arXiv subjects

U. Kolb

Publications and source records attributed to U. Kolb.

At least 55 records · Page 3Linked to original sources

arXiv2005

A Potential Supernova Remnant/X-ray Binary Association in M31

The well-studied X-ray/Optical/Radio supernova remnant DDB 1-15 (CXOM31 J004327.8+411829; r3-63) in M31 has been investigated with archival XMM-Newton and Chandra observations. The timing data from XMM-Newton reveals a power density spectrum (PDS) characteristic of accreting compact objects in X-ray binaries (XRBs). The PDS shows features typical of Roche lobe overflow accretion, hinting that the XRB is low-mass. The Chandra observations resolve the SNR into a shell and show a variable count rate at the 94% confidence level in the northwest quadrant. Together, these XMM-Newton and Chandra data suggest that there is an XRB in the SNR r3-63 and that the XRB is located in the northwestern portion of the SNR. The currently-available X-ray and optical data show no evidence that the XRB is high-mass. If the XRB is low-mass, r3-63 would be the first SNR found to contain a low-mass X-ray binary.

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arXiv2004

Identifying a black hole X-ray transient in M31 with XMM-Newton and Chandra

Stochastic variability in two out of four XMM-Newton observations of XMMU J004303+4115 along with its power spectra and X-ray luminosities suggest a low-mass X-ray binary (LMXB) with a black hole primary. However, Chandra observations resolve the object into two point sources. We use data from 35 Chandra observations to analyse the contributions of each source, and attribute the variability to CXOM31 J004303.2+411528 (known as r2-3), which varies in intensity by a factor of ~100 between observations. We assume that the power density spectra of LMXBs are governed by the luminosity, and that the transition between types of power density spectra occurs at some critical luminosity in Eddington units, l_c, that applies to all LMXBs. We use results from these XMM-Newton observations and past results from the available literature to estimate this transition luminosity, and find that all results are consistent with l_c ~0.1 in the 0.3--10 keV band. CXOM31 J004303.2+411528 exhibits a low accretion rate power density spectrum at a 0.3--10 keV luminosity of (5.3+/-0.6)x10^{37} erg/s. Known stellar mass black holes have masses of 4--15 M_{\odot}; hence our observations of CXOM31 J004303.2+411528 are consistent with l_c ~0.1 if it has a black hole primary.

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arXiv2004

The orbital period distribution of wide binary millisecond pulsars

We present results of a binary population synthesis study on the orbital period distribution of wide binary millisecond pulsars forming through four evolutionary channels. In three of the channels, the progenitor of the millisecond pulsar undergoes a common envelope phase prior to the supernova explosion which gives birth to the neutron star. In the fourth channel, the primary avoids the common-envelope phase and forms a neutron star when it ascends the asymptotic giant branch. The four formation channels yield an orbital period distribution which typically shows a short-period peak below 10 days, a long-period peak around 100 days, and a cut-off near 200 days. The agreement with the orbital period distribution of observed binary millisecond pulsars in the Galactic disk is best when the common-envelope ejection is efficient, the mass-transfer phase responsible for spinning up the pulsar is highly non-conservative, and no or moderate supernova kicks are imparted to neutron stars at birth.

