arXiv subjects

Rasmus Voss

Publications and source records attributed to Rasmus Voss.

16 recordsLinked to original sources

arXiv2014

The influence of mass-transfer variability on the growth of white dwarfs, and the implications for supernova type Ia rates

White dwarfs (WDs) can increase their mass by accretion from companion stars, provided the mass-accretion rate is high enough to avoid nova eruptions. The accretion regimes that allow growth of the WDs are usually calculated assuming constant mass-transfer rates. However, it is possible that these systems are influenced by effects that cause the rate to fluctuate on various timescales. We investigate how long-term mass-transfer variability affects accreting WDs systems. We show that, if such variability is present, it expands the parameter space of binaries where the WD can effectively increase its mass. Furthermore, we find that the supernova type Ia (SNIa) rate is enhanced by a factor 2-2.5 to a rate that is comparable with the lower limit of the observed rates. The changes in the delay-time distribution allow for more SNIae in stellar populations with ages of a few Gyr. Thus, mass-transfer variability gives rise to a new formation channel of SNIa events that can significantly contribute to the SNIa rate. Mass-transfer variability is also likely to affect other binary populations through enhanced WD growth. For example, it may explain why WDs in cataclysmic variables are observed to be more massive than single WDs, on average.

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arXiv2013

On double-degenerate type Ia supernova progenitors as supersoft X-ray sources - A population synthesis analysis using SeBa

Context: The nature of the progenitors of type Ia supernova (SN) progenitors remains unclear. While it is usually agreed that single-degenerate progenitor systems would be luminous supersoft X-ray sources (SSSs), it was recently suggested that double-degenerate progenitors might also go through a SSS phase. Aims: We aim to examine the possibility of double-degenerate progenitor systems being SSSs, and place stringent upper limits on the maximally possible durations of any SSS phases and expected number of these systems in a galactic population. Method: We employ the binary population synthesis code SeBa to examine the mass-transfer characteristics of a possible SSS phase of double-degenerate type Ia SN progenitor systems for 1) the standard SeBa assumptions, and 2) an optimistic best-case scenario. The latter case establishes firm upper limits on the possible population of supersoft source double-degenerate type Ia SN progenitor systems. Results: Our results indicate that unlike what is expected for single-degenerate progenitor systems, the vast majority of the material accreted by either pure wind mass transfer or a combination of wind and RLOF mass transfer is helium rather than hydrogen. Even with extremely optimistic assumptions concerning the mass-transfer and retention efficiencies, the average mass accreted by systems that eventually become double-degenerate type Ia SNe is small. Consequently, the lengths of time that these systems may be SSSs are short, even under optimal conditions, and the expected number of such systems in a galactic population is negligible. Conclusions: The population of double-degenerate type Ia SN progenitors that are SSSs is at least an order of magnitude smaller than the population of single-degenerate progenitors expected to be SSSs, and the supersoft X-ray behaviour of double-degenerate systems typically ceases long before the SN explosions.

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arXiv2013

Spectral Properties of X-ray Binaries in Centaurus A

We present a spectral investigation of X-ray binaries in NGC 5128 (Cen A), using six 100 ks Chandra observations taken over two months in 2007. We divide our sample into thermally and non-thermally dominated states based on the behavior of the fitted absorption column, and present the spectral parameters of sources with L >2x10^37 erg/s. The majority of sources are consistent with being neutron star low mass X-ray binaries (NS LMXBs) and we identify three transient black hole (BH) LMXB candidates coincident with the dust lane, which is the remnant of a small late-type galaxy. Our results also provide tentative support for the apparent `gap' in the mass distribution of compact objects between ~2-5 Msol. We propose that BH LMXBs are preferentially found in the dust lane, and suggest this is because of the younger stellar population. The majority (~70-80%) of potential Roche-lobe filling donors in the Cen A halo are >12 Gyr old, while BH LMXBs require donors >1 Msol to produce the observed peak luminosities. This requirement for more massive donors may also explain recent results that claim a steepening of the X-ray luminosity function with age at Lx >= 5x10^38 erg/s for the XB population of early-type galaxies; for older stellar populations, there are fewer stars >1 Msol, which are required to form the more luminous sources.

