arXiv subjects

U. C. Kolb

Publications and source records attributed to U. C. Kolb.

At least 19 recordsLinked to original sources

arXiv2020

WASP-186 and WASP-187: two hot Jupiters discovered by SuperWASP and SOPHIE with additional observations by TESS

We present the discovery of two new hot Jupiters identified from the WASP survey, WASP-186b and WASP-187b (TOI-1494.01 and TOI-1493.01). Their planetary nature was established from SOPHIE spectroscopic observations, and additional photometry was obtained from TESS. Stellar parameters for the host stars are derived from spectral line, IRFM, and isochrone placement analyses. These parameters are combined with the photometric and radial velocity data in an MCMC method to determine the planetary properties. WASP-186b is a massive Jupiter (4.22 +/- 0.18 M_J, 1.11 +/-0.03 R_J) orbiting a mid-F star on a 5.03 day eccentric (e=0.327 +/- 0.008) orbit. WASP-187b is a low density (0.80 +/- 0.09 M_J, 1.64 +/- 0.05 R_J) planet in a 5.15 day circular orbit around a slightly evolved early F-type star.

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arXiv2020

Monitoring of transiting exoplanets and their host stars with small aperture telescopes

Exoplanet research is now target rich with a wide diversity of systems making it difficult for high demand observatories to undertake follow up observations over extended periods of time. We investigate the effectiveness of using 0.4m-class telescopes for monitoring transiting hot Jupiters and their host stars. We consider two representative case studies: WASP-52b with 13 new transits, and HAT-P-23b with 17 new transits and concurrent photometric monitoring covering 78 days. We present updated system parameters and combine our new transit times with previously published results to calculate new ephemerides for both systems. Our analysis of transit mid-times for WASP-52b results in a slight preference for a quadratic ephemeris ($Δχ_ν^2 = 0.07$, $ΔBIC = 1.53$ over a linear ephemeris. We discuss the reality of this quadratic ephemeris indicating a period change of $ δP / δt = -38.6\pm4{\rm ms~yr^{-1}}$ and consider possible causes. WASP-52 is known to be an active star with previous publications reporting many spot crossing events, however no such events are seen in our new photometry. Our analysis shows that WASP-52 is still active and that the latitude of the spots has likely migrated away from the transit chord. We confirm the inflated nature and circular orbit for HAT-P-23b. Our monitoring of HAT-P-23 reveals a periodicity of 7.015 days with an amplitude of 0.011 mag which we interpret as the rotation period of HAT-P-23. The photometric and transit timing precision achieved in the case studies shows that this class of telescope is capable of precise characterisation and long-term monitoring of transiting hot Jupiters in support of dedicated ongoing and future ground and space based observations.

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arXiv2020

Discovery and characterization of the exoplanets WASP-148b and c. A transiting system with two interacting giant planets

We present the discovery and characterization of WASP-148, a new extrasolar system that includes at least two giant planets. The host star is a slowly rotating inactive late-G dwarf with a V=12 magnitude. The planet WASP-148b is a hot Jupiter of 0.72 R_Jup and 0.29 M_Jup that transits its host with an orbital period of 8.80 days. We found the planetary candidate with the SuperWASP photometric survey, then characterized it with the SOPHIE spectrograph. Our radial velocity measurements subsequently revealed a second planet in the system, WASP-148c, with an orbital period of 34.5 days and a minimum mass of 0.40 M_Jup. No transits of this outer planet were detected. The orbits of both planets are eccentric and fall near the 4:1 mean-motion resonances. This configuration is stable on long timescales, but induces dynamical interactions so that the orbits differ slightly from purely Keplerian orbits. In particular, WASP-148b shows transit-timing variations of typically 15 minutes, making it the first interacting system with transit-timing variations that is detected on ground-based light curves. We establish that the mutual inclination of the orbital plane of the two planets cannot be higher than 35 degrees, and the true mass of WASP-148c is below 0.60 M_Jup. We present photometric and spectroscopic observations of this system that cover a time span of ten years. We also provide their Keplerian and Newtonian analyses; these analyses should be significantly improved through future TESS~observations.

