IM Normae: The Death Spiral of a Cataclysmic Variable?

We present a study of the orbital light curves of the recurrent nova IM Normae since its 2002 outburst. The broad “eclipses” recur with a 2.46 hr period, which increases on a timescale of 1.28(16) × 106 yr. Under the assumption of conservative mass transfer, this suggests a rate near 10−7 M ⊙ yr−1, and this agrees with the estimated accretion rate of the postnova, based on our estimate of luminosity. IM Nor appears to be a close match to the famous recurrent nova T Pyxidis. Both stars appear to have very high accretion rates, sufficient to drive the recurrent-nova events. Both have quiescent light curves, which suggest strong heating of the low-mass secondary, and very wide orbital minima, which suggest obscuration of a large “corona” around the primary. And both have very rapid orbital period increases, as expected from a short-period binary with high mass transfer from the low-mass component. These two stars may represent a final stage of nova—and cataclysmic variable—evolution, in which irradiation-driven winds drive a high rate of mass transfer, thereby evaporating the donor star in a paroxysm of nova outbursts.

‘Novae, supernovae, or something else?’ – (super-)nova highlights from Hoffmann & Vogt are quite certainly comets (ad 668 and 891)

ABSTRACT Galactic novae and supernovae can be studied by utilizing historical observations, yielding explosion time, location on sky, etc. Recent publications by Hoffmann & Vogt present CVs (Cataclysmic Variables), supernova remnants, planetary nebulae, etc. as potential counterparts based on their list of historically reported transients from the Classical Chinese text corpus. Since their candidate selection neglects the state-of-the-art (e.g. Stephenson & Green), and since it includes ‘broom stars’ and ‘fuzzy stars’, i.e. probable comets, we investigate their catalogue in more detail. We discuss here their two highlights, the suggestion of two ‘broom star’ records dated ad 667 and 668 as one historical supernova and of the ‘guest star’ of ad 891 as recurrent nova U Sco. The proposed positional search areas are not justified due to translation and dating problems, source omission, as well as misunderstandings of historical Chinese astronomy and unfounded textual interpretations. All sources together provide strong evidence for comet sightings in both ad 668 and 891 – e.g. there are no arguments for stationarity. The ad 667 record is a misdated doublet of 668. Our critique pertains more generally to their whole catalogue of ‘24 most promising events’: their speculations on counterparts lack a solid foundation and should not be used in follow-ups.

AstroSat soft X-ray observations of the symbiotic recurrent nova V3890 Sgr during its 2019 outburst

ABSTRACT 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 d 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 supersoft 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 dependent. 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 fitted 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 2019 eruption of recurrent nova V3890 Sgr: observations by Swift, NICER, and SMARTS

ABSTRACT V3890 Sgr is a recurrent nova that has been seen in outburst three times so far, with the most recent eruption occurring on 2019 August 27 ut. This latest outburst was followed in detail by the Neil Gehrels Swift Observatory, from less than a day after the eruption until the nova entered the Sun observing constraint, with a small number of additional observations after the constraint ended. The X-ray light curve shows initial hard shock emission, followed by an early start of the supersoft source phase around day 8.5, with the soft emission ceasing by day 26. Together with the peak blackbody temperature of the supersoft spectrum being ∼100 eV, these timings suggest the white dwarf mass to be high, $\sim 1.3\, {\rm M_{\odot }}$. The UV photometric light curve decays monotonically, with the decay rate changing a number of times, approximately simultaneously with variations in the X-ray emission. The UV grism spectra show both line and continuum emission, with emission lines of N, C, Mg, and O being notable. These UV spectra are best dereddened using a Small Magellanic Cloud extinction law. Optical spectra from SMARTS show evidence of interaction between the nova ejecta and wind from the donor star, as well as the extended atmosphere of the red giant being flash-ionized by the supersoft X-ray photons. Data from NICER reveal a transient 83 s quasi-periodic oscillation, with a modulation amplitude of 5 per cent, adding to the sample of novae that show such short variabilities during their supersoft phase.

