arXiv subjects

Iain A. Steele

Publications and source records attributed to Iain A. Steele.

At least 19 recordsLinked to original sources

arXiv2026

GBOT: ground based optical tracking of the Gaia satellite

Gaia, the 1 billion star, high precision, astrometric satellite will revolutionise our understanding in many areas of astronomy ranging from bodies in our Solar System to the formation and structure of our Galaxy. To fully achieve the ambitious goals of the mission, and to completely eliminate effects such as aberration, we must know the position and velocity vectors of the spacecraft as it orbits the Lagrange point to an accuracy greater than can be obtained by traditional radar techniques, leading to the decision to conduct astrometric observations of the Gaia satellite itself from the ground. Therefore the Ground Based Optical Tracking (GBOT) project was formed and a small worldwide network using 1-2 m telescopes established in order to obtain one measurement per day of a precision/accuracy of 20 mas. We will discuss all aspects of GBOT, setup, feasibility considerations, preliminary tests of observing methods, partner observatories, the pipeline/database

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arXiv2026

Brightness, Colour, Polarisation: A Multi-Instrument Observation Campaign of the Gorizont-6 Satellite

We present a coordinated multi-instrument photometric and polarimetric study of the defunct geosynchronous satellite, Gorizont-6. This observation campaign combined wide-field multi-colour observations with simultaneous multi-site photometry and linear polarimetry. Our results demonstrate that the combined simultaneous colour and polarimetric measurements aid in breaking the degeneracy between the fundamental spin period and its harmonics, enabling light curve features to be associated with specific reflecting surfaces. Using phase-dispersion minimisation with bootstrap resampling, we then measure a steadily increasing rotation period across six epochs spanning nine months, well described by a damped exponential curve consistent with Yarkovsky O'Keefe Radzievskii Paddack (YORP) driven spin-down. A geometrical analysis of the multi-site observations provides additional constraints on the spin-axis orientation based on paired glint events. Overall, multi-colour photometry yields efficient, robust period measurements compared with single-band data, while polarimetry supplies the decisive constraints needed for unambiguous rotation-state determination, highlighting the value of combined photometric-polarimetric strategies for characterising defunct satellites.

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arXiv2025

Optical and near-infrared polarization of the black hole X-ray binary A0620-00 in quiescence

We present simultaneous high-precision optical polarimetric and near-infrared (NIR) to ultraviolet (UV) photometric observations of low-mass black hole X-ray binary A0620-00 in the quiescent state. Subtracting interstellar polarization, estimated from a sample of field stars, we derive the intrinsic polarization of A0620-00. We show that the intrinsic polarization degree (PD) is variable with the orbital period with the amplitude of $\sim0.3\%$ at least in the $R$ band, where the signal-to-noise ratio of our observations is the best. This implies that some fraction of the optical polarization is produced by a scattering of stellar radiation off the matter that follows the black hole in its orbital motion. In addition, we see a rotation of the orbit-average intrinsic polarization angle (PA) with the wavelength from $163°$ in the $R$ to $177°$ in the $B$ band. All of the above, combined with the historical NIR-to-optical polarimetric observations, demonstrates the complex behavior of the average intrinsic polarization of A0620-00: the PA continuously rotates from the infrared to the blue band by $\sim53°$ in total, while the PD of $\sim1\%$ remains nearly constant over the entire spectral range. The spectral dependence of the PA can be described by Faraday rotation with the rotation measure of RM=$-0.2$ rad $μ$m$^{-2}$, implying a few Gauss magnetic field in the plasma surrounding the black hole accretion disk. However, our preferred interpretation of the peculiar wavelength dependence is an interplay between two polarized components with different PAs. Polarimetric measurements in the UV range can help in distinguishing between these scenarios.

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arXiv2025

Unveiling blazar synchrotron emission: a multiwavelength polarimetric study of HSP and LSP populations

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.

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arXiv2025

Determining the origin of the X-ray emission in blazars through multiwavelength polarization

The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes that are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polarization observations promise to finally distinguish between emission models. We use extensive multiwavelength and polarization campaigns as well as state-of-the-art polarized spectral energy distribution models to attack this problem by focusing on two X-ray polarization observations of blazar BL Lacertae in flaring and quiescent $γ$-ray states. We find that regardless of the jet composition and underlying emission model, inverse-Compton scattering from relativistic electrons dominates at X-ray energies.

