arXiv subjects

Fraser Lewis

Publications and source records attributed to Fraser Lewis.

33 records · Page 2Linked to original sources

arXiv2018

The long-term optical evolution of the black hole candidate MAXI J1659-152

We present 5 years of optical and infrared data of the black hole candidate MAXI J1659-152 covering its 2010 outburst, decay and quiescence. Combining optical data taken during the outburst decay, we obtain an orbital period of 2.414 $\pm$ 0.005 h, in perfect agreement with the value previously measured from X-ray dips. In addition, we detect a clear H$α$ excess in MAXI J1659-152 with data taken during the outburst decay. We also detect a single hump modulation most likely produced by irradiation. Assuming that the maximum occurs at orbital phase 0.5, we constrain the phase of the X-ray dips to be ~ 0.65. We also detect the quiescent optical counterpart at r' = 24.20 $\pm$ 0.08, I = 23.32 $\pm$ 0.02 and H = 20.7 $\pm$ 0.1. These magnitudes provide colour indices implying an M2-M5 donor star assuming 60% contribution from a disc component in the r'-band.

astro-ph.HE↗

arXiv2017

An Empirical Comparison of Parametric and Permutation Tests for Regression Analysis of Randomized Experiments

Hypothesis tests based on linear models are widely accepted by organizations that regulate clinical trials. These tests are derived using strong assumptions about the data-generating process so that the resulting inference can be based on parametric distributions. Because these methods are well understood and robust, they are sometimes applied to data that depart from assumptions, such as ordinal integer scores. Permutation tests are a nonparametric alternative that require minimal assumptions which are often guaranteed by the randomization that was conducted. We compare analysis of covariance (ANCOVA), a special case of linear regression that incorporates stratification, to several permutation tests based on linear models that control for pretreatment covariates. In simulations of randomized experiments using models which violate some of the parametric regression assumptions, the permutation tests maintain power comparable to ANCOVA. We illustrate the use of these permutation tests alongside ANCOVA using data from a clinical trial comparing the effectiveness of two treatments for gastroesophageal reflux disease. Given the considerable costs and scientific importance of clinical trials, an additional nonparametric method, such as a linear model permutation test, may serve as a robustness check on the statistical inference for the main study endpoints.

stat.AP↗

arXiv2017

Multiwavelength monitoring of a very active dwarf nova AX J1549.8$-$5416 with an unusually high duty cycle

We present the results of our analysis of new optical, ultraviolet (UV) and X-ray observations of a highly variable source $-$ AX J1549.8$-$5416. Both the detection of several fast rise, exponential decay outbursts in the optical light curve and the lack of He II emission lines in the optical spectra suggest AX J1549.8$-$5416 is a cataclysmic variable of the dwarf nova (DN) type. The multiwavelength analysis of three mini-outbursts and one normal outburst represent one of the most complete multiwavelength studies of a DN and help to refine the relationship between the X-ray, UV and optical emission in this system. We find that the UV emission is delayed with respect to the optical by $1.0-5.4$ days during the rising phase of the outburst. The X-ray emission is suppressed during the peak of the optical outburst and recovers during the end of the outburst. From our analysis of archival Swift, Chandra and XMM-Newton observations of AX J1549.8$-$5416, we estimate this DN has a high duty cycle ($\sim50\%$), suggesting a quiescent X-ray luminosity larger than $10^{32}$ erg/s. We also find the X-ray and UV flux are roughly anti-correlated. Furthermore, we find that, at low X-ray fluxes, the X-ray spectrum is well described by a single temperature thermal plasma model, while at high X-ray fluxes, an isobaric cooling flow model also works. We find that the maximum temperature of the plasma in quiescence is significantly higher than that in outburst.

astro-ph.HE↗

arXiv2016

Polarized synchrotron emission in quiescent black hole X-ray transients

We present near-infrared polarimetric observations of the black hole X-ray binaries Swift J1357.2-0933 and A0620-00. In both sources, recent studies have demonstrated the presence of variable infrared synchrotron emission in quiescence, most likely from weak compact jets. For Swift J1357.2-0933 we find that the synchrotron emission is polarized at a level of 8.0 +- 2.5 per cent (a 3.2 sigma detection of intrinsic polarization). The mean magnitude and rms variability of the flux (fractional rms of 19-24 per cent in K_s-band) agree with previous observations. These properties imply a continuously launched (stable on long timescales), highly variable (on short timescales) jet in the Swift J1357.2-0933 system in quiescence, which has a moderately tangled magnetic field close to the base of the jet. We find that for A0620-00, there are likely to be three components to the optical-infrared polarization; interstellar dust along the line of sight, scattering within the system, and an additional source that changes the polarization position angle in the reddest (H and K_s) wave-bands. We interpret this as a stronger contribution of synchrotron emission, and by subtracting the line-of-sight polarization, we measure an excess of ~ 1.25 +- 0.28 per cent polarization and a position angle of the magnetic field vector that is consistent with being parallel with the axis of the resolved radio jet. These results imply that weak jets in low luminosity accreting systems have magnetic fields which possess similarly tangled fields compared to the more luminous, hard state jets in X-ray binaries.

