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Gijs Nelemans

Publications and source records attributed to Gijs Nelemans.

107 records · Page 6Linked to original sources
arXiv2008

Limits on the X-ray and optical luminosity of the progenitor of the type Ia supernova SN2007sr

We present HST/WFPC2, GALEX and Chandra observations of the position of the type Ia supernova SN2007sr in the Antennae galaxies, taken before the explosion. No source is found in any of the observations, allowing us to put interesting constraints on the progenitor luminosity. In total there is about 450 kilosecond of Chandra data, spread over 7 different observations. Limiting magnitudes of FUV (23.7 AB mag), NUV (23.8 AB mag), F555W (26.5 Vega mag) and F814W (24.5-25 Vega mag) are derived. The distance to the Antennae is surprisingly poorly known, with almost a factor of 2 difference between the latest distance based on the tip of the red giant branch (13.3 Mpc) and the distance derived from the SN2007sr light curve (25 Mpc). Using these distances we derive limits on absolute optical and UV magnitudes of any progenitor but these are still above the brightest (symbiotic) proposed progenitors. From the Chandra data a 3 sigma upper limit to the X-ray luminosity of 0.5 -- 8.0 10^37 erg/s in the 0.3-1 keV range is found. This is below the X-ray luminosity of the potential progenitor of the type Ia supernova 2007on that we recently discovered and for which we report a corrected X-ray luminosity. If that progenitor is confirmed it suggests the two supernovae have different progenitors. The X-ray limit is comparable to the brightest supersoft X-ray sources in the Galaxy, the LMC and the SMC and significantly below the luminosities of the brightest supersoft and quasi-soft X-ray sources found in nearby galaxies, ruling out such sources as progenitors of this type Ia supernova.

astro-ph↗
arXiv2008

Discovery of the progenitor of the type Ia supernova 2007on

Type Ia supernovae are exploding stars that are used to measure the accelerated expansion of the Universe and are responsible for most of the iron ever produced. Although there is general agreement that the exploding star is a white dwarf in a binary system, the exact configuration and trigger of the explosion is unclear, which could hamper their use for precision cosmology. Two families of progenitor models have been proposed. In the first, a white dwarf accretes material from a companion until it exceeds the Chandrasekhar mass, collapses and explodes. Alternatively, two white dwarfs merge, again causing catastrophic collapse and an explosion. It has hitherto been impossible to determine if either model is correct. Here we report the discovery of an object in pre-supernova archival X-ray images at the position of the recent type Ia supernova (2007on) in the elliptical galaxy NGC 1404. Deep optical images (also archival) show no sign of this object. From this we conclude that the X-ray source is the progenitor of the supernova, which favours the accretion model for this supernova, although the host galaxy is older (6-9 Gyr) than the age at which the explosions are predicted in the accreting models.

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arXiv2007

Hydrogen deficient donors in low-mass X-ray binaries

A number of X-ray binaries (neutron stars or black holes accreting from a companion star) have such short orbital periods that ordinary, hydrogen rich, stars do not fit in. Instead the mass-losing star must be a compact, evolved star, leading to the transfer of hydrogen deficient material to the neutron star. I discuss the current knowledge of these objects, with focus on optical spectroscopy.

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arXiv2007

Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries

Helium that accretes onto a Carbon/Oxygen white dwarf in the double white dwarf AM Canum Venaticorum (AM CVn) binaries undergoes unstable thermonuclear flashes when the orbital period is in the 3.5-25 minute range. At the shortest orbital periods (and highest accretion rates, Mdot > 10^-7 Msol/yr), the flashes are weak and likely lead to the Helium equivalent of classical nova outbursts. However, as the orbit widens and Mdot drops, the mass required for the unstable ignition increases, leading to progressively more violent flashes up to a final flash with Helium shell mass ~ 0.02-0.1 Msol. The high pressures of these last flashes allow the burning to produce the radioactive elements 48Cr, 52Fe, and 56Ni that power a faint (M_V in the range of -15 to -18) and rapidly rising (few days) thermonuclear supernova. Current galactic AM CVn space densities imply one such explosion every 5,000-15,000 years in 10^11 Msol of old stars (~ 2-6% of the Type Ia rate in E/SO galaxies). These ".Ia" supernovae (one-tenth as bright for one-tenth the time as a Type Ia supernovae) are excellent targets for deep (e.g. V=24) searches with nightly cadences, potentially yielding an all-sky rate of 1,000 per year.