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arXiv2025

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

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arXiv2024

A Two-Week $IXPE$ Monitoring Campaign on Mrk 421

X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including a $\sim90^\circ$ angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets.

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arXiv2024

Distinguishing radiation mechanisms and particle populations in blazar jets through long-term multi-band monitoring with RINGO3 and Fermi

We present the results of seven years of multicolour photometric monitoring of a sample of 31 $γ$-ray bright blazars using the RINGO3 polarimeter on the Liverpool Telescope from 2013--2020. We explore the relationships between simultaneous observations of flux in three optical wavebands along with Fermi $γ$-ray data in order to explore the radiation mechanisms and particle populations in blazar jets. We find significant correlations between optical and $γ$-ray flux with no detectable time lag, suggesting leptonic emission processes in the jets of these sources. Furthermore, we find the spectral behaviour against optical and $γ$-ray flux for many sources is best fit logarithmically. This is suggestive of a transition between bluer-/redder-when-brighter into stable-when-brighter behaviour during high activity states; a behaviour that might be missed in poorly sampled data, resulting in apparent linear relationships.

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arXiv2024

Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations

Variability can be the pathway to understanding the physical processes in astrophysical jets, however, the high-cadence observations required to test particle acceleration models are still missing. Here we report on the first attempt to produce continuous, >24 hour polarization light curves of blazars using telescopes distributed across the globe and the rotation of the Earth to avoid the rising Sun. Our campaign involved 16 telescopes in Asia, Europe, and North America. We observed BL Lacertae and CGRaBS J0211+1051 for a combined 685 telescope hours. We find large variations in the polarization degree and angle for both sources in sub-hour timescales as well as a ~180 degree rotation of the polarization angle in CGRaBS J0211+1051 in less than two days. We compared our high-cadence observations to Particle-In-Cell magnetic reconnection and turbulent plasma simulations. We find that although the state of the art simulation frameworks can produce a large fraction of the polarization properties, they do not account for the entirety of the observed polarization behavior in blazar jets.

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arXiv2024

The extreme coronal line emitter AT 2022fpx: Varying optical polarization properties and late-time X-ray flare

Supermassive black holes disrupt passing stars, producing outbursts called tidal disruption events (TDEs). TDEs have recently gained attention due to their unique dynamics and emission processes, which are still not fully understood. Especially, the so-called optical TDEs, are of interest as they often exhibit delayed or obscured X-ray emission from the accretion disk, making the origin of the prompt emission unclear. In this paper, we present multiband optical polarization observations and optical spectrometry of a recent TDE candidate AT 2022fpx, alongside monitoring observations in optical, ultraviolet and X-rays. The optical spectra of AT 2022fpx show Bowen fluorescence as well as highly-ionized iron emission lines, which are characteristic of extreme coronal line emitters. Additionally, the source exhibits variable but low-polarized continuum emission at the outburst peak, with a clear rotation of the polarization angle. X-ray emission observed approximately 250 days after the outburst peak in the decay appear flare-like but is consistent with constant temperature black-body emission. The overall outburst decay is slower than for typical TDEs, and resembles more the ones seen from Bowen fluorescence flares. These observations suggest that AT 2022fpx could be a key source in linking different long-lived TDE scenarios. Its unique characteristics, such as extreme coronal line emission, variable polarization, and delayed X-ray flare, can be attributed to the outer shock scenario or a clumpy torus surrounding the supermassive black hole. Further studies, especially in the context of multi-wavelength observations, are crucial to fully understand the dynamics and emission mechanisms of these intriguing astrophysical events.