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arXiv2004

Detached white dwarf main-sequence star binaries

We considered the formation of detached white dwarf main-sequence star (WDMS) binaries through seven evolutionary channels subdivided according to the evolutionary process that gives rise to the formation of the white dwarf or its helium-star progenitor: dynamically stable Roche-lobe overflow (Algol-type evolution), dynamically unstable Roche-lobe overflow (common-envelope evolution), or stellar winds (single star evolution). We examine the sensitivity of the population to changes in the amount of mass lost from the system during stable Roche-lobe overflow, the common-envelope ejection efficiency, and the initial mass ratio or initial secondary mass distribution. In the case of a flat initial mass ratio distribution, the local space density of WDMS binaries is of the order of 10^{-3}/pc^3. This number decreases to 10^{-4}/pc^3 when the initial mass ratio distribution is approximately proportional to the inverse of the initial mass ratio. More than 75% of the WDMS binary population stems from wide systems in which both components evolve as if they were single stars. The remaining part of the population is dominated by systems in which the white dwarf is formed in a common-envelope phase. The birthrate of WDMS binaries forming through a common-envelope phase is about 10 times larger than the birthrate of WDMS binaries forming through a stable Roche-lobe overflow phase. The ratio of the number of helium white dwarf systems to the number of carbon/oxygen or oxygen/neon/magnesium white dwarf systems derived from large samples of observed WDMS binaries by, e.g., future planet-search missions such as SuperWASP, COROT, and Kepler may furthermore constrain the common-envelope ejection efficiency.

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arXiv2003

Tracing a Z-track in the M31 X-ray Binary RX J0042.6+4115

Four XMM-Newton observations of the core of M31, spaced at 6 month intervals, show that the brightest point X-ray source, RX J0042.6+4115, has a 0.4-10 keV luminosity of ~5 10^38 erg/s, and exhibits significant variability in intensity and X-ray spectrum over a time scale of ~100 s including hard flares; such behaviour is only observed in Z-sources and transient blackhole binaries in our Galaxy. The lightcurves, X-ray spectra and hardness-intensity data from the four XMM-Newton observations all strongly suggest that it is a Z-source, bringing the total number of known Z-sources to nine.

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arXiv2003

On the detection of pre-low-mass X-ray binaries

We explore the population of candidate pre-low-mass X-ray binaries in which a neutron star accretes mass from the wind of a low-mass companion (mass < 2Msun) in the framework of a binary population synthesis study. The simulated accretion-luminosity distribution shows a primary peak close to 1e31 erg/s and a secondary peak near 1e28 erg/s. The relative contribution of the two peaks depends primarily on the magnitude of the kick velocity imparted to the neutron star at birth. The secondary peak is negligible for average kick velocities larger than 200 km/s, but becomes dominant for average kick velocities smaller than 50 km/s. Regardless of the relative contributions of the two peaks, our calculations suggest that pre-low-mass X-ray binaries may provide a non-negligible contribution to the population of discrete low-luminosity X-ray sources in the Galaxy.

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arXiv2003

RX J0042.3+4115: a stellar mass black hole binary identified in M31

Four XMM-Newton X-ray observations of the central region of the Andromeda Galaxy (M31) have revealed an X-ray source that varies in luminosity over \~1--3, 10^{38} erg s^{-1} between observations and also displays significant variability over time-scales of a few hundred seconds. The power density spectra of lightcurves obtained in the 0.3--10 keV energy band from the three EPIC instruments on board XMM-Newton are typical of disc-accreting X-ray binaries at low accretion rates, observed in neutron star binaries only at much lower luminosities (~10^{36} erg s^{-1}). However X-ray binaries with massive black hole primaries have exhibited such power spectra for luminosities >10^{38} erg s^{-1}. We discuss alternative possibilities where RX J0042.3+4115 may be a background AGN or foreground object in the field of view, but conclude that it is located within M31 and hence use the observed power spectra and X-ray luminosities to identify the primary as a black hole candidate.

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arXiv2003

Catalogue of cataclysmic binaries, low-mass X-ray binaries and related objects (Seventh edition)

The catalogue lists coordinates, apparent magnitudes, orbital parameters, and stellar parameters of the components and other characteristc properties of 472 cataclysmic binaries, 71 low-mass X-ray binaries and 113 related objects with known or suspected orbital periods together with a comprehensive selection of the relevant recent literature. In addition the catalogue contains a list of references to published finding charts for 635 of the 656 objects, and a cross-reference list of alias object designations. Literature published before 1 January 2003 has, as far as possible, been taken into account. All data can be accessed via the dedicated catalogue webpage at http://www.mpa-garching.mpg.de/RKcat/ (MPA) and http://physics.open.ac.uk/RKcat/ (OU). We will update the information given on the catalogue webpage regularly, initially every six months.