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arXiv2006

XMM-Newton observations of AM CVn binaries: V396 Hya & SDSS J1240-01

We present the results of XMM-Newton observations of two AM CVn systems - V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions.

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arXiv2005

The Thermal State of the Accreting White Dwarf in AM Canum Venaticorum Binaries

We calculate the heating and cooling of the accreting white dwarf (WD) in the ultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD can contribute significantly to their optical and ultraviolet emission. We estimate the WD's effective temperature, Teff, using the optical continuum for a number of observed binaries, and show that it agrees well with our theoretical calculations. Driven by gravitational radiation losses, the time averaged accretion rate, , decreases monotonically with increasing Porb, covering six orders of magnitude. If the short period (Porb<10 min) systems accrete at a rate consistent with gravitational radiation via direct impact, we predict their unpulsed optical/UV light to be that of the Teff>50,000$ K accreting WD. At longer Porb we calculate the Teff and absolute visual magnitude, M_V, that the accreting WD will have during low accretion states, and find that the WD naturally crosses the pulsational instability strip. Discovery and study of pulsations could allow for the measurement of the accumulated helium mass on the accreting WD, as well as its rotation rate. Accretion heats the WD core, but for Porb>40 minutes, the WD's Teff is set by its cooling as plummets. For the two long period AM CVn binaries with measured parallaxes, GP Com and CE 315, we show that the optical broadband colors and intensity are that expected from a pure helium atmosphere WD. This confirms that the WD brightness sets the minimum light in wide AM CVn binaries, allowing for meaningful constraints on their population density from deep optical searches, both in the field and in Globular Clusters.

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arXiv2005

Cygnus X-3 and the problem of the missing Wolf-Rayet X-ray binaries

Cygnus X-3 is a strong X-ray source (L_X about 10^38 erg/s) which is thought to consist of a compact object, accreting matter from a helium star. We find analytically that the estimated ranges of mass-loss rate and orbital-period derivative for Cyg X-3 are consistent with two models: i) the system is detached and the mass loss from the system comes from the stellar wind of a massive helium star, of which only a fraction that allows for the observed X-ray luminosity is accreted, or ii) the system is semidetached and a Roche-lobe-overflowing low- or moderate-mass helium donor transfers mass to the compact object, followed by ejection of its excess over the Eddington rate from the system. These analytical results appear to be consistent with evolutionary calculations. By means of population synthesis we find that currently in the Galaxy there may exist ~1 X-ray binary with a black hole that accretes from a >~ 7 MSun Wolf-Rayet star and ~1 X-ray binary in which a neutron star accretes matter from a Roche-lobe-overflowing helium star with mass <~ 1.5 MSun. Cyg X-3 is probably one of these systems.

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arXiv2005

AM CVn stars

I review our observational and theoretical knowledge of AM CVn stars, focusing on recent developments. These include newly discovered systems, the possibility that two recently discovered extremely short period objects are AM CVn stars and an update on X-ray, UV an optical studies. Theoretical advances include the study of the details of both the donor and accretor, and the physics of the helium accretion discs. I review our (limited) knowledge of the formation of AM CVn stars and the apparent success of the now more than 25 year old suggestion that in these objects the mass transfer is driven by gravitational wave radiation losses. The exciting prospect of directly detecting these gravitational waves and the possibilities this brings conclude this contribution.