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arXiv2015

The doubly eclipsing quintuple low-mass star system 1SWASP J093010.78+533859.5

Our discovery of 1SWASP J093010.78+533859.5 as a probable doubly eclipsing quadruple system containing a contact binary with P~0.23 d and a detached binary with P~1.31 d was announced in 2013. Subsequently Koo et al. confirmed the detached binary spectroscopically and identified a fifth set of static spectral lines at its location, corresponding to a further non-eclipsing component of the system. Here we present new spectroscopic and photometric observations, allowing confirmation of the contact binary and improved modelling of all four eclipsing components. The detached binary is found to contain components of masses 0.837(8) and 0.674(7) M_sol, with radii of 0.832(18) and 0.669(18) R_sol and effective temperatures of 5185(-20,+25) and 4325(-15,+20) K respectively, the contact system has masses 0.86(2) and 0.341(11) M_sol, radii of 0.79(4) and 0.52(5) R_sol respectively, and a common T_eff of 4700(50) K. The fifth star is of similar temperature and spectral type to the primaries in the two binaries. Long-term photometric observations indicate the presence of a spot on one component of the detached binary, moving at an apparent rate of approximately one rotation every two years. Both binaries have consistent system velocities around -11 to -12 km/s, which match the average radial velocity of the fifth star, consistent distance estimates for both subsystems of d=78(3) and d=73(4) pc are also found, and (with some further assumptions) of d=83(9) pc for the fifth star. These findings strongly support the claim that both binaries (and very probably all five stars) are gravitationally bound in a single system. The consistent angles of inclination found for the two binaries (88.2(3) and 86(4) degrees) may also indicate that they originally formed by fragmentation (~9-10 Gyr ago) from a single protostellar disk and subsequently remained in the same orbital plane.

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arXiv2015

XMM-Newton and Chandra observations of the M31 globular cluster black hole candidate XB135: a heavyweight contender cut down to size

CXOM31 J004252.030+413107.87 is one of the brightest X-ray sources within the D_25 region of M31, and associated with a globular cluster (GC) known as B135; we therefore call this X-ray source XB135. XB135 is a low mass X-ray binary (LMXB) that apparently exhibited hard state characteristics at 0.3--10 keV luminosities 4--6 E+38 erg/s, and the hard state is only observed below ~10% Eddington. If true, the accretor would be a high mass black hole (BH) (> ~50 M_Sun); such a BH may be formed from direct collapse of a metal-poor, high mass star, and the very low metalicity of B135 (0.015 Z_Sun) makes such a scenario plausible. We have obtained new XMM-Newton and Chandra HRC observations to shed light on the nature of this object. We find from the HRC observation that XB135 is a single point source located close to the center of B135. The new XMM-Newton spectrum is consistent with a rapidly spinning ~10--20 M_Sun BH in the steep power law or thermal dominant state, but inconsistent with the hard state that we previously assumed. We cannot formally reject three component emission models that have been associated with high luminosity neutron star LMXBs (known as Z-sources); however, we prefer a BH accretor. We note that deeper observation of XB135 could discriminate against a neutron star accretor.

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arXiv2014

Energy-dependent evolution in IC10 X-1: hard evidence for an extended corona, and implications

We have analyzed a ~130 ks XMM-Newton observation of the dynamically confirmed black hole + Wolf-Rayet (BH+WR) X-ray binary (XB) IC10 X-1, covering ~1 orbital cycle. This system experiences periodic intensity dips every ~35 hours. We find that energy-independent evolution is rejected at a >5 sigma level. The spectral and timing evolution of IC10 X-1 are best explained by a compact disk blackbody and an extended Comptonized component, where the thermal component is completely absorbed and the Comptonized component is partially covered during the dip. We consider three possibilities for the absorber: cold material in the outer accretion disk, as is well documented for Galactic neutron star (NS) XBs at high inclination; a stream of stellar wind that is enhanced by traveling through the L1 point; and a spherical wind. We estimated the corona radius (r_ADC) for IC10 X-1 from the dip ingress to be ~1 E+6 km, assuming absorption from the outer disk, and found it to be consistent with the relation between r_m ADC and 1--30 keV luminosity observed in Galactic NS XBs that spans 2 orders of magnitude. For the other two scenarios, the corona would be larger. Prior BH mass (M_BH) estimates range over 23--38 M_Sun, depending on the inclination and WR mass. For disk absorption, the inclination, i, is likely to be ~60--80 degrees, with M_BH ~24--41 M_Sun. Alternatively, the L1-enhanced wind requires i ~80 degrees, suggesting ~24--33 M_Sun. For a spherical absorber, i ~ 40 degrees, and M_BH ~50--65 M_Sun.

