High White Dwarf Masses Research Articles

X-ray measurement of a high-mass white dwarf and its spin for the intermediate polar IGR J18434−0508

ABSTRACT IGR J18434−0508 is a Galactic Intermediate Polar (IP) type Cataclysmic Variable (CV) previously classified through optical spectroscopy. The source is already known to have a hard Chandra spectrum. In this paper, we have used follow-up XMM–Newton and NuSTAR observations to measure the white dwarf (WD) mass and spin period. We measure a spin period of P = 304.4 ± 0.3 s based on the combined MOS1, MOS2, and pn light curve. Although this is twice the optical period found previously, we interpret this value to be the true spin period of the WD. The source has an $8\pm 2~{{\ \rm per\ cent}}$ pulsed fraction in the 0.5–10 keV XMM–Newton data and shows strong dips in the soft energy band (0.5–2 keV). The XMM–Newton and NuSTAR joint spectrum is consistent with a thermal bremsstrahlung continuum model with an additional partial covering factor, reflection, and Fe-line Gaussian components. Furthermore, we fit the joint spectrum with the post-shock region ‘ipolar’ model which indicates a high WD mass ≳1.36 M⊙, approaching the Chandrasekhar limit.

The nova KT Eri Is a recurrent nova with a recurrence time-scale of 40–50 yr

ABSTRACT KT Eridani was a very fast nova in 2009 peaking at V = 5.42 mag. We marshal large data sets of photometry to finally work out the nature of KT Eri. From the TESS light curve, as confirmed with our radial velocity curve, we find an orbital period of 2.61595 d. With our 272 spectral energy distributions from simultaneous BVRIJHK measures, the companion star has a temperature of 6200 ± 500 K. Our century-long average in quiescence has V = 14.5. With the Gaia distance (5110$^{+920}_{-430}$ pc), the absolute magnitude is $M_{V_q}$ = +0.7 ± 0.3. We converted this absolute magnitude (corrected to the disc light alone) to accretion rates, $\dot{M}$, with a full integration of the α-disc model. This $\dot{M}$ is very high at 3.5 × 10−7 M⊙ yr−1. Our search and analysis of archival photographs shows that no eruption occurred from 1928 to 1954 or after 1969. With our analysis of the optical light curve, the X-ray light curve, and the radial velocity curve, we derive a white dwarf mass of 1.25 ± 0.03 M⊙. With the high white dwarf mass and very-high $\dot{M}$, KT Eri must require a short time to accumulate the required mass to trigger the next nova event. Our detailed calculations give a recurrence time-scale of 12 yr with a total range of 5–50 yr. When combined with the archival constraints, we conclude that the recurrence time-scale must be between 40 and 50 yr. So, KT Eri is certainly a recurrent nova, with the prior eruption remaining undiscovered in a solar gap of coverage from 1959 to 1969.

Triggering micronovae through magnetically confined accretion flows in accreting white dwarfs

ABSTRACT Rapid bursts at optical wavelengths have been reported for several accreting white dwarfs. In these bursts, the optical luminosity can increase by up to a factor of 30 in less than an hour, before fading on time-scales of several hours, and the energy release can reach ~1039 erg (‘micronovae’). Several systems have also shown these bursts to be semirecurrent on time-scales of days to months, and the temporal profiles of these bursts strongly resemble those observed in Type-I X-ray bursts in accreting neutron stars. It has been suggested that the observed micronovae may be the result of localized thermonuclear runaways in the surface layers of accreting white dwarfs. Here we propose a model in which the magnetic confinement of accretion streams on to the accreting magnetic white dwarf may trigger localized thermonuclear runaways. The model proposed to trigger micronovae appears to favour magnetic systems with both a high white dwarf mass and a high mass-transfer rate.

Far Ultraviolet Spectroscopy of Old Novae I. V603 Aquila.

We present the results of a synthetic spectral analysis of the far ultraviolet archival IUE, HST and FUSE observations of the fast old nova V603 Aql, obtained some 90 years after its 1918 nova outburst. Our analysis utilizes the new Hubble FGS parallax distance for this nearly face-on old nova, a high white dwarf mass and a low reddening. Our analysis includes non-truncated optically thick accretion disks since V603 Aql is neither a polar nor an intermediate polar. Our synthetic spectral modeling of the FUSE and HST spectra analyzed separately indicate a mass transfer rate [Formula: see text] for the FUSE and HST spectra respectively, assuming a WD mass of 1.2M⊙. The mass accretion rate also depends on the assumed WD mass, and increases by a factor of two for a WD mass of 0.8M⊙. Combining the FUSE and HST spectra together lead to the same results. Potential implications are discussed.

