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Title: NuSTAR observations of the Dwarf Nova GK Persei in 2015: comparison between outburst and quiescent phases
Author: Yuuki Wada, Takayuki Yuasa, Kazuhiro Nakazawa, Kazuo Makishima, Takayuki Hayashi, Manabu Ishida

We report on NuSTAR observations of the Intermediate Polar GK Persei which also behaves as a Dwarf Nova. It exhibited a Dwarf Nova outburst in 2015 March-April. The object was observed in 3-79 keV X-rays with NuSTAR, once at the outburst peak, and again in 2015 September during quiescence. The 5-50 keV flux during the outburst was 26 times higher than that during the quiescence. With a multi-temperature emission model and a reflection model, we derived the post-shock temperature as 19.2 +/- 0.7 keV in the outburst, and 38.5 +4.1/-3.6 keV in the quiescence. This temperature difference is considered to reflect changes in the radius at which the accreting matter, forming an accretion disk, is captured by the magnetosphere of the white dwarf (WD). Assuming that this radius scales as the power of -2/7 of the mass accretion rate, and utilizing the two temperature measurements, as well as the standard mass-radius relation of WDs, we determined the WD mass in GK Persei as 0.90 +/- 0.06 solar masses. The magnetic field is estimated as 4*10^5 G.

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ATel 7244: INTEGRAL hard X-ray observation of the nova GK Per during its 2015 outburst



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GK Persei
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GK Persei: "Mini Supernova" Explosion Could Have Big Impact

A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a "classical nova," an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star.
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ATel 7217: Optical Spectroscopy of GK Per (Nova Per 1901) during the 2015 Outburst



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Spoiler



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(Nova) GK Persei has brightened to magnitude 12.3, which indicates it may be going into outburst.
The Variable star reached magnitude 9.7 in March 2010.



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Nova Persei 1901
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Nova Persei 1901  (now known as GK Per) was discovered by Scottish clergyman and amateur astronomer Dr. Thomas David Anderson 'with neither binocular nor telescope' at 21 East Claremont Street in Edinburgh, at 02.40 UT, 22nd February 1901 .



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A 3D View of the Remnant of Nova Persei 1901



GK Per (Nova Persei 1901) nova remnant is the result of a remarkable nearby nova exploded in 1901 which reached the brightness of the star Vega at its maximum. It was the first object around which, superluminal light echoes where observed. The actual ejecta of the outburst became visible 15 years later in 1916. Ever since, the ejecta has been monitored in several wavelengths in narrow- and broad-band filters.
This study, conducted by astronomers from Spain and Estonia, shows that the nova ejecta is a thick knotty shell in which knots expand with a significant range of velocities, mostly between 600 and 1000 km/s.

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Title: A 3D view of the remnant of Nova Persei 1901 (GK Per)
Authors: T. Liimets, R. L. M. Corradi, M. Santander-García, E. Villaver, P. Rodríguez-Gil, K. Verro, I. Kolka

We present a kinematical study of the optical ejecta of GK Per. It is based on proper motions measurements of 282 knots from ~20 images spanning 25 years. Doppler-shifts are also computed for 217 knots. The combination of proper motions and radial velocities allows a unique 3-D view of the ejecta to be obtained. The main results are: (1) the outflow is a thick shell in which knots expand with a significant range of velocities, mostly between 600 and 1000 km/s; (2) kinematical ages indicate that knots have suffered only a modest deceleration since their ejection a century ago; (3) no evidence for anisotropy in the expansion rate is found; (4) velocity vectors are generally aligned along the radial direction but a symmetric pattern of non-radial velocities is also observed at specific directions; (5) the total Halpha+[NII] flux has been linearly decreasing at a rate of 2.6 % per year in the last decade. The Eastern nebular side is fading at a slower rate than the Western one. Some of the knots displayed a rapid change of brightness during the 2004-2011 period. Over a longer timescale, a progressive circularisation and homogenisation of the nebula is taking place; (6) a kinematic distance of 400±30 pc is determined. These results raise some problems to the previous interpretations of the evolution of GK Per. In particular, the idea of a strong interaction of the outflow with the surrounding medium in the Southwest quadrant is not supported by our data.

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Title: GK Per (Nova Persei 1901): HST Imagery and Spectroscopy of the Ejecta, and First Spectrum of the Jet-Like Feature
Authors: Michael M. Shara, David Zurek, Orsola De Marco, Trisha Mizusawa, Robert Williams, Mario Livio

We have imaged the ejecta of GK Persei (Nova Persei 1901 A.D.) with the Hubble Space Telescope (HST), revealing hundreds of cometary-like structures. One or both ends of the structures often show a brightness enhancement relative to the structures' middle sections, but there is no simple regularity to their morphologies (in contrast with the Helix nebula). Some of the structures' morphologies suggest the presence of slow-moving or stationary material with which the ejecta is colliding, while others suggest shaping from a wind emanating from GK Per itself. A detailed expansion map of the nova's ejecta was created by comparing HST images taken in successive years. WFPC2 narrowband images and STIS spectra demonstrate that the physical conditions in the ejecta vary strongly on spatial scales much smaller than those of the ejecta. Directly measuring accurate densities and compositions, and hence masses of this and other nova shells, will demand data at least as resolved spatially as those presented here. The filling factor the ejecta is < 1%, and the nova ejecta mass must be less than 10^{-4} solar masses. A few of the nebulosities vary in brightness by up to a factor of two on timescales of one year. Finally, we present the deepest images yet obtained of a jet-like feature outside the main body of GK Per nebulosity, and the first spectrum of that feature. Dominated by strong, narrow emission lines of [NII], [OII], [OIII], and [SII], this feature is probably a shock due to ejected material running into stationary ISM, slowly moving ejecta from a previous nova episode, or circum-binary matter present before 1901. An upper limit to the mass of the jet is of order a few times 10^{-6} solar masses. The jet might be an important, or even dominant mass sink from the binary system. The jet's faintness suggests that similar features could easily have been missed in other cataclysmic binaries.

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