astro-ph↗
arXiv2005

XMM-Newton observations of AM CVn binaries

We present the results of XMM-Newton observations of four AM CVn systems -- AM CVn, CR Boo, HP Lib and GP Com. Their light curves show very different characteristics. The X-ray light curves show no coherent pulsations, suggesting the accreting white dwarfs have relatively low magnetic field strengths. Their spectra were best modelled using a multi-temperature emission model and a strong UV component. We find that CR Boo and HP Lib have X-ray spectra with abundances consistent with relatively low temperature CNO processed material, while AM CVn and GP Com show an enhancement of nitrogen. A large fraction of the accretion luminosity is emitted in the UV. We determine accretion luminosities of ~1.6x10^{33} ergs/s and 1.7x10^{31} ergs/s for AM CVn and GP Com respectively. Comparing the implied mass transfer rates with that derived using model fits to optical and UV spectra, we find evidence that in the case of AM CVn, we do not detect a significant proportion of the accretion energy. This missing component could be lost in the form of a wind.

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arXiv2005

The White Dwarf -- White Dwarf galactic background in the LISA data

LISA (Laser Interferometer Space Antenna) is a proposed space mission, which will use coherent laser beams exchanged between three remote spacecraft to detect and study low-frequency cosmic gravitational radiation. In the low-part of its frequency band, the LISA strain sensitivity will be dominated by the incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own galaxy. In order to estimate the magnitude of the LISA response to this background, we have simulated a synthesized population that recently appeared in the literature. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time, reaching a minimum equal to about twice that of the LISA noise for a period of about two months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. Since the galactic white-dwarfs background will be observed by LISA not as a stationary but rather as a cyclostationary random process with a period of one year, we summarize the theory of cyclostationary random processes, present the corresponding generalized spectral method needed to characterize such process, and make a comparison between our analytic results and those obtained by applying our method to the simulated data. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarfs binary systems present in our Galaxy.

gr-qc↗
arXiv2005

Simulation of the White Dwarf -- White Dwarf galactic background in the LISA data

LISA (Laser Interferometer Space Antenna) is a proposed space mission, which will use coherent laser beams exchanged between three remote spacecraft to detect and study low-frequency cosmic gravitational radiation. In the low-part of its frequency band, the LISA strain sensitivity will be dominated by the incoherent superposition of hundreds of millions of gravitational wave signals radiated by inspiraling white-dwarf binaries present in our own galaxy. In order to estimate the magnitude of the LISA response to this background, we have simulated a synthesized population that recently appeared in the literature. We find the amplitude of the galactic white-dwarf binary background in the LISA data to be modulated in time, reaching a minimum equal to about twice that of the LISA noise for a period of about two months around the time when the Sun-LISA direction is roughly oriented towards the Autumn equinox. Since the galactic white-dwarfs background will be observed by LISA not as a stationary but rather as a cyclostationary random process with a period of one year, we summarize the theory of cyclostationary random processes and present the corresponding generalized spectral method needed to characterize such process. We find that, by measuring the generalized spectral components of the white-dwarf background, LISA will be able to infer properties of the distribution of the white-dwarfs binary systems present in our Galaxy.

gr-qc↗
arXiv2005

Arbitrarily Degenerate Helium White Dwarfs as Donors in AM CVn Binaries

We apply the Deloye & Bildsten (2003) isentropic models for donors in ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact, interacting binaries. The mass-radius relations of these systems' donors in the mass range of interest ($M_2<0.1 \msun$) are not single-valued, but parameterized by the donor's specific entropy. This produces a range in the relationships between system observables, such as orbital period, $\Porb$, and mass transfer rate, $\Mdot$. For a reasonable range in donor specific entropy, $\Mdot$ can range over several orders of magnitude at fixed $\Porb$. We determine the unique relation between $\Mdot$ and $M_2$ in the AM CVn systems with known donor to accretor mass ratios, $q=M_2/M_1$. We use structural arguments, as well as each system's photometric behavior, to place limits on $\Mdot$ and $M_2$ in each. Most systems allow a factor of about 3 variation in $\Mdot$, although V803 Cen, if the current estimates of its $q$ are accurate, is an exception and must have $M_2 \approx 0.02 \msun$ and $\Mdot \approx 10^{-10} \msun$ yr$^{-1}$. Our donor models also constrain each donor's core temperature, $T_c$, range and correlate $T_c$ with $M_2$. We examine how variations in donor specific entropy across the white dwarf family \citep{nele01a} of AM CVn systems affects this population's current galactic distribution. Allowing for donors that are not fully degenerate produces a shift in systems towards longer $\Porb$ and higher $\Mdot$ increasing the parameter space in which these systems can be found. This shift increases the fraction of systems whose $\Porb$ is long enough that their gravity wave (GW) signal is obscured by the background of detached double white dwarf binaries that dominate the GW spectrum below a frequency $\approx 2$ mHz.

