arXiv subjects

R. Fender

Publications and source records attributed to R. Fender.

At least 37 records · Page 2Linked to original sources

arXiv2023

Discovery of an Extremely Intermittent Periodic Radio Source

We report the serendipitous discovery of an extremely intermittent radio pulsar, PSR J1710-3452, with a relatively long spin period of 10.4 s. The object was discovered through the detection of 97 bright radio pulses in only one out of 66 epochs of observations spanning almost three years. The bright pulses have allowed the source to be localised to a precision of 0.5" through radio imaging. We observed the source location with the Swift X-ray telescope but did not detect any significant X-ray emission. We did not identify any high-energy bursts or multi-frequency counterparts for this object. The solitary epoch of detection hinders the calculation of the surface magnetic field strength, but the long period and the microstructure in the single-pulses resembles the emission of radio-loud magnetars. If this is indeed a magnetar, it is located at a relatively high Galactic latitude (2.9 degree), making it potentially one of the oldest and the most intermittent magnetars known in the Galaxy. The very short activity window of this object is unique and may point towards a yet undetected population of long period, highly transient radio emitting neutron stars.

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arXiv2023

Day-timescale variability in the radio light curve of the Tidal Disruption Event AT2022cmc: confirmation of a highly relativistic outflow

Tidal disruption events (TDEs) are transient, multi-wavelength events in which a star is ripped apart by a supermassive black hole. Observations show that in a small fraction of TDEs, a short-lived, synchrotron emitting jet is produced. We observed the newly discovered TDE AT2022cmc with a slew of radio facilities over the first 100 days after its discovery. The light curve from the AMI-LA radio interferometer shows day-timescale variability which we attribute to a high brightness temperature emitting region as opposed to scintillation. We measure a brightness temperature of 2x10^15 K, which is unphysical for synchrotron radiation. We suggest that the measured high brightness temperature is a result of relativistic beaming caused by a jet being launched at velocities close to the speed of light along our line of sight. We infer from day-timescale variability that the jet associated with AT2022cmc has a relativistic Doppler factor of at least 16, which corresponds to a bulk Lorentz factor of at least 8 if we are observing the jet directly on axis. Such an inference is the first conclusive evidence that the radio emission observed from some TDEs is from relativistic jets because it does not rely on an outflow model. We also compare the first 100 days of radio evolution of AT2022cmc with that of the previous bright relativistic TDE, Swift J1644, and find a remarkable similarity in their evolution.

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arXiv2023

The Co-Ordinated Radio and Infrared Survey for High-Mass Star Formation. V. The CORNISH-South Survey and Catalogue

We present the first high spatial resolution radio continuum survey of the southern Galactic plane. The CORNISH project has mapped the region defined by $295^{\circ} < l < 350^{\circ}$; $|b| < 1^{\circ}$ at 5.5-GHz, with a resolution of 2.5$^{''}$ (FWHM). As with the CORNISH-North survey, this is designed to primarily provide matching radio data to the Spitzer GLIMPSE survey region. The CORNISH-South survey achieved a root mean square noise level of $\sim$ 0.11 mJy beam$^{-1}$, using the 6A configuration of the Australia Telescope Compact Array (ATCA). In this paper, we discuss the observations, data processing and measurements of the source properties. Above a 7$σ$ detection limit, 4701 sources were detected, and their ensemble properties show similar distributions with their northern counterparts. The catalogue is highly reliable and is complete to 90 per cent at a flux density level of 1.1 mJy. We developed a new way of measuring the integrated flux densities and angular sizes of non-Gaussian sources. The catalogue primarily provides positions, flux density measurements and angular sizes. All sources with IR counterparts at 8$μm$ have been visually classified, utilizing additional imaging data from optical, near-IR, mid-IR, far-IR and sub-millimetre galactic plane surveys. This has resulted in the detection of 524 H II regions of which 255 are ultra-compact H II regions, 287 planetary nebulae, 79 radio stars and 6 massive young stellar objects. The rest of the sources are likely to be extra-galactic. These data are particularly important in the characterization and population studies of compact ionized sources such as UCHII regions and PNe towards the Galactic mid-plane.

