M. J. Darnley

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high-velocity ejecta interact with a preexisting red giant wind, setting up shock systems analogous to those seen in supernova remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t = 0 to t ~ 6 days. XRT observations from 0.3 to 10 keV started 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission, which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column (NH)W, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, (NH)W is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard phase I remnant evolution terminated by t ~ 6 days and the remnant then rapidly evolved to display behavior characteristic of phase III. Around t = 26 days, however, a new, luminous, and highly variable soft X-ray source began to appear, whose origin will be explored in a subsequent paper.

Classical novae from the POINT-AGAPE microlensing survey of M31 : II. Rate and statistical characteristics of the nova population

The POINT-AGAPE (Pixel-lensing Observations with the Isaac Newton Telescope- Andromeda Galaxy Amplified Pixels Experiment) survey is an optical search for gravitational microlensing events towards the Andromeda galaxy (M31). As well as microlensing, the survey is sensitive to many different classes of variable stars and transients. In our first paper of this series, we reported the detection of 20 classical novae (CNe) observed in Sloan rand i � passbands. An analysis of the maximum magnitude versus rate of decline (MMRD) relationship in M31 is performed using the resulting POINT-AGAPE CN catalogue. Within the limits of the uncertainties of extinction internal to M31, good fits are produced to the MMRD in two filters. The MMRD calibration is the first to be performed for Sloan rand ifilters. However, we are unable to verify that novae have the same absolute magnitude 15 d after peak (the t15 relationship), nor any similar relationship for either Sloan filter. The subsequent analysis of the automated pipeline has provided us with the most thorough knowledge of the completeness of a CN survey to date. In addition, the large field of view of the survey has permitted us to probe the outburst rate well into the galactic disc, unlike previous CCD imaging surveys. Using this analysis, we are able to probe the CN distribution of M31 and evaluate the global nova rate. Using models of the galactic surface brightness of M31, we show that the observed CN distribution consists of a separate bulge and disc population. We also show that the M31 bulge CN eruption rate per unit rflux is more than five times greater

Exquisite Nova Light Curves from the Solar Mass Ejection Imager (SMEI)

We present light curves of three classical novae (KT Eridani, V598 Puppis, V1280 Scorpii) and one recurrent nova (RS Ophiuchi) derived from data obtained by the Solar Mass Ejection Imager (SMEI) on board the Coriolis satellite. SMEI provides near complete sky-map coverage with precision visible-light photometry at 102-minute cadence. The light curves derived from these sky maps offer unprecedented temporal resolution around, and especially before, maximum light, a phase of the nova eruption normally not covered by ground-based observations. They allow us to explore fundamental parameters of individual objects including the epoch of the initial explosion, the reality and duration of any pre-maximum halt (found in all three fast novae in our sample), the presence of secondary maxima, speed of decline of the initial light curve, plus precise timing of the onset of dust formation (in V1280 Sco) leading to estimation of the bolometric luminosity, white dwarf mass and object distance. For KT Eri, Liverpool Telescope SkyCamT data confirm important features of the SMEI light curve and overall our results add weight to the proposed similarities of this object to recurrent rather than to classical novae. In RS Oph, comparison with hard X-ray data from the 2006 outburst implies that the onset of the outburst coincides with extensive high velocity mass-loss. It is also noted that two of the four novae we have detected (V598 Pup and KT Eri) were only discovered by ground-based observers weeks or months after maximum light, yet these novae reached peak magnitudes of 3.46 and 5.42 respectively. This emphasizes the fact that many bright novae per year are still overlooked, particularly those of the very fast speed class. Coupled with its ability to observe novae in detail even when relatively close to the Sun in the sky, we estimate that as many as 5 novae per year may be detectable by SMEI.

