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Title: The 2011 Outburst of Recurrent Nova T Pyx: X-ray Observations Expose the White Dwarf Mass and Ejection Dynamics
Author: Laura Chomiuk, Thomas Nelson, Koji Mukai, J. L. Sokoloski, Michael P. Rupen, Kim L. Page, Julian P. Osborne, Erik Kuulkers, Amy J. Mioduszewski, Nirupam Roy, Jennifer Weston, Miriam I. Krauss

The recurrent nova T Pyx underwent its sixth historical outburst in 2011, and became the subject of an intensive multi-wavelength observational campaign. We analyze data from the Swift and Suzaku satellites to produce a detailed X-ray light curve augmented by epochs of spectral information. X-ray observations yield mostly non-detections in the first four months of outburst, but both a super-soft and hard X-ray component rise rapidly after Day 115. The super-soft X-ray component, attributable to the photosphere of the nuclear-burning white dwarf, is relatively cool (~45 eV) and implies that the white dwarf in T Pyx is significantly below the Chandrasekhar mass (~1 M_sun). The late turn-on time of the super-soft component yields a large nova ejecta mass (>~10^-5 M_sun), consistent with estimates at other wavelengths. The hard X-ray component is well fit by a ~1 keV thermal plasma, and is attributed to shocks internal to the 2011 nova ejecta. The presence of a strong oxygen line in this thermal plasma on Day 194 requires a significantly super-solar abundance of oxygen and implies that the ejecta are polluted by white dwarf material. The X-ray light curve can be explained by a dual-phase ejection, with a significant delay between the first and second ejection phases, and the second ejection finally released two months after outburst. A delayed ejection is consistent with optical and radio observations of T Pyx, but the physical mechanism producing such a delay remains a mystery.

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Title: The 2011 Outburst of Recurrent Nova T Pyx: Radio Observations Reveal the Ejecta Mass and Hint at Complex Mass Loss
Author: Thomas Nelson, Laura Chomiuk, Nirupam Roy, J. L. Sokoloski, Koji Mukai, Miriam I. Krauss, Amy J. Mioduszewski, Michael P. Rupen, Jennifer Weston

Despite being the prototype of its class, T Pyx is arguably the most unusual and poorly understood recurrent nova. Here, we use radio observations from the Karl G. Jansky Very Large Array to trace the evolution of the ejecta over the course of the 2011 outburst of T Pyx. The radio emission is broadly consistent with thermal emission from the nova ejecta. However, the radio flux began rising surprisingly late in the outburst, indicating that the bulk of the radio-emitting material was either very cold, or expanding very slowly, for the first ~50 days of the outburst. Considering a plausible range of volume filling factors and geometries for the ejecta, we find that the high peak flux densities of the radio emission require a massive ejection of 1-30 x 10^{-5} solar masses. This ejecta mass is much higher than the values normally associated with recurrent novae, and is more consistent with a nova on a white dwarf well below the Chandrasekhar limit.

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Hubble Maps 3-D Structure of Ejected Material Around Erupting Star

After 45 years of peaceful bliss, the nova T Pyxidis erupted again in 2011. Astronomers took advantage of a flash of light accompanying the blast to map the ejecta from previous outbursts surrounding the double-star system. The team used NASA's Hubble Space Telescope to trace the light as it sequentially illuminated different parts of the disk, a phenomenon called a light echo. Contrary to some predictions, the astronomers were somewhat surprised to find that the ejecta stayed in the vicinity of the star and formed a disk of debris. The discovery suggests that material continues expanding outward along the system's orbital plane, but it does not escape the system.
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Title: The Recurrent Nova T Pyx: Distance and Remnant Geometry from Light Echoes
Authors: J. L. Sokoloski (1), Arlin P. S. Crotts (1), Helena Uthas (1), Stephen Lawrence (2) ((1) Columbia, (2) Hofstra)

