2005, for the various low-frequency QPO types). Their frequencies increase from ∼0.01 Hz to ∼10 Hz as the spectrum softens. At some point during the transition the type-C QPOs and arXiv:2003.01012v1 [astro-ph.HE]

We present Zwicky Transient Facility (ZTF) observations of the tidal disruption flare AT2018zr/PS18kh reported by Holoien et al. and detected during ZTF commissioning. The ZTF light curve of the tidal disruption event (TDE) samples the rise-to-peak exceptionally well, with 50 days of g-and r-band detections before the time of maximum light. We also present our multi-wavelength 2 van Velzen et al.follow-up observations, including the detection of a thermal (kT ≈ 100 eV) X-ray source that is two orders of magnitude fainter than the contemporaneous optical/UV blackbody luminosity, and a stringent upper limit to the radio emission. We use observations of 128 known active galactic nuclei (AGN) to assess the quality of the ZTF astrometry, finding a median host-flare distance of 0. 2 for genuine nuclear flares. Using ZTF observations of variability from known AGN and supernovae we show how these sources can be separated from TDEs. A combination of light-curve shape, color, and location in the host galaxy can be used to select a clean TDE sample from multi-band optical surveys such as ZTF or LSST.

The candidate black hole X-ray binary Swift J1753.5−0127 faded to quiescence in 2016 November after a prolonged outburst that was discovered in 2005. Nearly three months later, the system displayed renewed activity that lasted through 2017 July. Here, we present radio and X-ray monitoring over 3 » months of the renewed activity to study the coupling between the jet and the inner regions of the disk/jet system. Our observations cover low X-ray luminosities that have not historically been well-sampled (L 2 10 10 erg s ; X 33 36 1»´--1-10 keV), including time periods when the system was both brightening and fading. At these low luminosities, Swift J1753.5 −0127 occupies a parameter space in the radio/X-ray luminosity plane that is comparable to "canonical" systems (e.g., GX 339−4), regardless of whether the system was brightening or fading, even though during its 11year outburst, Swift J1753.5−0127 emitted less radio emission from its jet than expected. We discuss implications for the existence of a single radio/X-ray luminosity correlation for black hole X-ray binaries at the lowest luminosities (L 10 erg s X 35 1  -), and we compare to supermassive black holes. Our campaign includes the lowest luminosity quasi-simultaneous radio/X-ray detection to date for a black hole X-ray binary during its rise out of quiescence, thanks to early notification from optical monitoring combined with fast responses from sensitive multiwavelength facilities.