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arXiv2024

IXPE observation of PKS 2155-304 reveals the most highly polarized blazar

We report the X-ray polarization properties of the high-synchrotron-peaked (HSP) blazar PKS 2155$-$304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7, 2023. We also conducted an extensive contemporaneous multiwavelength (MW) campaign. We find that during the first half ($T_1$) of the IXPE pointing, the source exhibited the highest X-ray polarization degree detected for an HSP blazar thus far, (30.7$\pm$2.0)%, which dropped to (15.3$\pm$2.1)% during the second half ($T_2$). The X-ray polarization angle remained stable during the IXPE pointing at 129.4$^\circ$$\pm$1.8$^\circ$ and 125.4$^\circ$$\pm$3.9$^\circ$ during $T_1$ and $T_2$, respectively. Meanwhile, the optical polarization degree remained stable during the IXPE pointing, with average host-galaxy-corrected values of (4.3$\pm$0.7)% and (3.8$\pm$0.9)% during the $T_1$ and $T_2$, respectively. During the IXPE pointing, the optical polarization angle changed achromatically from $\sim$140$^\circ$ to $\sim$90$^\circ$ and back to $\sim$130$^\circ$. Despite several attempts, we only detected (99.7% conf.) the radio polarization once (during $T_2$, at 225.5 GHz): with degree (1.7$\pm$0.4)% and angle 112.5$^\circ$$\pm$5.5$^\circ$. The direction of the broad pc-scale jet is rather ambiguous and has been found to point to the east and south at different epochs; however, on larger scales (> 1.5 pc) the jet points toward the southeast ($\sim$135$^\circ$), similar to all of the MW polarization angles. Moreover, the X-ray to optical polarization degree ratios of $\sim$7 and $\sim$4 during $T_1$ and $T_2$, respectively, are similar to previous IXPE results for several HSP blazars. These findings, combined with the lack of correlation of temporal variability between the MW polarization properties, agree with an energy-stratified shock-acceleration scenario in HSP blazars.

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arXiv2024

Detection of an intranight optical hard-lag with colour variability in blazar PKS 0735+178

Blazars are a highly variable subclass of active galactic nuclei that have been observed to vary significantly during a single night. This intranight variability remains a debated phenomenon, with various mechanisms proposed to explain the behaviour including jet energy density evolution or system geometric changes. We present the results of an intranight optical monitoring campaign of four blazars: TXS 0506+056, OJ287, PKS 0735+178, and OJ248 using the Carlos Sánchez Telescope. We detect significant but colourless behaviour in OJ287 and both bluer- and redder-when-brighter colour trends in PKS 0735+178. Additionally, the g band shows a lag of ~10 min with respect to the r,i,z_s bands for PKS 0735+178 on 2023 January 17. This unexpected hard-lag in PKS 0735+178 is not in accordance with the standard synchrotron shock cooling model (which would predict a soft lag) and instead suggests the variability may be a result of changes in the jet's electron energy density distribution, with energy injection from Fermi acceleration processes into a post-shocked medium.

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arXiv2023

Automated SpectroPhotometric Image REDuction (ASPIRED)

We provide a suite of public open-source spectral data-reduction software to rapidly obtain scientific products from all forms of long-slit-like spectroscopic observations. Automated SpectroPhotometric REDuction (ASPIRED) is a Python-based spectral data-reduction toolkit. It is designed to be a general toolkit with high flexibility for users to refine and optimize their data-reduction routines for the individual characteristics of their instruments. The default configuration is suitable for low-resolution long-slit spectrometers and provides a quick-look quality output. However, for repeatable science-ready reduced spectral data, some moderate one-time effort is necessary to modify the configuration. Fine-tuning and additional (pre)processing may be required to extend the reduction to systems with more complex setups. It is important to emphasize that although only a few parameters need updating, ensuring their correctness and suitability for generalization to the instrument can take time due to factors such as instrument stability. We compare some example spectra reduced with ASPIRED to published data processed with iraf-based and STARLINK-based pipelines, and find no loss in the quality of the final product. The Python-based, iraf-free ASPIRED can significantly ease the effort of an astronomer in constructing their own data-reduction workflow, enabling simpler solutions to data-reduction automation. This availability of near-real-time science-ready data will allow adaptive observing strategies, particularly important in, but not limited to, time-domain astronomy.

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arXiv2022

An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t_E~270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of the Earth's motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 +/- 1.3 Msun and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a 'natal kick' from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial-velocity measurements of Galactic X-ray binaries, and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.