astro-ph.HE↗

arXiv2016

HERschel Observations of Edge-on Spirals (HEROES). III. Dust energy balance study of IC 2531

We investigate the dust energy balance for the edge-on galaxy IC 2531, one of the seven galaxies in the HEROES sample. We perform a state-of-the-art radiative transfer modelling based, for the first time, on a set of optical and near-infrared galaxy images. We show that taking into account near-infrared imaging in the modelling significantly improves the constraints on the retrieved parameters of the dust content. We confirm the result from previous studies that including a young stellar population in the modelling is important for explaining the observed stellar energy distribution. However, the discrepancy between the observed and modelled thermal emission at far-infrared wavelengths, the so-called dust energy balance problem, is still present: the model underestimates the observed fluxes by a factor of about two. We compare two different dust models, and find that dust parameters and thus the spectral energy distribution in the infrared domain are sensitive to the adopted dust model. In general, the THEMIS model reproduces the observed emission in the infrared wavelength domain better than the popular Zubko et al. BARE-GR-S model. Our study of IC 2531 is a pilot case for detailed and uniform radiative transfer modelling of the entire HEROES sample, which will shed more light on the strength and origins of the dust energy balance problem.

astro-ph.GA↗

arXiv2015

Properties and Evolution of the Redback Millisecond Pulsar Binary PSR J2129-0429

PSR J2129-0429 is a "redback" eclipsing millisecond pulsar binary with an unusually long 15.2 hour orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified Fermi gamma-ray sources. The pulsar companion is optically bright (mean $m_R = 16.6$ mag), allowing us to construct the longest baseline photometric dataset available for such a system. We present ten years of archival and new photometry of the companion from LINEAR, CRTS, PTF, the Palomar 60-inch, and LCOGT. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200-inch indicates that the pulsar is massive: $1.74\pm0.18 M_\odot$. The G-type pulsar companion has mass $0.44\pm0.04 M_\odot$, one of the heaviest known redback companions. It is currently 95\% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr$^{-1}$ secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical lightcurve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129-0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar--white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.

astro-ph.SR↗

arXiv2014

Multiwavelength observations of the black hole transient Swift J1745-26 during the outburst decay

We characterized the broad-band X-ray spectra of Swift J1745-26 during the decay of the 2013 outburst using INTEGRAL ISGRI, JEM-X and Swift XRT. The X-ray evolution is compared to the evolution in optical and radio. We fit the X- ray spectra with phenomenological and Comptonization models. We discuss possible scenarios for the physical origin of a ~50 day flare observed both in optical and X- rays ~170 days after the peak of the outburst. We conclude that it is a result of enhanced mass accretion in response to an earlier heating event. We characterized the evolution in the hard X-ray band and showed that for the joint ISGRI-XRT fits, the e-folding energy decreased from 350 keV to 130 keV, while the energy where the exponential cut-off starts increased from 75 keV to 112 keV as the decay progressed.We investigated the claim that high energy cut-offs disappear with the compact jet turning on during outburst decays, and showed that spectra taken with HEXTE on RXTE provide insufficient quality to characterize cut-offs during the decay for typical hard X-ray fluxes. Long INTEGRAL monitoring observations are required to understand the relation between the compact jet formation and hard X-ray behavior. We found that for the entire decay (including the flare), the X-ray spectra are consistent with thermal Comptonization, but a jet synchrotron origin cannot be ruled out.

astro-ph.HE↗

arXiv2012

Quiescent X-Ray/Optical Counterparts of the Black Hole Transient H 1705-250

We report the result of a new Chandra observation of the black hole X-ray transient H 1705-250 in quiescence. H 1705-250 was barely detected in the new 50 ks Chandra observation. With 5 detected counts, we estimate the source quiescent luminosity to be Lx~9.1e30 erg/s in the 0.5-10 keV band (adopting a distance of 8.6 kpc). This value is in line with the quiescent luminosities found among other black hole X-ray binaries with similar orbital periods. By using images taken with the Faulkes Telescope North, we derive a refined position of H 1705-250. We also present the long-term lightcurve of the optical counterpart from 2006 to 2012, and show evidence for variability in quiescence.