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arXiv2014

Period and amplitude variations in post-common-envelope eclipsing binaries observed with SuperWASP

Period or amplitude variations in eclipsing binaries may reveal the presence of additional massive bodies in the system, such as circumbinary planets. Here, we have studied twelve previously-known eclipsing post-common-envelope binaries for evidence of such light curve variations, on the basis of multi-year observations in the SuperWASP archive. The results for HW Vir provided strong evidence for period changes consistent with those measured by previous studies, and help support a two-planet model for the system. ASAS J102322-3737.0 exhibited plausible evidence for a period increase not previously suggested; while NY Vir, QS Vir and NSVS 14256825 afforded less significant support for period change, providing some confirmation to earlier claims. In other cases, period change was not convincingly observed; for AA Dor and NSVS 07826147, previous findings of constant period were confirmed. This study allows us to present hundreds of new primary eclipse timings for these systems, and further demonstrates the value of wide-field high-cadence surveys like SuperWASP for the investigation of variable stars.

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arXiv2014

Parameters of two low-mass contact eclipsing binaries near the short-period limit

The two objects 1SWASP J150822.80-054236.9 and 1SWASP J160156.04+202821.6 were initially detected from their SuperWASP archived light curves as candidate eclipsing binaries with periods close to the short-period cut-off of the orbital period distribution of main sequence binaries, at ~0.2 d. Here, using INT spectroscopic data, we confirm them as double-lined spectroscopic and eclipsing binaries, in contact configuration. Following modelling of their visual light curves and radial velocity curves, we determine their component and system parameters to precisions between ~2 and 11%. The former system contains 1.07 and 0.55 M_sun components, with radii of 0.90 and 0.68 R_sun respectively; its primary exhibits pulsations with period 1/6 the orbital period of the system. The latter contains 0.86 and 0.57 M_sun components, with radii of 0.75 and 0.63R_sun respectively.

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arXiv2013

One, two or three stars? An investigation of an unusual eclipsing binary candidate undergoing dramatic period changes

We report our investigation of 1SWASP J234401.81-212229.1, a variable with a 18461.6 s period. After identification in a 2011 search of the SuperWASP archive for main-sequence eclipsing binary candidates near the distribution's short-period limit of approx. 0.20 d, it was measured to be undergoing rapid period decrease in our earlier work, though later observations supported a cyclic variation in period length. Spectroscopic data obtained in 2012 with the Southern African Large Telescope did not, however, support the interpretation of the object as a normal eclipsing binary. Here, we consider three possible explanations consistent with the data: a single-star oblique rotator model in which variability results from stable cool spots on opposite magnetic poles; a two-star model in which the secondary is a brown dwarf; and a three-star model involving a low-mass eclipsing binary in a hierarchical triple system. We conclude that the latter is the most likely model.

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arXiv2013

Near-UV Absorption, Chromospheric Activity, and Star-Planet Interactions in the WASP-12 system

We observed the extreme close-in hot Jupiter system WASP-12 with HST. Near-UV transits up to three times deeper than the optical transit of WASP-12b reveal extensive diffuse gas, extending well beyond the Roche lobe. The distribution of absorbing gas varies between visits. The deepest NUV transits are at wavelength ranges with strong photospheric absorption, implying the absorbing gas may have temperature and composition similar to the stellar photosphere. Our spectra reveal significantly enhanced absorption (greater than 3 σbelow the median) at ~200 wavelengths on each of two HST visits; 65 of these wavelengths are consistent between the two visits, using a strict criterion for velocity matching which excludes matches with velocity shifts exceeding ~20 km/s. Excess transit depths are robustly detected throughout the inner wings of the MgII resonance lines independently on both HST visits. We detected absorption in FeII 2586A, the heaviest species yet detected in an exoplanet transit. The MgII line cores have zero flux, emission cores exhibited by every other observed star of similar age and spectral type are conspicuously absent. WASP-12 probably produces normal MgII profiles, but the inner portions of these strong resonance lines are likely affected by extrinsic absorption. The required Mg+ column is an order of magnitude greater than expected from the ISM, though we cannot completely dismiss that possibility. A more plausible source of absorption is gas lost by WASP-12b. We show that planetary mass loss can produce the required column. Our Visit 2 NUV light curves show evidence for a stellar flare. We show that some of the possible transit detections in resonance lines of rare elements may be due instead to non-resonant transitions in common species. We present optical observations and update the transit ephemeris.