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arXiv2020

Confirmation of Monoperiodicity Above 20 Seconds for Two Blue Large-Amplitude Pulsators

Blue Large-Amplitude Pulsators (BLAPs) are a new class of pulsating variable star. They are located close to the hot subdwarf branch in the Hertzsprung-Russell diagram and have spectral classes of late O or early B. Stellar evolution models indicate that these stars are likely radially pulsating, driven by iron group opacity in their interiors. A number of variable stars with a similar driving mechanism exist near the hot subdwarf branch with multi-periodic oscillations caused by either pressure (p) or gravity (g) modes. No multi-periodic signals were detected in the OGLE discovery light curves since it would be difficult to detect short period signals associated with higher-order p modes with the OGLE cadence. Using the RISE instrument on the Liverpool Telescope, we produced high cadence light curves of two BLAPs, OGLE-BLAP-009 ($m_{\mathrm{v}}=15.65$ mag) and OGLE-BLAP-014 ($m_{\mathrm{v}}=16.79$ mag) using a $720$ nm longpass filter. Frequency analysis of these light curves identify a primary oscillation with a period of $31.935\pm0.0098$ mins and an amplitude from a Fourier series fit of $0.236$ mag for BLAP-009. The analysis of BLAP-014 identifies a period of $33.625\pm0.0214$ mins and an amplitude of $0.225$ mag. Analysis of the residual light curves reveals no additional short period variability down to an amplitude of $15.20\pm0.26$ mmag for BLAP-009 and $58.60\pm3.44$ mmag for BLAP-014 for minimum periods of $20$ s and $60$ s respectively. These results further confirm that the BLAPs are monoperiodic.

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arXiv2020

Characterization of a dual-beam, dual-camera optical imaging polarimeter

Polarization plays an important role in various time-domain astrophysics to understand the magnetic fields, geometry, and environments of spatially unresolved variable sources. In this paper we present the results of laboratory and on-sky testing of a novel dual-beam, dual-camera optical imaging polarimeter (MOPTOP) exploiting high sensitivity, low-noise CMOS technology and designed to monitor variable and transient sources with low systematic errors and high sensitivity. We present a data reduction algorithm that corrects for sensitivity variations between the cameras on a source-by-source basis. Using our data reduction algorithm, we show that our dual-beam, dual-camera technique delivers the benefits of low and stable instrumental polarization ($<0.05$\% for lab data and $<0.25$\% for on sky data) and high throughput while avoiding the additional sky brightness and image overlap problems associated with dual-beam, single-camera polarimeters.

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arXiv2019

Uncooled Microbolometer Arrays for Ground Based Astronomy

We describe the design and commissioning of a simple prototype, low-cost 10$μ$m imaging instrument. The system is built using commercially available components including an uncooled microbolometer array as a detector. The incorporation of adjustable germanium reimaging optics rescale the image to the appropriate plate scale for the 2-m diameter Liverpool Telescope. From observations of bright solar system and stellar sources, we demonstrate a plate scale of 0.75$^{\prime\prime}$ per pixel and confirm the optical design allows diffraction limited imaging. We record a $\sim$ 10$\%$ photometric stability due to sky variability. We measure a $3 σ$ sensitivity of $7 \times 10^{3}$ Jy for a single, $\sim$ 0.11 second exposure. This corresponds to a sensitivity limit of $3 \times 10^{2}$ Jy for a 60 second total integration. We present an example science case from observations of the 2019 Jan total lunar eclipse and show that the system can detect and measure the anomalous cooling rate associated with the features Bellot and Langrenus during eclipse.

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arXiv2018

Laying the Groundwork for the Development of the Data Archive of the New Robotic Telescope

The Liverpool Telescope has been in fully autonomous operation since 2004. The supporting data archive facility has largely been untouched. The data provision service has not been an issue although some modernisation of the system is desirable. This project is timely. Not only does it suit the upgrade of the current LT data archive, it is in line with the design phase of the New Robotic Telescope which will be online in the early-2020s; and with the development of a new data archive facility for a range of telescopes at the National Astronomical Research Institute of Thailand. The Newton Fund enabled us to collaborate in designing a new versatile generic system that serves all purposes. In the end, we conclude that a single system would not meet the needs of all parties and only adopt similar front-ends while the back-ends are bespoke to our respective systems and data-flows.

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