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arXiv2002

AE Aquarii: how CVs descend from supersoft binaries

AE Aquarii is a propeller system. It has the shortest spin period among cataclysmic variables, and this is increasing on a 10^7 yr timescale. Its UV spectrum shows very strong carbon depletion vs nitrogen and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spindown timescale of AE Aqr requires a high birthrate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of order one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the CIV1550 / NV1238 ratio, in determining how large the observed fraction is.

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arXiv2002

Population synthesis of wide binary millisecond pulsars

Four evolutionary channels leading to the formation of wide binary millisecond pulsars are investigated. The majority of binary millisecond pulsars are found to descend from systems in which the most massive component undergoes a common-envelope phase prior to the supernova explosion leading to the birth of the neutron star. The orbital period distribution of simulated samples of wide binary millisecond pulsars is compared with the observed distribution of Galactic binary millisecond pulsars for a variety of parameters describing the formation and evolution of binaries. The distribution functions typically show a short-period peak below 10 days and a long-period peak around 100 days. The observed distribution is best reproduced by models with highly non-conservative mass transfer, common-envelope efficiencies equal to or larger than unity, a critical mass ratio for the delayed dynamical instability larger than 3, and no or moderate supernova kicks at the birth of the neutron star. Few systems are found with orbital periods longer than 200 days, irrespective of the accretion efficiency of neutron stars. This occurs as a result of the upper limit on the initial orbital periods beyond which the binary avoids the common-envelope phase prior to the supernova explosion of the primary.

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arXiv2002

XMM-Newton detection of Nova Muscae 1991 in Quiescence

The soft X-ray transient GU Mus has been detected by XMM-Newton in the quiescent state. The source is very faint, with a 0.5-10.0 keV unabsorbed flux of $\simeq 1.1 \times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$. The spectra is well fit by an absorbed powerlaw with a photon index of $ α= 1.6 \pm 0.4$, close to the value seen when the source was in the low/hard state in Aug. 1991. From our observed luminosity, it seems unlikely that the quiescent state emission is dominated by coronal X-rays from the secondary. The flux also appears to be in agreement with the ADAF model of BH-transients in quiescence.

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arXiv2002

Distortion of secondaries in semi-detached binaries and the cataclysmic variable period minimum

Based on SPH simulations, we quantify the geometrical distortion effect due to tidal and rotational forces on polytropic secondaries in semi-detached binaries. The main effect is an expansion of the polytropic star, with an effect on the radius of $\sim$ 5%-12%, depending on the polytropic index and the mass ratio. We apply such distortion effects to the secular evolution of secondaries in cataclysmic variable systems. We focus on systems below the 2-3h period gap and that approach the minimum period. We find a significant increase of the predicted minimum period ($\sim$ 4% if changes in the secondary's thermal relaxation are approximately taken into account). Though an improvement, the effect is not big enough to solve the mismatch between predicted and observed minimum period at 80 min.

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arXiv2002

The systemic velocities of four long-period cataclysmic variable stars

Although a large number of orbital periods of cataclysmic variable stars (CVs) have been measured, comparison of period and luminosity distributions with evolutionary theory is affected by strong selection effects. A test has been discovered which is independent of these selection effects and is based upon the kinematics of CVs (Kolb & Stehle, 1996). If the standard models of evolution are correct then long-period (P_orb > 5 hrs) CVs should be typically less than 1.5 Gyr old, and their line-of-sight velocity dispersion ($σ_γ$) should be small. We present results from a pilot study which indicate that this postulate is indeed true. Four long-period dwarf novae (EM Cyg, V426 Oph, SS Cyg and AH Her) were observed over a complete orbit, in order that accurate radial velocities be obtained. We find values of -1.7, 5.4, 15.4 and 1.8 km/s with uncertainties of order 3 km/s, referred to the dynamical Local Standard of Rest (LSR), leading to a dispersion of ~ 8 km/s. Calculation of a 95 per cent confidence interval gives the result 4 < σ_γ< 28 km/s compared to a prediction of 15 km/s. We also have an improved determination of mass donor spectral type, K_2 and q for the four systems.

