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Register a new userEVLA Nova Project Observations of the Classical Nova V1723 Aquilae
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We present radio light curves and spectra of the classical nova V1723 Aql obtained with the Expanded Very Large Array (EVLA). This is the first paper to showcase results from the EVLA Nova Project, which comprises a team of observers and theorists utilizing the greatly enhanced sensitivity and frequency coverage of EVLA radio observations, along with observations at other wavelengths, to reach a deeper understanding of the energetics, morphology, and temporal characteristics of nova explosions. Our observations of V1723 Aql span 1-37 GHz in frequency, and we report on data from 14-175 days following the time of the nova explosion. The broad frequency coverage and frequent monitoring show that the radio behavior of V1723 Aql does not follow the classic Hubble-flow model of homologous spherically expanding thermal ejecta. The spectra are always at least partially optically thin, and the flux rises on faster timescales than can be reproduced with linear expansion. Therefore, any description of the underlying physical processes must go beyond this simple picture. The unusual spectral properties and light curve evolution might be explained by multiple emitting regions or shocked material. Indeed, X-ray observations from Swift reveal that shocks are likely present.
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The radio light curves of novae rise and fall over the course of months to years, allowing for detailed observations of the evolution of the nova shell. However, the main parameter determined by radio models of nova explosions - the mass of the ejecta - often seems to exceed theoretical expectations by an order of magnitude. With the recent technological improvements on the Karl G. Jansky Very Large Array (VLA), new observations can test the assumptions upon which ejecta mass estimates are based. Early observations of the classical nova V1723 Aql showed an unexpectedly rapid rise in radio flux density and a distinct bump in the radio light curve on the rise to radio maximum, which is inconsistent with the simple model of spherical ejecta expelled in a single discrete event. This initial bump appears to indicate the presence of shocked material in the outer region of the ejected shell, with the emission from the shocks fading over time. We explore possible origins for this emission and its relation to the mass loss history of the nova. The evolution of the radio spectrum also reveals the density profile, the mass of the ejected shell, and other properties of the ejecta. These observations comprise one of the most complete, longterm set of multi-wavelength radio observations for any classical nova to date.
Near Infrared (NIR) and optical photometry and spectroscopy are presented for the nova V1831 Aquilae, covering the early decline and dust forming phases during the first $sim$90 days after its discovery. The nova is highly reddened due to interstellar extinction. Based solely on the nature of NIR spectrum we are able to classify the nova to be of the Fe II class. The distance and extinction to the nova are estimated to be 6.1 $pm$ 0.5 kpc and $A_{rm v}$ $sim$ 9.02 respectively. Lower limits of the electron density, emission measure and ionized ejecta mass are made from a Case B analysis of the NIR Brackett lines while the neutral gas mass is estimated from the optical [OI] lines. We discuss the cause for a rapid strengthening of the He I 1.0830 $mu$m line during the early stages. V1831 Aql formed a modest amount of dust fairly early ($sim$ 19.2 days after discovery); the dust shell is not seen to be optically thick. Estimates are made of the dust temperature, dust mass and grain size. Dust formation commences around day 19.2 at a condensation temperature of 1461 $pm$ 15 K, suggestive of a carbon composition, following which the temperature is seen to gradually decrease to 950K. The dust mass shows a rapid initial increase which we interpret as being due to an increase in the number of grains, followed by a period of constancy suggesting the absence of grain destruction processes during this latter time. A discussion is made of the evolution of these parameters, including certain peculiarities seen in the grain radius evolution.
The importance of shocks in nova explosions has been highlighted by Fermis discovery of gamma-ray producing novae. Over three years of multi-band VLA radio observations of the 2010 nova V1723 Aql show that shocks between fast and slow flows within the ejecta led to the acceleration of particles and the production of synchrotron radiation. Soon after the start of the eruption, shocks in the ejecta produced an unexpected radio flare, resulting in a multi-peaked radio light curve. The emission eventually became consistent with an expanding thermal remnant with mass $2 times 10^{-4} M_odot$ and temperature $10^4$ K. However, during the first two months, the $gtrsim 10^6$ K brightness temperature at low frequencies was too high to be due to thermal emission from the small amount of X-ray producing shock-heated gas. Radio imaging showed structures with velocities of 400 km s$^{-1}$ (d/6 kpc) in the plane of the sky, perpendicular to a more elongated 1500 km s$^{-1}$ (d/6 kpc) flow. The morpho-kinematic structure of the ejecta from V1723 Aql appears similar to nova V959 Mon, where collisions between a slow torus and a faster flow collimated the fast flow and gave rise to gamma -ray producing shocks. Optical spectroscopy and X-ray observations of V1723 Aql during the radio flare are consistent with this picture. Our observations support the idea that shocks in novae occur when a fast flow collides with a slow collimating torus. Such shocks could be responsible for hard X-ray emission, gamma -ray production, and double-peaked radio light curves from some classical novae.
We report on our Chandra and RXTE observations of the bright old nova, V603 Aql, performed in 2001 April, supplemented by our analysis of archival X-ray data on this object. We find that the RXTE data are contaminated by the Galactic Ridge X-ray emission. After accounting for this effect, we find a high level of aperiodic variability in the RXTE data, at a level consistent with the uncontaminated Chandra data. The Chandra HETG spectrum clearly originates in a multi-temperature plasma. We constrain the possible emission measure distribution of the plasma through a combination of global and local fits. The X-ray luminosity and the spectral shape of V603 Aql resemble those of SS Cyg in transition between quiescence and outburst. The fact that the X-ray flux variability is only weakly energy dependent can be interpreted by supposing that the variability is due to changes in the maximum temperature of the plasma. The plasma density is likely to be high, and the emission region is likely to be compact. Finally, the apparent overabundance of Ne is consistent with V603 Aql being a young system.
Low- and medium resolution spectra of the fast nova, Nova (V1494) Aql 1999 No.2 obtained approximately 6, 7, 19 and 28 days after the maximum brightness are presented and discussed. The spectrum covering the whole optical range at day 6 shows the principal plus diffuse-enhanced spectrum. The presence of strong Fe II multiplets with P-Cyg profiles suggest that V1494 Aql belongs to the ``Fe II class defined by Williams (1992). The medium-resolution profiles (lambda/Delta lambda approx 7000) of the Hgamma and Hdelta lines show well-defined sharp absorption features with the same radial velocities, while the Halpha split into two distinct emission peaks in the last two spectra (Delta t=19 and 28 days). The observed behaviour suggests an expanding equatorial ring with possible small-scale clumpiness in the nova shell. The visual lightcurve is used to deduce M_V by the maximum magnitude versus rate of decline relationship. The resulting parameters are: t_2=6.6+/-0.5 days, t_3=16+/-0.5 days, M_V=-8.8+/-0.2 mag. Adopting this value, a distance d=3.6+/-0.3 kpc is determined.
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