The recurrent nova T Pyxidis (T Pyx) is well known for its small binary separation, its unusually high luminosity in quiescence, and the spectacular Hubble Space Telescope (HST) images of its surrounding remnant. In 2011 April, T Pyx erupted for the first time since 1966. Here we describe HST observations in late 2011 of a transient reflection nebula around the erupting white dwarf (WD). Our observations of this light echo in the pre-existing remnant show that it is dominated by a clumpy ring with a radius of about 5", and an inclination of 30 to 40 degrees, with the eastern edge tilted toward the observer. The delay times between the direct optical light from the central source, and the scattering of this light from dust in several clumps with the same foreground distance as the central source, give a distance to T Pyx of 4.8 ± 0.5 kpc. Given past evidence from two-dimensional optical spectra that the remnant contains a shell-like component, it must actually consist of a ring embedded within a quasi-spherical shell. The large distance of 4.8 kpc supports the contention that T Pyx has an extraordinarily high rate of mass transfer in quiescence, and thus that nova explosions themselves can enhance mass loss from a donor star, and reduce the time between eruptions in a close binary.

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Title: The mystery of T Pyx; the 2011 explosion
Authors: A. Ederoclite

T Pyx is a recurrent nova which has undergone eruptions on an almost regular basis every 20 years until reaching a long lasting quiescence between 1967 and 2011. We observed the long awaited 2011 explosion in the optical and near infrared with intermediate spectral resolution. In this paper we report on the change in the spectral type of the nova (both during its rise and during its fading), as well as the observed changes in the expansion velocities. We also present an interpretation of these changes and set them in the general framework of a new understanding of nova classification.

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Title: The Death Spiral of T Pyxidis
Authors: Joseph Patterson (1), Arto Oksanen (2), Berto Monard (2), Robert Rea (2), Franz-Josef Hambsch (2), Jennie McCormick (2), Peter Nelson (2), Jonathan Kemp (1), William Allen (2), Thomas Krajci (2), Simon Lowther (2), Shawn Dvorak (2), Thomas Richards (2), Gordon Myers (2), Greg Bolt (2) ((1) Columbia Univ., (2) Center for Backyard Astrophysics)

We report a long campaign to track the 1.8 hr photometric wave in the recurrent nova T Pyxidis, using the global telescope network of the Centre for Backyard Astrophysics. During 1996-2011, that wave was highly stable in amplitude and waveform, resembling the orbital wave commonly seen in supersoft binaries. The period, however, was found to increase on a timescale P/P-dot=3x10^5 yr. This suggests a mass transfer rate of ~10^-7 M_sol/yr in quiescence. The orbital signal became vanishingly weak (<0.003 mag) near maximum light of the 2011 eruption. After it returned to visibility near V=11, the orbital period had increased by 0.0054(6) %. This is a measure of the mass ejected in the nova outburst. For a plausible choice of binary parameters, that mass is at least 3x10^-5 M_sol, and probably more. This represents >300 yr of accretion at the pre-outburst rate, but the time between outbursts was only 45 yr. Thus the erupting white dwarf seems to have ejected at least 6x more mass than it accreted. If this eruption is typical, the white dwarf must be eroding, rather than growing, in mass -- dashing the star's hopes of ever becoming famous via a supernova explosion. Instead, it seems likely that the binary dynamics are basically a suicide pact between the eroding white dwarf and the low-mass secondary, excited and rapidly whittled down, probably by the white dwarf's EUV radiation.