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arXiv2012

Period and period change measurements for 143 SuperWASP eclipsing binary candidates near the short-period limit and discovery of a doubly eclipsing quadruple system

Building on previous work, a new search of the SuperWASP archive was carried out to identify eclipsing binary systems near the short-period limit. 143 candidate objects were detected with orbital periods between 16000 and 20000 s, of which 97 are new discoveries. Period changes significant at 1 sigma or more were detected in 74 of these objects, and in 38 the changes were significant at 3 sigma or more. The significant period changes observed followed an approximately normal distribution with a half-width at half-maximum of ~0.1 s/yr. There was no apparent relationship between period length and magnitude or direction of period change. Amongst several interesting individual objects studied, 1SWASP J093010.78+533859.5 is presented as a new doubly eclipsing quadruple system, consisting of a contact binary with a 19674.575 s period and an Algol-type binary with a 112799.109 s period, separated by 66.1 AU, being the sixth known system of this type.

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arXiv2012

Period decrease in three SuperWASP eclipsing binary candidates near the short-period limit

SuperWASP light curves for 53 W UMa-type eclipsing binary (EB) candidates, identified in previous work as being close to the contact binary short-period limit, were studied for evidence of period change. The orbital periods of most of the stars were confirmed, and period decrease, significant at more than 5 sigma, was observed in three objects: 1SWASP J174310.98+432709.6 (-0.055 \pm0.003 s/yr), 1SWASP J133105.91+121538.0 (-0.075 \pm0.013 s/yr) and 1SWASP J234401.81-212229.1 (-0.313 \pm0.019 s/yr). The magnitudes of the observed period changes cannot be explained by magnetic braking or gravitational radiation effects, and are most likely primarily due to unstable mass transfer from primary to secondary components, possibly accompanied by unstable mass and angular momentum loss from the systems. If these period decreases persist, the systems could merge on a relatively short timescale.

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arXiv2008

New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%, and its 0.3--7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44$\pm$0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities $\la$10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star.

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arXiv2008

NGC300 X-1 and IC10 X-1: a new breed of black hole binary?

[ABRIDGED] IC10 X-1 has recently been confirmed as a black hole (BH) + Wolf-Rayet (WR) X-ray binary, and NGC300 X-1 is thought to be. IC10 X-1 and NGC300 X-1 have similar X-ray properties, with luminosities ~10^38 erg/s, and orbital periods ~30 hr. We investigate similarities between these two, as well as differences between them and the known Galactic BH binary systems. We have examined XMM-Newton observations of NGC300 X-1 and IC10 X-1. We extracted lightcurves and spectra; power density spectra (PDS) were constructed from the lightcurves, and the X-ray emission spectra were modeled. Each source exhibits PDS that are characteristic of disc-accreting X-ray binaries (XBs) in the high state. In this state, Galactic XBs with known BH primaries have soft, thermal emission; however the emission spectra of our targets are predominantly non-thermal. Furthermore, the Observation 1 spectrum of NGC300 X-1 is strikingly similar to that of IC10 X-1. The remarkable similarity between the behaviour of NGC300 X-1 in Observation 1 and that of IC10 X-1 lends strong evidence for NGC300 X-1 being a (BH+WR) binary. The unusual spectra of NGC300 X-1 and IC10 X-1 may be due to these systems existing in a persistently high state, whereas all known BH LMXBs are transient. BH XBs in a persistent high state could retain their corona, and hence exhibit a large non-thermal component. LMC X-1 is a BH XB that has only been observed in the high state, and its spectrum is remarkably similar to those of our targets. We therefore classify NGC300 X-1, IC10 X-1 and perhaps LMC X-1 as a new breed of BH XB, defined by their persistently high accretion rates and consequent stable disc configuration and corona. This scenario may also explain the lack of ultraluminous X-ray sources in the canonical soft state.

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arXiv2007

On the artificial nature of aperiodic variability in XMM-Newton observations of M31 X-ray sources and the ultraluminous X-ray source NGC 4559 ULX-7