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arXiv2001

Braking and bouncing

I discuss the case for an efficient orbital braking mechanism in CVs, and the effect of nova outbursts on the observed range of mass transfer rates. I review the continuing problem that models have in interpreting the short-period cut-off of the CV period distribution.

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arXiv2001

A first look at cataclysmic variable stars from the 2dF QSO survey

The 2dF QSO survey is a spectroscopic survey of 48,000 point-sources selected by colour with magnitudes in the range 18.35 < B < 20.95. Amongst QSOs, white dwarfs, narrow-line galaxies and other objects are some cataclysmic variables (CVs). This survey should be sensitive to intrinsically faint CVs. In the standard picture of CV evolution, these form the majority of the CV population. We present the spectra of 6 CVs from this survey. Four have the spectra of dwarf novae and two are magnetic CVs. We present evidence that suggests that the dwarf novae have period P < 2 h and are indeed intrinsically less luminous than average. However, it is not clear yet whether these systems are present in the large numbers predicted.

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arXiv2000

Mass estimates in short-period compact binaries

Using stellar models we investigate the relation between mass and the spectral type of the secondary star in low-mass short-period compact binaries such as cataclysmic variables and soft X-ray transients. Allowing for different mass transfer rates and different system ages prior to mass transfer we find that the secondaries should populate a band in the spectral type vs. mass plane. The mass M_ms of a ZAMS star with the same spectral type as the donor is effectively an upper limit to the donor mass. The lower mass limit for a given spectral type depends on the mixing length parameter. If this is large, there is no lower limit if the spectral type is later than K6. The band width decreases from 0.4 M_sun at K6 to less than 0.2 M_sun at K0. If the mixing length parameter is small, there is no lower mass limit if the spectral type is later than M2, and the band width decreases from 0.2 M_sun at M2 to less than 0.1 M_sun for types earlier than K0. We also point out an error in the method suggested by Beekman et al. (1997) to estimate the primary mass in a soft X-ray transient.

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arXiv2000

The minimum orbital period in thermal-timescale mass transfer

We show that the usual picture of supersoft X-ray binary evolution as driven by conservative thermal-timescale mass transfer cannot explain the short orbital periods of RX J0537.7-7034 (3.5 hr) and 1E 0035.4-7230 (4.1 hr). Non-conservative evolution may produce such periods, but requires very significant mass loss, and is highly constrained.

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arXiv2000

Irradiation and mass transfer in low-mass compact binaries

We study the reaction of low-mass stars to anisotropic irradiation and its importance for the long-term evolution of compact binaries binaries. We show by means of a simple homology model that if the energy outflow through the surface layers of a low-mass main sequence star is blocked over a fraction s_eff < 1 of its surface it will inflate only modestly and that the maximum contribution to mass transfer is s_eff times what one obtains in the isotropic case. The duration of this mass transfer phase is roughly -0.1 ln(1-s_eff) times the thermal time scale of the convective envelope. Numerical computations confirm these results. Second, we present a simple analytic one-zone model for computing the blocking effect by irradiation which gives results in acceptable quantitative agreement with detailed numerical computations. Third, we show that unless mass transfer is strongly enhanced by consequential angular momentum losses, cataclysmic variables are stable against mass transfer if the mass of the main sequence donor is less than about 0.7 solar masses. Otherwise systems may be unstable, subject to the efficiency of irradiation. We argue that low-mass X-ray binaries are even less susceptible to this instability. In case of instability, mass transfer must evolve through a limit cycle in which phases of high mass transfer alternate with phases of small (or no) mass transfer.

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