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Title: The spectroscopic evolution of the recurrent nova T Pyxidis during its 2011 outburst. II.The optically thin phase and the structure of the ejecta in recurrent novae
Authors: S. N. Shore (Univ. of Pisa and INFN-Pisa), G. J. Schwarz (AAS), I. De Gennaro Aquino (Univ. of Pisa), T. Augusteijn (NOT), F. M. Walter (SUNY - SB), S. Starrfield (ASU), E. M. Sion (Villanova)

We continue our study of the physical properties of the recurrent nova T Pyx, focussing on the structure of the ejecta in the nebular stage of expansion during the 2011 outburst. The nova was observed contemporaneously with the Nordic Optical Telescope (NOT), at high resolution spectroscopic resolution (R ~ 65000) on 2011 Oct. 11 and 2012 Apr. 8 (without absolute flux calibration), and with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope, at high resolution (R ~ 30000) on 2011 Oct. 10 and 2012 Mar. 28 (absolute fluxes). We use standard plasma diagnostics (e.g. [O III] and [N II] line ratios and the H \beta line fluxes) to constrain electron densities and temperatures. Using Monte Carlo modelling of the ejecta, we derive the structure and filling factor from comparisons to the optical and ultraviolet line profiles. The ejecta can be modelled using an axisymmetric conical -- bipolar -- geometry with a low inclination of the axis to the line of sight, i=15±5 degrees, compatible with published results from high angular resolution optical spectro-interferometry. The structure is similar to that observed in the other short orbital period recurrent novae during their nebular stages. We show that the electron density scales as t^{-3} as expected from a ballistically ejected constant mass shell; there is no need to invoke a continuing mass outflow following the eruption. The derived mass for the ejecta with filling factor f ~ 3%, M_ej ~ 2E-6 solar masses is similar to that obtained for other recurrent nova ejecta but inconsistent with the previously reported extended optically thick epoch of the explosion. We suggest that the system underwent a common envelope phase following the explosion that produced the recombination event. Implications for the dynamics of the recurrent novae are discussed.

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Title: Discovery of an X-ray Emitting Nebula around the Recurrent Nova T Pyxidis
Authors: Solen Balman (METU)

I resolved and detected an X-ray nebulosity around the recurrent nova T Pyx using a 98.8 ksec observation with the ACIS-S detector on-board the Chandra Observatory during the quiescent phase of the nova before its outburst in 2011. The nebula shows an elliptical shape with an inner semi-major axis \sim 0.45 arc sec and an outer semi-major axis \sim 0.9 arc sec which indicates a torus-like or a ring-like shell structure around the nova. There is also a (conical) elongation towards the southern direction of about 1.85 arc sec. This structure may be part of a bipolar outflow from the source/nova. The count rate of the nebulosity is 0.0025 ±0.0010 c s^{-1} and that of the central binary is \sim 0.003 c s^{-1} over the 0.2-9.0 keV energy range. The best fitted spectrum of the X-ray nebula is a two-component plasma model (e.g., a double MEKAL) with \sim 0.6 keV and \sim 2.2 keV along with two different neutral hydrogen column densities of (0.2-0.9) x 10^{22} cm^{-2} and (3.0-26.0) x 10^{22} cm^{-2} for the two temperatures, respectively. I calculate an absorbed X-ray flux of (0.6-10.0) x 10^{-14} erg cm^{-2} s^{-1} with a luminosity of (0.08-2.0) x 10^{32} erg s^{-1} (at 3.5 kpc) for the X-ray nebula. The estimated shocked mass is \le1.8 x 10^{-5} solar masses. The central source spectrum can be fitted by a single MEKAL model with a temperature 9.2^{<}_{-5.4} keV yielding a luminosity of about 5.2 x 10^{31} erg s^{-1}. The orbital period of the system is detected in the Chandra light curve.

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Title: The 2011 outburst of the recurrent novaT Pyx. Evidence for a face-on bipolar ejection
Authors: Olivier Chesneau (FIZEAU), A. Meilland (FIZEAU), D. P. K. Banerjee (PRL), Jean-Baptiste Le Bouquin (IPAG), H. A. Mcalister (CHARA), F. Millour (FIZEAU), S.T. Ridgway (NOAO), A. Spang (FIZEAU), T.A. Ten Brummelaar (CHARA), M. Wittkowski (ESO), N. M. Ashok (PRL), M. Benisty (MPIA), J.-P. Berger (ESO), T. S. Boyajian, C. Farrington (CHARA), P.J. Goldfinger (CHARA), A. Mérand (ESO), N. Nardetto (FIZEAU), R. Petrov (FIZEAU), T. Rivinius (ESO), G. Schaefer, G. Zins (IPAG)