Context: Power density spectra (PDS) that are characteristic of low mass X-ray binaries (LMXBs) have been previously reported for M31 X-ray sources, observed by XMM-Newton. However, we have recently discovered that these PDS result from the improper addition/subtraction of non-simultaneous lightcurves. Aims: To understand the properties and origins of the artefact. Methods: We re-analysed our XMM-Newton observations of M31 with non-simultaneous and simultaneous lightcurves, then combined simulated lightcurves at various intensities with various offsets and found that the artefact is more dependent on the offset than the intensity. Results: The lightcurves produced by the XMM-Newton Science Analysis Software (SAS) are non-synchronised by default. This affects not only the combination of lightcurves from the three EPIC detectors (MOS1, MOS2 and pn), but also background subtraction in the same CCD. It is therefore imperative that all SAS-generated lightcurves are synchronised by time filtering, even if the whole observation is to be used. We also find that the reported timing behaviour for NGC 4559 ULX-7 was also contaminated by the artefact; there is no significant variability in the correctly-combined lightcurves of NGC 4559 ULX-7. Hence, the classification of this source as an intermediate-mass black hole is no longer justified. Conclusions: While previous timing results from M31 have been proven wrong, and also the broken power law PDS in NGC 4559 ULX-7, XMM-Newton was able to detect aperiodic variability in just 3 ks of observations of NGC 5408 ULX1. Hence XMM-Newton remains a viable tool for analysing variability in extra-galactic X-ray sources.

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arXiv2006

Artificial variability in XMM-Newton observations of X-ray sources: M31 as a case study

Power density spectra (PDS) that are characteristic of low mass X-ray binaries (LMXBs) have been previously reported for M31 X-ray sources observed by XMM-Newton. However, we have recently discovered that these PDS are false positives resulting from the improper manipulation of non-simultaneous lightcurves. The lightcurves produced by the XMM-Newton Science Analysis Software (SAS) are non-synchronised by default. This affects not only the combination of lightcurves from the three EPIC detectors (MOS1, MOS2 and pn), but also background subtraction in the same CCD. It is therefore imperative that all SAS-generated lightcurves are synchronised by time filtering, even if the whole observation is to be used. We combined simulated lightcurves at various intensities with various offsets and found that the artefact is more dependent on the offset than the intensity. While previous timing results from M31 have been proven wrong, and also the broken power law PDS in NGC 4559 ULX-7, XMM-Newton was able to detect aperiodic variability in just 3 ks of observations of NGC 5408 ULX1. Hence XMM-Newton remains a viable tool for analysing variability in extra-galactic X-ray sources.

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arXiv2004

Black hole hunting in the Andromeda Galaxy

We present a new technique for identifying stellar mass black holes in low mass X-ray binaries (LMXBs), and apply it to XMM-Newton observations of M31. We examine X-ray time series variability seeking power density spectra (PDS) typical of LMXBs accreting at a low accretion rate (which we refer to as Type A PDS); these are very similar for black hole and neutron star LMXBs. Galactic neutron star LMXBs exhibit Type A PDS at low luminosities (~10^36--10^37 erg/s) while black hole LMXBs can exhibit them at luminosities >10^38 erg/s. We propose that Type A PDS are confined to luminosities below a critical fraction of the Eddington limit, $l_c$ that is constant for all LMXBs; we have examined asample of black hole and neutron star LMXBs and find they are all consistent with $l_c$ = 0.10+/-0.04 in the 0.3--10 keV band. We present luminosity and PDS data from 167 observations of X-ray binaries in M31 that provide strong support for our hypothesis. Since the theoretical maximum mass for a neutron star is \~3.1 M_Sun, we therefore assert that any LMXB that exhibits a Type A PDS at a 0.3--10 keV luminosity greater than 4 x 10^37 erg/s is likely to contain a black hole primary. We have found eleven new black hole candidates in M31 using this method. We focus on XMM-Newton observations of RX J0042.4+4112, an X-ray source in M31 and find the mass of the primary to be 7+/-2 M_Sun, if our assumptions are correct. Furthermore, RX J0042.4+4112 is consistently bright in \~40 observations made over 23 years, and is likely to be a persistently bright LMXB; by contrast all known Galactic black hole LMXBs are transient. Hence our method may be used to find black holes in known, persistently bright Galactic LMXBs and also in LMXBs in other galaxies.

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arXiv2002

The minimum period problem in cataclysmic variables

We investigate if consequential angular momentum losses (CAML) or an intrinsic deformation of the donor star in CVs could increase the CV bounce period from the canonical theoretical value ~65 min to the observed value $P_{min} \approx77$ min, and if a variation of these effects in a CV population could wash out the theoretically predicted accumulation of systems near the minimum period (the period spike). We are able to construct suitably mixed CV model populations that a statisticial test cannot rule out as the parent population of the observed CV sample. However, the goodness of fit is never convincing, and always slightly worse than for a simple, flat period distribution. Generally, the goodness of fit is much improved if all CVs are assumed to form at long orbital periods. The weighting suggested by King, Schenker & Hameury (2002) does not constitute an improvment if a realistically shaped input period distribution is used. Put your abstract here.

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