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arXiv2012

Feedback by massive stars and the emergence of superbubbles I. Energy efficiency & Vishniac instabilities

We study the evolution of isolated and merging interstellar bubbles of three stars (25, 32 and 60 M\odot) in a homogeneous background medium with a density of 10 mp /ccm via 3D-hydrodynamic simulations with standard ISM thermodynamics (optically thin radiative cooling and photo-electric heating) and time dependent energy and mass input according to stellar evolutionary tracks. We vary the position of the three stars relative to each other to compare the energy response for cases of isolated, merging and initially cospatial bubbles. Due to mainly the Vishniac instability, our simulated bubbles develop thick shells and filamentary internal structures in column density. The shell widths reach tens of per cent of the outer bubble radius, which compares favourably to observations. More energy is retained in the ISM for more closely packed groups, by up to a factor of three and typically a factor of two for intermediate times after the first supernova. Once the superbubble is established, different positions of the contained stars make only a minor difference to the energy tracks. For our case of three massive stars, the energy deposition varies only very little for distances up to about 30 pc between the stars. Energy injected by supernovae is entirely dissipated in a superbubble on a timescale of about 1 Myr, which increases slightly with the superbubble size at the time of the explosion. The Vishniac instability may be responsible for the broadening of the shells of interstellar bubbles. Massive star winds are significant energetically due to their - in the long run - more efficient, steady energy injection and because they evacuate the space around the massive stars. For larger scale simulations, the feedback effect of close groups of stars or clusters may be subsumed into one effective energy input with insignificant loss of energy accuracy.

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arXiv2012

Obscuration of Supersoft X-ray Sources by Circumbinary Material - A Way to Hide Type Ia Supernova Progenitors?

The progenitors of supernovae (SNe) type Ia are usually assumed to be either a single white dwarf (WD) accreting from a non-degenerate companion (the SD channel) or the result of two merging WDs (DD channel). However, no consensus currently exists as to which progenitor scenario is the correct one, or whether the observed SN Ia rate is produced by a combination of both channels. Unlike a DD progenitor a SD progenitor is expected to emit supersoft X-rays for a prolonged period of time (~1 Myr) as a result of the burning of accreted matter on the surface of the WD. An argument against the SD channel as a significant producer of SNe type Ia has been the lack of observed supersoft X-ray sources (SSS) and the lower-than-expected integrated soft X-ray flux from elliptical galaxies. We wish to determine if it is possible to obscure the supersoft X-ray emission from a nuclear burning white dwarf in an accreting single degenerate binary system. In case of obscured systems we wish to determine their general observational characteristics. We examine the emergent X-ray emission from a canonical SSS system surrounded by a spherically symmetric configuration of material, assuming a black body spectrum with T_BB=50 eV and L=10^38 erg/s. The circumbinary material is assumed to be of solar chemical abundances, and we leave the mechanism behind the mass loss into the circumbinary region unspecified. If steadily accreting, nuclear burning WDs are canonical SSS our analysis suggests that they can be obscured by relatively modest circumbinary mass loss rates. This may explain the discrepancy of SSS compared to the SN Ia rate inferred from observations if the SD progenitor scenario contributes significantly to the SN Ia rate. Recycled emissions from obscured systems may be visible in other wavebands than X-rays. It may also explain the lack of observed SSS in symbiotic binary systems.

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arXiv2012

Upper limits on bolometric luminosities of ten type Ia supernova progenitors from Chandra observations

We present an analysis of Chandra observations of the position of ten nearby (< 25 Mpc) type Ia supernovae, taken before the explosions. No sources corresponding to progenitors were found in any of the observations. We calculated upper limits on the bolometric luminosities of the progenitors assuming black-body X-ray spectra with temperatures of 30-150 eV. This is inspired by the fact that luminous super-soft X-ray sources have been suggested as the direct progenitors of type Ia supernovae. The upper limits of two supernovae in our sample are comparable to the luminosities of the brightest observed super-soft sources, ruling out such sources as the progenitors of these supernovae. In contrast to Liu et al (2012) we find that for SN2011fe we can rule out Eddington luminosity systems for black body temperatures as low as 40 eV. Our findings are consistent with statistical studies comparing the observed type Ia supernova rate to the number of super-soft sources or the integrated X-ray luminosity in external galaxies. This suggest that either the progenitors of type Ia supernovae are not accreting, nuclear burning white dwarfs, or that they do not look like the classical super-soft sources, e.g. because they are obscured.

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arXiv2012

A Transient Sub-Eddington Black Hole X-ray Binary Candidate in the Dust Lanes of Centaurus A

We report the discovery of a bright X-ray transient, CXOU J132527.6-430023, in the nearby early-type galaxy NGC 5128. The source was first detected over the course of five Chandra observations in 2007, reaching an unabsorbed outburst luminosity of 1-2*10^38 erg/s in the 0.5-7.0 keV band before returning to quiescence. Such luminosities are possible for both stellar-mass black hole and neutron star X-ray binary transients. Here, we attempt to characterize the nature of the compact object. No counterpart has been detected in the optical or radio sky, but the proximity of the source to the dust lanes allows for the possibility of an obscured companion. The brightness of the source after a >100 fold increase in X-ray flux makes it either the first confirmed transient non-ULX black hole system in outburst to be subject to detailed spectral modeling outside the Local Group, or a bright (>10^38 erg/s) transient neutron star X-ray binary, which are very rare. Such a large increase in flux would appear to lend weight to the view that this is a black hole transient. X-ray spectral fitting of an absorbed power law yielded unphysical photon indices, while the parameters of the best-fit absorbed disc blackbody model are typical of an accreting ~10 Msol black hole in the thermally dominant state.

