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Register a new userCollimation and asymmetry of the hot blast wave from the recurrent nova V745 Scorpii
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The recurrent symbiotic nova V745 Sco exploded on 2014 February 6 and was observed on February 22 and 23 by the Chandra X-ray Observatory Transmission Grating Spectrometers. By that time the supersoft source phase had already ended and Chandra spectra are consistent with emission from a hot, shock-heated circumstellar medium with temperatures exceeding 10^7K. X-ray line profiles are more sharply peaked than expected for a spherically-symmetric blast wave, with a full width at zero intensity of approximately 2400 km/s, a full width at half maximum of 1200 +/- 30 km/s and an average net blueshift of 165 +/- 10 km/s. The red wings of lines are increasingly absorbed toward longer wavelengths by material within the remnant. We conclude that the blast wave was sculpted by an aspherical circumstellar medium in which an equatorial density enhancement plays a role, as in earlier symbiotic nova explosions. Expansion of the dominant X-ray emitting material is aligned close to the plane of the sky and most consistent with an orbit seen close to face-on. Comparison of an analytical blast wave model with the X-ray spectra, Swift observations and near-infrared line widths indicates the explosion energy was approximately 10^43 erg, and confirms an ejected mass of approximately 10^-7 Msun. The total mass lost is an order of magnitude lower than the accreted mass required to have initiated the explosion, indicating the white dwarf is gaining mass and is a supernova Type 1a progenitor candidate.
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The recurrent nova (RN) V745 Scorpii underwent its third known outburst on 2014 February 6. Infrared monitoring of the eruption on an almost daily basis, starting from 1.3d after discovery, shows the emergence of a powerful blast wave generated by the high velocity nova ejecta exceeding 4000 kms$^{-1}$ plowing into its surrounding environment. The temperature of the shocked gas is raised to a high value exceeding 10$^{8}$K immediately after outburst commencement. The energetics of the outburst clearly surpass those of similar symbiotic systems like RS Oph and V407 Cyg which have giant secondaries. The shock does not show a free-expansion stage but rather shows a decelerative Sedov-Taylor phase from the beginning. Such strong shockfronts are known to be sites for $gamma$ ray generation. V745 Sco is the latest nova, apart from five other known novae, to show $gamma$ ray emission. It may be an important testbed to resolve the crucial question whether all novae are generically $gamma$ ray emitters by virtue of having a circumbinary reservoir of material that is shocked by the ejecta rather than $gamma$ ray generation being restricted to only symbiotic systems with a shocked red giant (RG) wind. The lack of a free-expansion stage favors V745 Sco to have a density enhancement around the white dwarf (WD), above that contributed by a RG wind. Our analysis also suggests that the WD in V745 Sco is very massive and a potential progenitor for a future SN Ia explosion.
Near-IR spectroscopy is presented for Nova Scorpii 2014. It is shown that the outburst occurred in a symbiotic binary system - an extremely rare configuration for a classical nova outburst to occur in but appropriate for the eruption of a recurrent nova of the T CrB class. We estimate the spectral class of secondary as M5III $pm$ (two sub-classes). The maximum magnitude versus rate of decline (MMRD) relations give an unacceptably large value of 37.5 kpc for the distance. The spectra are typical of the He/N class of novae with strong HeI and H lines. The profiles are broad and flat topped with full width at zero intensities (FWZIs) approaching 9000-10000 km s$^{-1}$ and also have a sharp narrow component superposed which is attributable to emission from the giants wind. Hot shocked gas, accompanied by X-rays and $gamma$ rays, is expected to form when the high velocity ejecta from the nova plows into the surrounding giant wind. Although X-ray emission was observed no $gamma$-ray emission was reported. It is also puzzling that no signature of a decelerating shock is seen in the near-infrared (NIR), seen in similar systems like RS Oph, V745 Sco and V407 Cyg, as rapid narrowing of the line profiles. The small outburst amplitude and the giant secondary strongly suggest that Nova Sco 2014 could be a recurrent nova.
VLT and SALT spectroscopy of U Sco were obtained $sim$18 and $sim$30 months after the 2010 outburst. From these spectra the accretion disc is shown to take at least 18 months to become fully reformed. The spectral class of the companion is constrained to be F8$^{+5}_{-6}$,IV-V at the 95% confidence level when the irradiated face of the companion is visible.
Stellar explosions such as novae and supernovae produce most of the heavy elements in the Universe. Although the onset of novae from runaway thermonuclear fusion reactions on the surface of a white dwarf in a binary star system is understood[1], the structure, dynamics, and mass of the ejecta are not well known. In rare cases, the white dwarf is embedded in the wind nebula of a red-giant companion; the explosion products plow through the nebula and produce X-ray emission. Early this year, an eruption of the recurrent nova RS Ophiuchi[2,3] provided the first opportunity to perform comprehensive X-ray observations of such an event and diagnose conditions within the ejecta. Here we show that the hard X-ray emission from RS Ophiuchi early in the eruption emanates from behind a blast wave, or outward-moving shock wave, that expanded freely for less than 2 days and then decelerated due to interaction with the nebula. The X-rays faded rapidly, suggesting that the blast wave deviates from the standard spherical shell structure[4-6]. The early onset of deceleration indicates that the ejected shell had a low mass, the white dwarf has a high mass[7], and that RS Ophiuchi is a progenitor of the type of supernova integral to studies of the expansion of the universe.
The eruption of the recurrent nova U Scorpii on 28 January 2010 is now the all-time best observed nova event. We report 36,776 magnitudes throughout its 67 day eruption, for an average of one measure every 2.6 minutes. This unique and unprecedented coverage is the first time that a nova has any substantial amount of fast photometry. With this, two new phenomena have been discovered: the fast flares in the early light curve seen from days 9-15 (which have no proposed explanation) and the optical dips seen out of eclipse from days 41-61 (likely caused by raised rims of the accretion disk occulting the bright inner regions of the disk as seen over specific orbital phases). The expanding shell and wind cleared enough from days 12-15 so that the inner binary system became visible, resulting in the sudden onset of eclipses and the turn-on of the supersoft X-ray source. On day 15, a strong asymmetry in the out-of-eclipse light points to the existence of the accretion stream. The normal optical flickering restarts on day 24.5. For days 15-26, eclipse mapping shows that the optical source is spherically symmetric with a radius of 4.1 R_sun. For days 26-41, the optical light is coming from a rim-bright disk of radius 3.4 R_sun. For days 41-67, the optical source is a center-bright disk of radius 2.2 R_sun. Throughout the eruption, the colors remain essentially constant. We present 12 eclipse times during eruption plus five just after the eruption.
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