Isotopic ratios in the red giant component of the recurrent nova T Coronae Borealis

ABSTRACT We report the determination of abundances and isotopic ratios for C, O, and Si in the photosphere of the red giant (RG) component of the recurrent nova (RN) T Coronae Borealis from new 2.284–2.402 μm and 3.985–4.155 μm spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the RG interior which are brought to the surface during dredge-up and (ii) contamination of the RG, either during the common envelope phase of the binary evolution or by material synthesized in RN eruptions, or a combination of the two. We find that the abundances of C, O, and Si are reasonably consistent with the expected composition of an RG after first dredge-up, as is the 16O/17O ratio. The 28Si/29Si ratio is found to be 8.6 ± 3.0, and that for 28Si/30Si is 21.5 ± 3.0. The 12C/13C ratio (10 ± 2) is somewhat lower than expected for first dredge-up. The 16O/18O ratio (41 ± 3) is highly inconsistent with that expected either from RG evolution (∼550) or from contamination of the RG by the products of a nova thermonuclear runaway. In particular, the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in an RN system of contamination by ejecta having anomalous abundances and isotopic ratios.

Supersoft X-ray phases of recurrent novae as an indicator of their white dwarf masses

We have examined the optical/X-ray light curves of seven well-observed recurrent novae, V745 Sco, M31N 2008-12a, LMC N 1968, U Sco, RS Oph, LMC N 2009a, T Pyx, and one recurrent nova candidate LMC N 2012a. Six novae out of the eight show a simple relation that the duration of supersoft X-ray source (SSS) phase is 0.70 times the total duration of the outburst (= X-ray turnoff time), i.e., tSSS = 0.70 toff, the total duration of which ranges from 10 to 260 d. These six recurrent novae show a broad rectangular X-ray light curve shape, the first half-period of which is highly variable in the X-ray count rate. The SSS phase also corresponds to an optical plateau phase that indicates a large accretion disk irradiated by a hydrogen-burning white dwarf (WD). The two other recurrent novae, T Pyx and V745 Sco, show a narrow triangular-shaped X-ray light curve without an optical plateau phase. Their relations between tSSS and toff are rather different from the above six recurrent novae. We also present theoretical SSS durations for recurrent novae with various WD masses and stellar metallicities (Z = 0.004, 0.01, 0.02, and 0.05) and compare them with the observed durations of these recurrent novae. We show that SSS duration is a good indicator of WD mass in recurrent novae with a broad rectangular X-ray light curve shape.

Counterparts of Far Eastern Guest Stars: Novae, supernovae, or something else?

ABSTRACT Historical observations of transients are crucial for studies of their long-term evolution. This paper forms part of a series of papers in which we develop methods for the analysis of ancient data of transient events and their usability in modern science. Prior research on this subject by other authors has focused on looking for historical supernovae and our earlier work focused on cataclysmic binaries as classical novae. In this study we consider planetary nebulae, symbiotic stars, supernova remnants, and pulsars in the search fields of our test sample. We present the possibilities for these object types to flare up visually, give a global overview on their distribution, and discuss the objects in our search fields individually. To summarize our results, we provide a table of the most likely identifications of the historical sightings in our test sample and outline our method in order to apply it to further historical records in future works. Highlights of our results include a re-interpretation of two separate sightings as one supernova observation from May 667 to June 668 CE, the remnant of which could possibly be SNR G160.9+02.6. We also suggest the recurrent nova U Sco as a candidate for the appearance observed between Scorpius and Ophiuchus in 891, which could point towards a long-term variability of eruption amplitudes. In addition, we find that the ‘shiny bright’ sighting in 1431 can be linked to the symbiotic binary KT Eri, which erupted as a naked eye classical nova in 2009.

A search for the modern counterparts of the Far Eastern guest stars 369 CE, 386 CE and 393 CE

ABSTRACT In this study, we apply our previously developed method to investigate ancient transient sightings in order to derive consequences for modern astrophysical problems. We present case studies of three observations of so-called guest stars in the fourth century CE, which lasted several months each. These three observations had been discussed and suggested as possible supernovae, but slow novae are also viable alternatives. Our careful re-interpretation of the historical texts and the currently known objects in the given fields shed new light on this topic. In particular, for the two events in 386 and 393 CE we suggest possible supernova identifications, while in all three cases there are interesting candidates for past classical or recurrent nova eruptions among known cataclysmic variables (CVs) and/or symbiotic stars. We suggest that the transient of 369 can be explained as a classical and possibly recurrent nova instead of a supernova. The most plausible candidates are BZ Cam, a CV with a possible nova shell, or CQ Dra, a naked-eye multiple system perhaps able to permit an overwhelmingly bright nova with day-time visibility.