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Register a new userNear-IR studies of recurrent nova V745 Scorpii during its 2014 outburst
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Dipankar P. K. Banerjee Dr
Publication date
2014
fields
Physics
and research's language is
English
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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.
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We present near-IR observations of the 2010 outburst of U Sco. JHK photometry is presented on ten consecutive days starting from 0.59 days after outburst. Such photometry can gainfully be integrated into a larger database of other multi-wavelength data which aim to comprehensively study the evolution of U Sco. Early near-IR spectra, starting from 0.56 days after outburst, are presented and their general characteristics discussed. Early in the eruption, we see very broad wings in several spectral lines, with tails extending up to ~10000km/s along the line of sight; it is unexpected to have a nova with ejection velocities equal to those usually thought to be exclusive to supernovae. From recombination analysis, we estimate an upper limit of 10^-4.64[+0.92.-0.74]Msun for the ejected mass.
We present multi-epoch near-infrared photo-spectroscopic observations of Nova Cephei 2014 and Nova Scorpii 2015, discovered in outburst on 2014 March 8.79 UT and 2015 February 11.84 UT respectively. Nova Cep 2014 shows the conventional NIR characteristics of a Fe II class nova characterized by strong CI, HI and O I lines, whereas Nova Sco 2015 is shown to belong to the He/N class with strong He I, HI and OI emission lines. The highlight of the results consists in demonstrating that Nova Sco 2015 is a symbiotic system containing a giant secondary. Leaving aside the T CrB class of recurrent novae, all of which have giant donors, Nova Sco 2015 is shown to be only the third classical nova to be found with a giant secondary. The evidence for the symbiotic nature is three-fold; first is the presence of a strong decelerative shock accompanying the passage of the novas ejecta through the giants wind, second is the H$alpha$ excess seen from the system and third is the spectral energy distribution of the secondary in quiescence typical of a cool late type giant. The evolution of the strength and shape of the emission line profiles shows that the ejecta velocity follows a power law decay with time ($t^{-1.13 pm 0.17}$). A Case B recombination analysis of the H I Brackett lines shows that these lines are affected by optical depth effects for both the novae. Using this analysis we make estimates for both the novae of the emission measure $n_e^2L$, the electron density $n_e$ and the mass of the ejecta.
We present optical and near-IR linear polarimetry of V404 Cyg during its 2015 outburst and in quiescence. We obtained time resolved r-band polarimetry when the source was in outburst, near-IR polarimetry when the source was near quiescence and multiple wave-band optical polarimetry later in quiescence. The optical to near-IR linear polarization spectrum can be described by interstellar dust and an intrinsic variable component. The intrinsic optical polarization, detected during the rise of one of the brightest flares of the outburst, is variable, peaking at 4.5 per cent and decaying to 3.5 per cent. We present several arguments that favour a synchrotron jet origin to this variable polarization, with the optical emission originating close to the jet base. The polarization flare occurs during the initial rise of a major radio flare event that peaks later, and is consistent with a classically evolving synchrotron flare from an ejection event. We conclude that the optical polarization flare represents a jet launching event; the birth of a major ejection. For this event we measure a rather stable polarization position angle of -9 degrees E of N, implying that the magnetic field near the base of the jet is approximately perpendicular to the jet axis. This may be due to the compression of magnetic field lines in shocks in the accelerated plasma, resulting in a partially ordered transverse field that have now been seen during the 2015 outburst. We also find that this ejection occurred at a similar stage in the repetitive cycles of flares.
Two long AstroSat Soft X-ray Telescope observations were taken of the third recorded outburst of the Symbiotic Recurrent Nova, V3890 Sgr. The first observing run, 8.1-9.9 days after the outburst, initially showed a stable intensity level with a hard X-ray spectrum that we attribute to shocks between the nova ejecta and the pre-existing stellar companion. On day 8.57, the first, weak, signs appeared of Super Soft Source (SSS) emission powered by residual burning on the surface of the White Dwarf. The SSS emission was observed to be highly variable on time scales of hours. After day 8.9, the SSS component was more stable and brighter. In the second observing run, on days 15.9-19.6 after the outburst, the SSS component was even brighter but still highly variable. The SSS emission was observed to fade significantly during days 16.8-17.8 followed by re-brightening. Meanwhile the shock component was stable leading to increase in hardness ratio during the period of fading. AstroSat and XMM-Newton observations have been used to study the spectral properties of V3890 Sgr to draw quantitative conclusions even if their drawback is model-dependence. We used the xspec to fit spectral models of plasma emission, and the best fits are consistent with the elemental abundances being lower during the second observing run compared to the first for spectra >1 keV. The SSS emission is well fit by non-local thermal equilibrium model atmosphere used for white dwarfs. The resulting spectral parameters, however, are subject to systematic uncertainties such as completeness of atomic data.
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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