Hubble Space Telescope Imaging of the Expanding Nebular Remnant of the 2006 Outburst of the Recurrent Nova RS Ophiuchi

We report Hubble Space Telescope imaging obtained 155 days after the 2006 outburst of RS Ophiuchi. We detect extended emission in both [O III] λ5007 and [Ne V] λ3426 lines. In both lines, the remnant has a double ring structure. The east-west orientation and total extent of these structures (580 ± 50 AU at d = 1.6 kpc) is consistent with that expected due to expansion of emitting regions imaged earlier in the outburst at radio wavelengths. Expansion at high velocity appears to have been roughly constant in the east-west direction (vexp = 3200 ± 300 km s-1 in the plane of the sky), with tentative evidence of deceleration north-south. We present a bipolar model of the remnant whose inclination is consistent with that of the central binary. The true expansion velocities of the polar components are then v = 5600 ± 1100 km s-1. We suggest that the bipolar morphology of the remnant results from interaction of the outburst ejecta with a circumstellar medium that is significantly denser in the equatorial regions of the binary than at the poles. This is also consistent with observations of shock evolution in the X-ray and the possible presence of dust in the infrared. Furthermore, it is in line with models of the shaping of planetary nebulae with close binary central systems, and also with recent observations relating to the progenitors of Type Ia supernovae, for which recurrent novae are a proposed candidate. Our observations also reveal more extended structures to the south and east of the remnant whose possible origin is briefly discussed.

A SPECTROSCOPIC AND PHOTOMETRIC SURVEY OF NOVAE IN M31

We report the results of a multi-year spectroscopic and photometric survey of novae in M31 that resulted in a total of 53 spectra of 48 individual nova candidates. Two of these, M31N 1995-11e and M31N 2007-11g, were revealed to be long-period Mira variables, not novae. These data double the number of spectra extant for novae in M31 through the end of 2009 and bring to 91 the number of M31 novae with known spectroscopic classifications. We find that 75 novae (82%) are confirmed or likely members of the Fe II spectroscopic class, with the remaining 16 novae (18%) belonging to the He/N (and related) classes. These numbers are consistent with those found for Galactic novae. We find no compelling evidence that spectroscopic class depends sensitively on spatial position or population within M31 (i.e., bulge versus disk), although the distribution for He/N systems appears slightly more extended than that for the Fe II class. We confirm the existence of a correlation between speed class and ejection velocity (based on line width), as in the case of Galactic novae. Follow-up photometry allowed us to determine light-curve parameters for a total of 47 of the 91 novae with known spectroscopic class. We confirm that more luminous novae generally fade the fastest and that He/N novae are typically faster and brighter than their Fe II counterparts. In addition, we find a weak dependence of nova speed class on position in M31, with the spatial distribution of the fastest novae being slightly more extended than that of slower novae.

On the Progenitors of Galactic Novae

Of the approximately 400 known Galactic classical novae, only 10 of them, the recurrent novae, have been seen to erupt more than once. At least eight of these recurrents are known to harbor evolved secondary stars, rather than the main-sequence secondaries typical in classical novae. In this paper, we propose a new nova classification system, based solely on the evolutionary state of the secondary and not (like the current schemes) based on the properties of the outbursts. Using archival optical and near-infrared photometric observations of a sample of 38 quiescent Galactic novae we show that the evolutionary state of the secondary star in a quiescent system can be predicted and several objects are identified for follow-up observations: CI Aql, V2487 Oph, DI Lac, and EU Sct.

Classical novae from the POINT–AGAPE microlensing survey of M31 – I. The nova catalogue

The POINT-AGAPE (Pixel-lensing Observations with the Isaac Newton Telescope-Andromeda Galaxy Amplified Pixels Experiment) survey is an optical search for gravitational microlensing events towards the Andromeda galaxy (M31). As well as microlensing, the survey is sensitive to many different classes of variable stars and transients. Here we describe the automated detection and selection pipeline used to identify M31 classical novae (CNe) and we present the resulting catalogue of 20 CN candidates observed over three seasons. CNe are observed both in the bulge region as well as over a wide area of the M31 disc. Nine of the CNe are caught during the final rise phase and all are well sampled in at least two colours. The excellent light-curve coverage has allowed us to detect and classify CNe over a wide range Qf speed class, from very fast to very slow. Among the light curves is a moderately fast CN exhibiting entry into a deep transition minimum, followed by its final decline. We have also observed in detail a very slow CN which faded by only 0.01 mag d -1 over a 150-d period. We detect other interesting variable objects, including one of the longest period and most luminous Mira variables. The CN catalogue constitutes a uniquely well-sampled and objectively-selected data set with which to study the statistical properties of CNe in M31, such as the global nova rate, the reliability of novae as standard-candle distance indicators and the dependence of the nova population on stellar environment. The findings of this statistical study will be reported in a follow-up paper.