Cataclysmic variables below the period gap: mass determinations of 14 eclipsing systems

We present high-speed, three-colour photometry of the eclipsing cataclysmic variables CTCV 1300, CTCV 2354 and SDSS 1152. All three systems are below the observed "period gap" for cataclysmic variables. For each system we determine the system parameters by fitting a parameterised model to the observed eclipse light curve by chi-squared minimisation. We also present an updated analysis of all other eclipsing systems previously analysed by our group. New donor masses are generally between 1 and 2 sigma of those originally published, with the exception of SDSS 1502 and DV UMa. We note that the donor mass of SDSS 1501 has been revised upwards by 0.024Msun. This system was previously identified as having evolved passed the minimum orbital period for cataclysmic variables, but the new mass determination suggests otherwise. Our new analysis confirms that SDSS 1035 and SDSS 1433 have evolved past the period minimum for cataclysmic variables, corroborating our earlier studies. We find that the radii of donor stars are oversized when compared to theoretical models, by approximately 10 percent. We show that this can be explained by invoking either enhanced angular momentum loss, or by taking into account the effects of star spots. We are unable to favour one cause over the other, as we lack enough precise mass determinations for systems with orbital periods between 100 and 130 minutes, where evolutionary tracks begin to diverge significantly. We also find a strong tendency towards high white dwarf masses within our sample, and no evidence for any He-core white dwarfs. The dominance of high mass white dwarfs implies that erosion of the white dwarf during the nova outburst must be negligible, or that not all of the mass accreted is ejected during nova cycles, resulting in the white dwarf growing in mass. (Abridged)

Influence of Compton scattering on the broad-band X-ray spectra of intermediate polars

Context. The majority of cataclysmic variables observed in the hard X-ray energy band are intermediate polars where the magnetic field is strong enough to channel the accreting matter to the magnetic poles of the white dwarf. A shock above the stellar surface heats the gas to fairly high temperatures (10–100 keV). The post-shock region cools mostly via optically thin bremsstrahlung. Aims. We investigate the influence of Compton scattering on the structure and the emergent spectrum of the post-shock region. We also study the effect it has on the mass of the white dwarfs obtained from fitting the observed X-ray spectrum of intermediate polars. Methods. We construct the model of the post-shock region taking Compton scattering into account. The radiation transfer equation is solved in the plane-parallel approximation. The feedback of Compton scattering on the structure of the post-shock region is also accounted for. A set of the post-shock region model spectra for various white dwarf masses is calculated. Results. We find that Compton scattering does not change the emergent spectra significantly for low accretion rates or low white dwarf masses. However, it becomes important at high accretion rates and high white dwarf masses. The time-averaged, broad-band X-ray spectrum of intermediate polar V709 Cas obtained by the RXTE andINTEGRAL observatories is fitted using the set of computed

Non‐LTE Model Atmosphere Analysis of the Large Magellanic Cloud Supersoft X‐Ray Source CAL 83

We present a non-LTE (NLTE) model atmosphere analysis of Chandra High Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG) and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption-line spectrum from the hot white dwarf photosphere but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (MWD 1 M☉). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of Type Ia supernovae (SNe Ia).

THEORETICAL STATISTICS OF ZERO-AGE CATACLYSMIC VARIABLES

The distribution of the white-dwarf masses, the distribution of the mass ratios, and the distribution of the orbital periods in cataclysmic variables which are forming at the present time are calculated. These systems are referred to as cataclysmic variables. The results show that 60% of the systems being formed contain helium white dwarfs and 40% contain carbon-oxygen white dwarfs. The mean white-dwarf mass in those systems containing helium white dwarfs is 0.34 33IO. The mean white-dwarf mass in those systems containing carbonoxygen white dwarfs is 0.75 3KO. The distribution of white-dwarf masses in zero-age cataclysmic variables is similar to the distribution of masses in single white dwarfs for masses above approximately 0.6 37io. The distribution of white-dwarf masses in zero-age cataclysmic variables is not sufficient to account for the observed high white-dwarf masses in cataclysmic variables, emphasizing the possible importance of selection effects. The orbital period distribution identifies four main classes of zero-age cataclysmic variables: (1) short-period systems containing helium white dwarfs, (2) systems containing carbon-oxygen white dwarfs whose secondaries are convectively stable against rapid mass transfer to the white dwarf, (3) systems containing carbon-oxygen white dwarfs whose secondaries are radiatively stable against rapid mass transfer to the white dwarf, and (4) long-period systems with evolved secondaries. The ranges of progenitor lobe-filling giant masses and radii are also better identified.

Classical Novae in the Context of the Evolution of Cataclysmic Binaries

AbstractIn this paper we explore to what extent the TNR model of nova outbursts and our current concepts of the formation and secular evolution of cataclysmic binaries are compatible. Specifically we address the following questions: 1) whether observational selection can explain the high white dwarf masses attributed to novae, 2) whether novae on white dwarfs in the mass range 0.6M⊙ ≲ M ≲ 0.9M⊙ can occur and how much they could contribute to the observed nova frequency, and 3) whether the high mass transfer rates imposed on the white dwarf in systems above the period gap can be accommodated by the TNR model of nova outbursts.