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arXiv2022

Radio observations of the Black Hole X-ray Binary EXO 1846-031 re-awakening from a 34-year slumber

We present radio [1.3 GHz MeerKAT, 4-8 GHz Karl G. Jansky Very Large Array (VLA) and 15.5 GHz Arcminute Microkelvin Imager Large Array (AMI-LA)] and X-ray (Swift and MAXI) data from the 2019 outburst of the candidate Black Hole X-ray Binary (BHXB) EXO 1846-031. We compute a Hardness-Intensity diagram, which shows the characteristic q-shaped hysteresis of BHXBs in outburst. EXO 1846-031 was monitored weekly with MeerKAT and approximately daily with AMI-LA. The VLA observations provide sub-arcsecond-resolution images at key points in the outburst, showing moving radio components. The radio and X-ray light curves broadly follow each other, showing a peak on ~MJD 58702, followed by a short decline before a second peak between ~MJD 58731-58739. We estimate the minimum energy of these radio flares from equipartition, calculating values of $E_{\rm min} \sim$ 4$\times$10$^{41}$ and 5$\times$10$^{42}$ erg, respectively. The exact date of the return to `quiescence' is missed in the X-ray and radio observations, but we suggest that it likely occurred between MJD 58887 and 58905. From the Swift X-ray flux on MJD 58905 and assuming the soft-to-hard transition happened at 0.3-3 per cent Eddington, we calculate a distance range of 2.4-7.5\,kpc. We computed the radio:X-ray plane for EXO 1846-031 in the `hard' state, showing that it is most likely a `radio-quiet' BH, preferentially at 4.5 kpc. Using this distance and a jet inclination angle of $θ$=73$^{\circ}$, the VLA data place limits on the intrinsic jet speed of $β_{\rm int} = 0.29c$, indicating sub-luminal jet motion.

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arXiv2022

A refined dynamical mass for the black hole in the X-ray transient XTE J1859+226

We present two contiguous nights of simultaneous time-resolved GTC spectroscopy and WHT photometry of the black hole X-ray transient XTE J1859+226, obtained in 2017 July during quiescence. Cross-correlation of the individual spectra against a late K-type spectral template enabled us to constrain the orbital period to $0.276 \pm 0.003$ d and the radial velocity semi-amplitude of the donor star to $K_2 = 550 \pm 59$ km s$^{-1}$. An ellipsoidal modulation is detected in the photometric $r$- and $i$-band light curves, although it is strongly contaminated by flickering activity. By exploiting correlations between the properties of the double-peaked H$α$ emission-line profile and the binary parameters, we derived an orbital inclination of $66.6 \pm 4.3$ deg, a refined $K_2 = 562 \pm 40$ km s$^{-1}$ and mass ratio $q = M_2/M_1 = 0.07 \pm 0.01$. From these values we obtained an updated black hole mass of $M_1 = 7.8 \pm 1.9$ M$_\odot$. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst QPO triplet being explained by the relativistic precession model. We also obtained a companion star mass $M_2 = 0.55 \pm 0.16$ M$_\odot$, which is consistent with its K5-K7 V spectral type.

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arXiv2022

Search and identification of transient and variable radio sources using MeerKAT observations: a case study on the MAXI J1820+070 field

