A common origin of magnetism from planets to white dwarfs

This is the final version of the article. Available from the publisher via the DOI in this record.Isolated magnetic white dwarfs have field strengths ranging from kilogauss to gigagauss. However, the origin of the magnetic field has not been hitherto elucidated. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of their progenitor stars, or are the result of binary interactions, or, finally, they are produced by other internal physical mechanisms during the cooling of the white dwarf itself, remains a mystery. At sufficiently low temperatures, white dwarfs crystallize. Upon solidification, phase separation of its main constituents, 12C and 16O, and of the impurities left by previous evolution occurs. This process leads to the formation of a Rayleigh–Taylor unstable liquid mantle on top of a solid core. This convective region, as it occurs in solar system planets like the Earth and Jupiter, can produce a dynamo able to yield magnetic fields of strengths of up to 0.1 MG, thus providing a mechanism that could explain magnetism in single white dwarfs.This work has been supported by MINECO grants ESP2013- 47637-P, ESP2015-66134-R (J.I.), and AYA2014-59084-P (E.G.-B.), by the European Union FEDER funds, by grants 2014SGR1458 (J.I.), 2014SGR0038 (E.G.-B.) of the AGAUR, and by the CERCS program of the Generalitat de Catalunya

A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?

It has been recently proposed that massive fast-rotating highly-magnetized white dwarfs could describe the observational properties of some of Soft Gamma-Ray Repeaters (SGRs) and Anomalous X-Ray Pulsars (AXPs). Moreover, it has also been shown that high-field magnetic (HFMWDs) can be the outcome of white dwarf binary mergers. The products of these mergers consist of a hot central white dwarf surrounded by a rapidly rotating disk. Here we show that the merger of a double degenerate system can explain the characteristics of the peculiar AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We also show that the observed properties of 4U 0142+6 are consistent with an approximately 1.2 M_{\sun} white dwarf, remnant of the coalescence of an original system made of two white dwarfs of masses 0.6\, M_{\sun} and 1.0\, M_{\sun}. Finally, we infer a post-merging age $\tau_{\rm WD}\approx 64$ kyr, and a magnetic field $B\approx 2\times 10^8$ G. Evidence for such a magnetic field may come from the possible detection of the electron cyclotron absorption feature observed between the $B$ and $V$ bands at $\approx 10^{15}$ Hz in the spectrum of 4U 0142+61.Comment: to appear in ApJ Letter

Component masses of young, wide, non-magnetic white dwarf binaries in the SDSS DR7

We present a spectroscopic component analysis of 18 candidate young, wide, non-magnetic, double-degenerate binaries identified from a search of the Sloan Digital Sky Survey Data Release 7 (DR7). All but two pairings are likely to be physical systems. We show SDSS J084952.47+471247.7 + SDSS J084952.87+471249.4 to be a wide DA+DB binary, only the second identified to date. Combining our measurements for the components of 16 new binaries with results for three similar, previously known systems within the DR7, we have constructed a mass distribution for the largest sample to date (38) of white dwarfs in young, wide, non-magnetic, double-degenerate pairings. This is broadly similar in form to that of the isolated field population with a substantial peak around M~0.6 Msun. We identify an excess of ultra-massive white dwarfs and attribute this to the primordial separation distribution of their progenitor systems peaking at relatively larger values and the greater expansion of their binary orbits during the final stages of stellar evolution. We exploit this mass distribution to probe the origins of unusual types of degenerates, confirming a mild preference for the progenitor systems of high-field-magnetic white dwarfs, at least within these binaries, to be associated with early-type stars. Additionally, we consider the 19 systems in the context of the stellar initial mass-final mass relation. None appear to be strongly discordant with current understanding of this relationship.Comment: 20 pages, 5 Tables, 7 figures. accepted for publication in MNRA

Observations of SN2011fe with INTEGRAL

SN2011fe was detected by the Palomar Transient Factory on August 24th 2011 in M101 few hours after the explosion. From the early spectra it was immediately realized that it was a Type Ia supernova thus making this event the brightest one discovered in the last twenty years. In this paper the observations performed with the instruments on board of INTEGRAL (SPI, IBIS/ISGRI, JEM-X and OMC) before and after the maximum of the optical light as well as the interpretation in terms of the existing models of $\gamma$--ray emission from such kind of supernovae are reported. All INTEGRAL high-energy have only been able to provide upper limits to the expected emission due to the decay of $^{56}$Ni. These bounds allow to reject explosions involving a massive white dwarf in the sub--Chandrasekhar scenario. On the other hand, the optical light curve obtained with the OMC camera suggests that the event was produced by a delayed detonation of a CO white dwarf that produced $\sim 0.5$ M$\odot$ of $^{56}$Ni. In this particular case, INTEGRAL would have only been able to detect the early $\gamma$--ray emission if the supernova had occurred at a distance of 2 -3 Mpc, although the brightest event could be visible up to distances larger by a factor two.Comment: Proceedings of "An INTEGRAL view of the high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October 15-19, 2012, Paris, France, in Proceedings of Science (INTEGRAL 2012), Eds. A. Goldwurm, F. Lebrun and C. Winkler, http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=176, id number PoS (INTEGRAL 2012) 103 (2013