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arXiv2011

Luminosity functions of LMXBs in different stellar environments

Based on the archival data from the Chandra observations of nearby galaxies, we study different sub populations of low-mass X-ray binaries (LMXBs) - dynamically formed systems in globular clusters (GCs) and in the nucleus of M31 and (presumably primordial) X-ray binaries in the fields of galaxies. Our aim is to produce accurate luminosity distributions of X-ray binaries in different environments, suitable for quantitative comparison with each other and with the output of population synthesis calculations. Our sample includes seven nearby galaxies (M31, Maffei 1, Centaurus A,M81, NGC 3379, NGC 4697, and NGC 4278) and the Milky Way, which together provide relatively uniform coverage down to the luminosity limit of E35 erg/s. In total we have detected 185 LMXBs associated with GCs, 35 X-ray sources in the nucleus of M31, and 998 field sources of which ~ 365 are expected to be background AGN. We combine these data, taking special care to accurately account for X-ray and optical incompleteness corrections and the removal of the contamination from the cosmic X-ray background sources, to produce luminosity distributions of X-ray binaries in different environments to far greater accuracy than has been obtained previously. We found that luminosity distributions of GC and field LMXBs differ throughout the entire luminosity range, the fraction of faint (log(Lx) < 37) sources among the former being ~ 4 times less than in the field population. The X-ray luminosity function (XLF) of sources in the nucleus of M31 is similar to that of GC sources at the faint end but differs at the bright end, with the M31 nucleus hosting significantly fewer bright sources. We discuss the possible origin and potential implications of these results.

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arXiv2011

Finding Supernova Ia Progenitors with the Chandra X-ray Observatory

We examine pre-supernova Chandra images to find X-ray luminosities of type Ia supernova progenitors. At present, we have one possible direct detection and upper limits for the X-ray luminosities of a number of other supernova progenitors. The method has also yielded a possible detection of a X-ray binary Wolf-Rayet system as the progenitor of a type Ib supernova.

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arXiv2010

Radioactive 26Al from the Scorpius-Centaurus Association

The Scorpius-Centaurus association is the most-nearby group of massive and young stars. As nuclear-fusion products are ejected by massive stars and supernovae into the surrounding interstellar medium, the search for characteristic gamma-rays from radioactivity is one way to probe the history of activity of such nearby massive stars on a My time scale through their nucleosynthesis. 26Al decays within ~1 My, 1809 keV gamma-rays from its decay can be measured with current gamma-ray telescopes, such as INTEGRAL's gamma-ray spectrometer SPI. Following earlier 26Al gamma-ray mapping with NASA's Compton observatory, we test spatial emission skymaps of 26Al for a component which could be attributed to ejecta from massive stars in the Scorpius-Centaurus group of stars. Such a model fit of spatial distributions for large-scale and local components is able to discriminate 26Al emission associated with Scorpius-Centaurus, in spite of the strong underlying nucleosynthesis signal from the Galaxy at large. We find an 26Al signal above 5 sigma significance, which we associate with the Sco-Cen group. The observed flux of 6 *10^{-5}ph cm^{-2} s^{-1} corresponds to 1.1 *10^{-4} M_sol of 26Al. This traces the nucleosynthesis ejecta of several massive stars within the past several million years. We confirm through direct detection of radioactive 26Al the recent ejection of massive-star nucleosynthesis products from the Sco-Cen association. Its youngest subgroup in Upper Scorpius appears to dominate 26Al contributions from this association. Our 26Al signal can be interpreted as a measure of the age and richness of this youngest subgroup. We also estimate a kinematic imprint of these nearby massive-star ejecta from the bulk motion of 26Al and compare this to other indications of Scorpius-Centaurus massive-star activity .