astro-ph.HE↗

arXiv2011

Rapid variations of polarization in low-mass X-ray binaries

Time-resolved optical and infrared polarimetric observations of black hole and neutron star low-mass X-ray binaries are presented. Data were acquired with the VLT, UKIRT and HIPPO on the SAAO 1.9-m. We find that for some sources in outburst, a rapidly variable component of polarization is evident that is stronger in the redder wavebands. We attribute this to the polarimetric signature of synchrotron emission from jets in these systems, the emission of which is known to dominate these redder bands. Such synchrotron emission from jets launched close to black holes and neutron stars can be highly linearly polarized, depending on the configuration of the magnetic field. The variability of the polarization is suggestive of a tangled and turbulent magnetic field at the location of the compact jet. For some sources the position angle of polarization is consistent with a magnetic field that is parallel to the observed radio jet. These are some of the first observational constraints of the geometry and magnetic structure at the inner regions of the outflow. We also present the first ever simultaneous optical polarization and X-ray campaign of an X-ray binary, using data taken simultaneously with HIPPO and RXTE with sub-second time resolution.

astro-ph.HE↗

arXiv2009

Robotic Astronomy with the Faulkes Telescopes and Las Cumbres Observatory Global Telescope

We present results from ongoing science projects conducted by members of the Faulkes Telescope (FT) team and Las Cumbres Observatory Global Telescope (LCOGT). Many of these projects incorporate observations carried out and analysed by FT users, comprising amateur astronomers and schools. We also discuss plans for the further development of the LCOGT network.

astro-ph.IM↗

arXiv2009

The Faulkes Telescope Project: Not Just Pretty Pictures

The Faulkes Telescope (FT) Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). As well as producing spectacular images of galaxies, nebulae, supernovae remnants, star clusters, etc., the FT team is involved in several projects pursuing scientific goals. Many of these projects also incorporate data collected and analysed by schools and amateur astronomers.

astro-ph.IM↗

arXiv2008

Unifying disc-jet behaviour in X-ray binaries: an optical/IR approach

Synchrotron emission from jets produced by X-ray binaries can be detected at optical and infrared (IR) frequencies. I show that optical/IR colour-magnitude diagrams of the outbursts of nine X-ray binaries successfully separate thermal disc emission from non-thermal jet emission, in both black hole and neutron star sources. A heated single-temperature blackbody is able to reproduce the observed relations between colour and magnitude, except when excursions are made to a redder colour than expected, which is due to jet emission. The general picture that is developed is then incorporated into the unified picture of disc-jet behaviour in black hole X-ray binaries. At a given position of a source in the X-ray hardness-intensity diagram, the radio, IR and optical properties can be inferred. Similarly, it is possible to predict the X-ray and radio luminosities and spectral states from optical/IR monitoring.

astro-ph↗

arXiv2008

Continued Monitoring of LMXBs with the Faulkes Telescopes

The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN). It has two 2-metre robotic telescopes, located at Haleakala on Maui (FT North) and Siding Spring in Australia (FT South). It is planned for these telescopes to be complemented by a research network of eighteen 1-metre telescopes, along with an educational network of twenty-eight 0.4-metre telescopes, providing 24 hour coverage of both northern and southern hemispheres. We have been conducting a monitoring project of 13 low-mass X-ray binaries (LMXBs) using FT North since early 2006. The introduction of FT South has allowed us to extend this to monitor a total of 30 LMXBs. New instrumentation will allow us to expand this project to include both infrared wavelengths (z and y band) and spectroscopy. Brighter targets (~ 16 - 18 mag.) are imaged weekly in V, R and i' band (SNR ~ 50), while fainter ones (> 18 mag.) are observed only in i' band (SNR ~ 20). We alter this cadence in response to our own analysis or Astronomers Telegrams (ATels).

astro-ph↗

arXiv2007

Monitoring LMXBs with the Faulkes Telescopes

We have been undertaking a monitoring project of 13 low-mass X-ray binaries (LMXBs) using FT North since early 2006. The introduction of FT South has allowed us to extend this monitoring to include 15 southern hemisphere LMXBs. With new instrumentation, we also intend to expand this monitoring to include both infrared wavelengths and spectroscopy.

astro-ph↗

arXiv2006

Monitoring LMXBs with the Faulkes Telescope

The Faulkes Telescope Project is the educational arm of the Las Cumbres Observatory Global Telescope Network (LCOGT). It currently has two 2-metre robotic telescopes, located at Haleakala on Maui (FT North) and Siding Spring in Australia (FT South). It is planned to increase this to six 2-metre telescopes in the future, complemented by a network of 30-40 smaller (0.4-1 metre) telescopes providing 24 hour coverage of both northern and southern hemispheres. We are undertaking a study of 10 low-mass X-ray binaries (LMXBs) using FT North to study the optical continuum behaviour of X-ray transients in quiescence. The introduction of FT South in September 2006 allows us to extend this monitoring to include 17 southern hemisphere LMXBs. With new instrumentation, we also intend to expand this monitoring to include both infrared wavelengths and spectroscopy.

astro-ph↗