Observation of SN2011fe with INTEGRAL. I. Pre--maximum phase

SN2011fe was detected by the Palomar Transient Factory on August 24th 2011 in M101 a few hours after the explosion. From the early optical spectra it was immediately realized that it was a Type Ia supernova thus making this event the brightest one discovered in the last twenty years. The distance of the event offered the rare opportunity to perform a detailed observation with the instruments on board of INTEGRAL to detect the gamma-ray emission expected from the decay chains of $^{56}$Ni. The observations were performed in two runs, one before and around the optical maximum, aimed to detect the early emission from the decay of $^{56}$Ni and another after this maximum aimed to detect the emission of $^{56}$Co. The observations performed with the instruments on board of INTEGRAL (SPI, IBIS/ISGRI, JEMX and OMC) have been analyzed and compared with the existing models of gamma-ray emission from such kind of supernovae. In this paper, the analysis of the gamma-ray emission has been restricted to the first epoch. Both, SPI and IBIS/ISGRI, only provide upper-limits to the expected emission due to the decay of $^{56}$Ni. These upper-limits on the gamma-ray flux are of 7.1 $\times$ 10$^{-5}$ ph/s/cm$^2$ for the 158 keV line and of 2.3 $\times$ 10$^{-4}$ ph/s/cm$^2$ for the 812 keV line. These bounds allow to reject at the $2\sigma$ level explosions involving a massive white dwarf, $\sim 1$ M$\odot$ in the sub--Chandrasekhar scenario and specifically all models that would have substantial amounts of radioactive $^{56}$Ni in the outer layers of the exploding star responsible of the SN2011fe event. The optical light curve obtained with the OMC camera also suggests that SN2011fe was the outcome of the explosion, possibly a delayed detonation although other models are possible, of a CO white dwarf that synthesized $\sim 0.55$ M$_\odot$ of $^{56}$Ni. For this specific model.Comment: Accepted for publication in A&A. 10 pages, 10 figure

A study of temeporal characteristics of selected Cataclysmic Variables: The broad-band noise structure between 2-20 kev

We present the preliminary analysis of the band-limited noise structure of Cataclysmic Variables (CVs) in the 220 keV energy band. We have currently analyzed Rossi X-ray Timing Explorer (RXTE) PCA data and derived time series from 30 CVs using the RXTE archive. In general, CVs of different types all show broad band noise which can be fitted with power laws, using exponential cut-offs, and Lorenzians in a similar way to power spectral (noise) characteristics of X-ray Binaries (XRBs). In general terms the power spectra show a power law index of (-)1.2-2. A rather large scale flattening of the power spectra exits in nonmagnetic systems in the low to very low frequency range. We observe that in low and high states/outbursts the noise in the high frequency range and low frequency range is changed. CVs show considerable very low frequency noise. In addition, we have recovered several possible QPOs in the X-ray wavelengths from CVs mainly from Intermediate Polar systems

A study of temeporal characteristics of selected Cataclysmic Variables: The broad-band noise structure between 2-20 kev

We present the preliminary analysis of the band-limited noise structure of Cataclysmic Variables (CVs) in the 220 keV energy band. We have currently analyzed Rossi X-ray Timing Explorer (RXTE) PCA data and derived time series from 30 CVs using the RXTE archive. In general, CVs of different types all show broad band noise which can be fitted with power laws, using exponential cut-offs, and Lorenzians in a similar way to power spectral (noise) characteristics of X-ray Binaries (XRBs). In general terms the power spectra show a power law index of (-)1.2-2. A rather large scale flattening of the power spectra exits in nonmagnetic systems in the low to very low frequency range. We observe that in low and high states/outbursts the noise in the high frequency range and low frequency range is changed. CVs show considerable very low frequency noise. In addition, we have recovered several possible QPOs in the X-ray wavelengths from CVs mainly from Intermediate Polar systems