The Angstrom Project: a microlensing survey of the structure and composition of the bulge of the Andromeda galaxy

The Andromeda Galaxy Stellar Robotic Microlensing Project (The Angstrom Project) aims to use stellar microlensing events to trace the structure and composition of the inner regions of the Andromeda Galaxy (M31). We present microlensing rate and time-scale predictions and spatial distributions for stellar and sub-stellar lens populations in combined disc and barred bulge models of M31. We show that at least half of the stellar microlenses in and around the bulge are expected to have characteristic durations between 1 and 10 d, rising to as much as 80 per cent for brown-dwarf dominated mass functions. These short-duration events are mostly missed by current microlensing surveys that are looking for Macho candidates in the M31 dark matter halo. Our models predict that an intensive monitoring survey programme, such as Angstrom, which will be able to detect events of durations upwards of a day, could detect around 30 events per season within ∼5 arcmin of the M31 centre due to ordinary low-mass stars and remnants. This yield increases to more than 60 events for brown-dwarf dominated mass functions. The overall number of events and their average duration are sensitive diagnostics of the bulge mass, in particular the contribution of low-mass stars and brown dwarfs. The combination of an inclined disc, an offset bar-like bulge, and differences in the bulge and disc luminosity functions results in a four-way asymmetry in the number of events expected in each quadrant defined by the M31 disc axes. The asymmetry is sensitive to the bar prolongation, orientation and mass.

M31N 2007-11d: A Slowly Rising, Luminous Nova in M31

We report a series of extensive photometric and spectroscopic observations of the luminous M31 nova M31N 2007-11d. Our photometric observations coupled with previous measurements show that the nova took at least 4 days to reach peak brightness at R ≃ 14.9 on 2007 November 20 UT. After reaching maximum, the time for the nova to decline 2 and 3 mag from maximum light (t 2 and t 3) was ~9.5 and ~13 days, respectively, establishing that M31N 2007-11d was a moderately fast declining nova. During the novas evolution, a total of three spectra were obtained. The first spectrum was obtained one day after maximum light (5 days post-discovery), followed by two additional spectra taken on the decline at two and three weeks post-maximum. The initial spectrum reveals narrow Balmer and Fe II emission with P Cygni profiles superimposed on a blue continuum. These data, along with the spectra obtained on the subsequent decline, clearly establish that M31N 2007-11d belongs to the Fe II spectroscopic class. The properties of M31N 2007-11d are discussed within the context of other luminous novae in M31, the Galaxy, and the LMC. Overall, M31N 2007-11d appears to be remarkably similar to Nova LMC 1991, which was another bright, slowly rising, Fe II nova. A comparison of the available data for luminous extragalactic novae suggests that the ≳ 4 day rise to maximum light seen in M31N 2007-11d may not be unusual, and that the rise times of luminous Galactic novae, usually assumed to be ≾ 2 days, may have been underestimated.

The Possibility of Detecting Planets in the Andromeda Galaxy

The Angstrom project is using a global network of 2 m class telescopes to conduct a high-cadence pixel microlensing survey of the bulge of the Andromeda galaxy (M31), with the primary aim of constraining its underlying bulge mass distribution and stellar mass function. Here we investigate the feasibility of using such a survey to detect planets in M31. We estimate the efficiency of detecting signals produced by planets with various masses and separations from the host star. We find that for a ~5MJ planet that is located within the lensing zone (~1-3 AU), detection is possible above 3 σ with detection efficiency ~6%. This corresponds to the yearly detection rate of ~3fLZ planets, where fLZ is the probability that a planet exists in the lensing zone. It is expected that most events with detectable planets are associated with giant source stars, and thus source size will have a significant effect on the planet detection efficiency. We also find that the planetary perturbations will be in nearly all cases caused by central caustics, and thus observational strategies focusing on these central perturbations would maximize planet detections. A dramatic improvement in the efficiency of ~30%-50% is expected if follow-up observations on an 8 m telescope are made possible by a real-time alert system.