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arXiv2001

The Quiescent Spectrum of the AM CVn star CP Eri

We used the 6.5m MMT to obtain a spectrum of the AM CVn star CP Eri in quiescence. The spectrum is dominated by He I emission lines, which are clearly double peaked with a peak-to-peak separation of ~1900 km/s. The spectrum is similar to that of the longer period AM CVn systems GP Com and CE 315, linking the short and the long period AM CVn systems. In contrast with GP Com and CE 315, the spectrum of CP Eri does not show a central 'spike' in the line profiles, but it does show lines of SiII in emission. The presence of these lines indicates that the material being transferred is of higher metallicity than in GP Com and CE 315, which, combined with the low proper motion of the system, probably excludes a halo origin of the progenitor of CP Eri. We constrain the primary mass to M_1>0.27 M_sun and the orbital inclination to 33 degr < i < 80 degr. The presence of the He I lines in emission opens up the possibility for phase resolved spectroscopic studies which allows a determination of the system parameters and a detailed study of helium accretion disks under highly varying circumstances.

astro-ph↗
arXiv2000

The population of close double white dwarfs in the Galaxy

We present a new model for the Galactic population of close double white dwarfs. The model accounts for the suggestion of the avoidance of a substantial spiral-in during mass transfer between a giant and a main-sequence star of comparable mass and for detailed cooling models. It agrees well with the observations of the local sample of white dwarfs if the initial binary fraction is close to 50% and an ad hoc assumption is made that white dwarfs with mass less than about 0.3 solar mass cool faster than the models suggest. About 1000 white dwarfs brighter than V=15 have to be surveyed for detection of a pair which has total mass greater than the Chandrasekhar mass and will merge within 10 Gyr.

astro-ph↗
arXiv2000

Population synthesis for double white dwarfs I.Close detached systems

We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies, due to two modifications. The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs. A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is ~ 50% and that the lowest mass white dwarfs (M < 0.3 Msun) cool faster than the most recently published cooling models predict. With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr^{-1}, a birth rate of AM CVn systems of 0.005 yr^{-1}, a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr^{-1} and a formation rate of planetary nebulae of 1 yr^{-1}. We estimate the total number of double white dwarfs in the Galaxy as 2.5 10^8. In an observable sample with a limiting magnitude V_lim = 15 we predict the presence of ~855 white dwarfs of which ~220 are close pairs. Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time.

astro-ph↗
arXiv2000

Reconstructing the evolution of double helium white dwarfs: envelope loss without spiral-in

The unique core-mass - radius relation for giants with degenerate helium cores enables us to reconstruct the evolution of three observed double helium white dwarfs with known masses of both components. The last mass transfer phase in their evolution must have been a spiral-in. In the formalism proposed by Webbink (1984), we can constrain the efficiency of the deposition of orbital energy into the envelope to be $1 \la α\la 6$, for an envelope structure parameter $λ=0.5$. We find that the two standard mass transfer types (stable mass transfer and spiral-in) are both unable to explain the first phase of mass transfer for these three binaries. We use a parametric approach to describe mass transfer in low-mass binaries, where both stars are of comparable mass and find that the orbital characteristics of the observed double helium white dwarfs can be well reproduced if the envelope of the primary is lost with ~1.5 times the specific angular momentum of the initial binary. In this case no substantial spiral-in occurs.

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