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arXiv2008

On the detection of the progenitor of the type Ia supernova 2007on

We present new Chandra X-ray observations and detailed astrometry of the field of the type Ia supernova 2007on, for which the detection of a likely progenitor in archival Chandra data was recently reported. No source is detected in the new Chandra images, taken six weeks after optical maximum. We calculate a 90-99% probability that any X-ray source near the position of the supernova (SN) is fainter than in the pre-outburst images, depending on the choice of aperture, which supports the identification of the archival X-ray source with the SN. Detailed astrometry of the X-ray and new optical images, however, gives an offset between the supernova and the measured X-ray source position of 1.15+/-0.27". Extensive simulations show that the probability of finding an offset of this magnitude is ~1%, equal to the (trial-corrected) probability of a chance alignment with any X-ray source in the field. This casts doubt on the identification of the X-ray source with the progenitor, although the scenario in which at least some of the observed X-rays are connected to the supernova may be the least unlikely based on all available data. After a brief review of the auxiliary evidence, we conclude that only future X-ray observations can shed further light on the proposed connection between the X-ray source and the progenitor of SN 2007on, and thus whether an accreting white dwarf scenario is truly favoured for this SN Ia.

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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.

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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

The dynamical formation of LMXBs in dense stellar environments: globular clusters and the inner bulge of M31

The radial distribution of luminous L_X>10^{36} erg/s X-ray point sources in the bulge of M31 is investigated using archival Chandra observations. We find a significant increase of the specific frequency of X-ray sources, per unit stellar mass, within 1 arcmin from the centre of the galaxy. The radial distribution of surplus sources in this region follows the density squared law, suggesting that they are low-mass X-ray binaries formed dynamically in the dense inner bulge. We investigate dynamical formation of LMXBs, paying particular attention to the high velocity regime characteristic for galactic bulges, which has not been explored previously. Our calculations suggest that the majority of the surplus sources are formed in tidal captures of black holes by main sequence stars of low mass, M<0.3-0.4 M_sol, with some contribution of NS systems of same type. Due to the small size of the accretion discs a fraction of such systems may be persistent X-ray sources. Some of sources may be ultra-compact X-ray binaries with helium star/white dwarf companions. We also predict a large number of faint transients, both NS and BH systems, within 1 arcmin from the M31 galactic centre. Finally, we consider the population of dynamically formed binaries in Galactic globular clusters, emphasizing the differences between these two types of stellar environments

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arXiv2007

A study of the population of LMXBs in the bulge of M31

The purpose of this study is to explore the population of X-ray point sources in the bulge of M31, with the primary goal to contrast properties of various subpopulations, such as persistent and transient sources, primordial LMXBs and dynamically formed ones. Based on the data from 26 archival Chandra observations we study the source content and properties of various subpopulations of X-ray sources to a maximum distance of 12 arcmin from the centre of M31. A study of the spatial distribution and the luminosity function of the X-ray sources shows that the distribution of primordial LMXBs is consistent with the distribution of the K-band light and that their luminosity function flattens below ~10^{37} erg/s to the dN/dL proportional to 1/L law in agreement with the behaviour found earlier for LMXBs in the Milky Way and in Cen A. The luminosity function of dynamically formed LMXBs shows a prominent fall-off below log(L_X)<36.5. Although the statistics is insufficient to claim a genuine low-luminosity cut-off in the luminosity function, the best fit powerlaw with a slope of -0.6+-0.2 is significantly flatter than the dN/dL proportional to 1/L law. We found 28 transient X-ray sources. Their spatial distribution follows the distribution of the persistent LMXBs within the accuracy allowed by the limited number of transients.

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arXiv2003

Galactic distribution of merging neutron stars and black holes - prospects for short GRB progenitors and LIGO/VIRGO

We have performed detailed population synthesis on a large number (20 million) of binary systems in order to investigate the properties of massive double degenerate binaries. We have included new important results in our input physics in order to obtain more reliable estimates of the merging timescales and relative formation rates. These improvements include refined treatment of the binding energy in a common envelope, helium star evolution and reduced kicks imparted to newborn black holes. The discovery and observations of GRB afterglows and the identification of host galaxies have allowed comparisons of theoretical distributions of merger sites with the observed distribution of afterglow positions relative to host galaxies. To help investigate the physical nature of short- and long-duration GRBs, we compute the distances of merging neutron stars (NS) and/or black holes (BH) from the centers of their host galaxies, as predicted by their formation scenario combined with motion in galactic potentials. Furthermore, we estimate the formation rate and merging rate of these massive double degenerate binaries. The latter is very important for the prospects of detecting gravitational waves with LIGO/VIRGO. We find that the expected detection rate for LIGO II is ~850 per year for galactic field sources and that this rate is completely dominated by merging BHBH binaries. Even LIGO I may detect such an event (~0.25 per year). Our preferred model estimate the Galactic field NSNS merger rate to be ~1.5*10^(-6) per year. For BHBH systems this model predicts a merger rate of ~9.7*10^{-6} per year. Our studies also reveal an accumulating numerous population of very wide orbit BHBH systems which never merge (t >> t_Hubble).

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