We report on near-IR interferometric observations of the outburst of the recurrent nova T Pyx. We obtained near-IR observations of T Pyx at dates ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC recombiner, located at the CHARA array, and with the PIONIER and AMBER recombiners, located at the VLTI array. These data are supplemented with near-IR photometry and spectra obtained at Mount Abu, India. Slow expansion velocities were measured (\leq 300km/s) before t=20d (assuming D=3.5kpc). From t=28d on, the AMBER and PIONIER continuum visibilities (K and H band, respectively) are best simulated with a two component model consisting of an unresolved source plus an extended source whose expansion velocity onto the sky plane is lower than 700km/s. The expansion of the Brgamma line forming region, as inferred at t=28d and t=35d is slightly larger, implying velocities in the range 500-800km/s, still strikingly lower than the velocities of 1300-1600km/s inferred from the Doppler width of the line. Moreover, a remarkable pattern was observed in the Brgamma differential phases. A semi-quantitative model using a bipolar flow with a contrast of 2 between the pole and equator velocities, an inclination of i=15 degrees and a position angle P.A.=110 degrees provides a good match to the AMBER observables (spectra, differential visibilities and phases). At t=48d, a PIONIER dataset confirms the two component nature of the H band emission, consisting of an unresolved stellar source and an extended region whose appearance is circular and symmetric within error bars. These observations are most simply interpreted within the frame of a bipolar model, oriented nearly face-on. This finding has profound implications for the interpretation of past, current and future observations of the expanding nebula.

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Title: The 2011 Eruption of the Recurrent Nova T Pyxidis; the Discovery, the Pre-eruption Rise, the Pre-eruption Orbital Period, and the Reason for the Long Delay
Authors: Bradley E. Schaefer, Arlo U. Landolt, Michael Linnolt, Rod Stubbings, Grzegorz Pojmanski, Alan Plummer, Stephen Kerr, Peter Nelson, Rolf Carstens, Margaret Streamer, Thomas Richards, Gordon Myers, William G. Dillon

We report the discovery by M. Linnolt on JD 2455665.7931 (UT 2011 April 14.29) of the sixth eruption of the recurrent nova T Pyxidis. This discovery was made just as the initial fast rise was starting, so with fast notification and response by observers worldwide, the entire initial rise was covered (the first for any nova) with fine time resolution and in three filters. The speed of the rise peaked at 9 mag/day, while the light curve is well fit over only the first two days by a model with a uniformly expanding sphere. We also report the discovery by R. Stubbings of a pre-eruption rise starting 11 days before the eruption, peaking 1.1 mag brighter than its long-time average, and then fading back towards quiescence five days before the eruption. This unique and mysterious behaviour is only the fourth known anticipatory rise/dip closely spaced before a nova eruption. We present 19 timings of photometric minima from 1986 to February 2011, where the orbital period is fast increasing with P/Pdot=+313,000 years. From 2008-2011, T Pyx had a small change in this rate of increase, so that the orbital period at the time of eruption was 0.07622916±0.00000008 days. This strong and steady increase of the orbital period can only come from mass transfer, for which we calculate a rate of 10^{-6.0±0.5} Msun/yr for mass leaving the companion star. We report 6114 magnitudes between 1890 and 2011, for an average B=15.59±0.01 from 1967-2011, which allows for an eruption in 2011 if the blue flux is nearly proportional to the accretion rate. We present a model for the infrared-to-X-ray emission during quiescence as being from a 34,000 K blackbody nearly filling the Roche lobe (caused by the very high accretion rate creating an extended envelope around the accretion column) plus a nu^0.9 non-thermal component (from the optically-thin outer and circumbinary regions).

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