Many transient and variable sources detected at multiple wavelengths are also observed to vary at radio frequencies. However, these samples are typically biased towards sources that are initially detected in wide-field optical, X-ray or gamma-ray surveys. Many sources that are insufficiently bright at higher frequencies are therefore missed, leading to potential gaps in our knowledge of these sources and missing populations that are not detectable in optical, X-rays or gamma-rays. Taking advantage of new state-of-the-art radio facilities that provide high quality wide-field images with fast survey speeds, we can now conduct unbiased surveys for transient and variable sources at radio frequencies. In this paper, we present an unbiased survey using observations obtained by MeerKAT, a mid-frequency ($\sim$1.4 GHz) radio array in South Africa's Karoo Desert. The observations used were obtained as part of a weekly monitoring campaign for X-ray binaries (XRBs) and we focus on the field of MAXI J1820+070. We develop methods to optimally filter transient and variable candidates that can be directly applied to other datasets. In addition to MAXI J1820+070, we identify four likely active galactic nuclei, one source that could be a Galactic source (pulsar or quiescent X-ray binary) or an AGN, and one variable pulsar. No transient sources, defined as being undetected in deep images, were identified leading to a transient surface density of $<3.7\times10^{-2}$ deg$^{-2}$ at a sensitivity of 1 mJy on timescales of one week at 1.4 GHz.

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arXiv2022

MeerKAT radio observations of the neutron star low-mass X-ray binary Cen X-4 at low accretion rates

Centaurus X-4 (Cen X-4) is a relatively nearby neutron star low-mass X-ray binary that showed outbursts in 1969 and 1979, but has not shown a full outburst since. Due to its proximity and sustained period of quiescence, it is a prime target to study the coupling between accretion and jet ejection in quiescent neutron star low-mass X-ray binaries. Here, we present four MeerKAT radio observations at 1.3 GHz of Cen X-4, combined with NICER and Swift X-ray monitoring. During the first and most sensitive observation, Cen X-4 was in a fully quiescent X-ray state. The three later and shorter observations targeted a brief period of faint X-ray activity in January 2021, which has been referred to as a 'mis-fired' outburst. Cen X-4 is not detected in any of the four MeerKAT observations. We place these radio non-detections on the X-ray -- radio luminosity diagram, improving the constraints on the correlation between the two luminosities from earlier quiescent radio studies. We confirm that Cen X-4 is radio fainter than the transitional milli-second pulsar PSR J1023+0038 at the same X-ray luminosity. We discuss the radio behaviour of accreting neutron stars at low X-ray luminosity more generally and finally comment on future observing campaigns.

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arXiv2022

A Late-Time Radio Flare following a Possible Transition in Accretion State in the Tidal Disruption Event AT 2019azh

We report here radio follow-up observations of the optical Tidal Disruption Event (TDE) AT 2019azh. Previously reported X-ray observations of this TDE showed variability at early times and a dramatic increase in luminosity, by a factor of $\sim 10$, about 8 months after optical discovery. The X-ray emission is mainly dominated by intermediate hard--soft X-rays and is exceptionally soft around the X-ray peak, which is $L_X \sim 10^{43} \rm \, erg \, s^{-1}$. The high cadence $15.5$ GHz observations reported here show an early rise in radio emission followed by an approximately constant light curve, and a late-time flare. This flare starts roughly at the time of the observed X-ray peak luminosity and reaches its peak about $110$ days after the peak in the X-ray, and a year after optical discovery. The radio flare peaks at $νL_ν \sim 10^{38} \rm \, erg \, s^{-1}$, a factor of two higher than the emission preceding the flare. In light of the late-time radio and X-ray flares, and the X-ray spectral evolution, we speculate a possible transition in the accretion state of this TDE, similar to the observed behavior in black hole X-ray binaries. We compare the radio properties of AT 2019azh to other known TDEs, and focus on the similarities to the late time radio flare of the TDE ASASSN-15oi.

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arXiv2022

VLBI observations of GRB 201015A, a relatively faint GRB with a hint of Very High Energy gamma-ray emission

GRB 201015A is a long-duration Gamma-Ray Burst (GRB) which was detected at very high energies (> 100 GeV) using the MAGIC telescopes. If confirmed, this would be the fifth and least luminous GRB ever detected at this energies. We performed a radio follow-up of GRB 201015A over twelve different epochs, from 1.4 to 117 days post-burst, with the Karl G. Jansky Very Large Array, e-MERLIN and the European VLBI Network. We included optical and X-rays observations, performed with the Multiple Mirror Telescope and the Chandra X-ray Observatory respectively, together with publicly available data. We detected a point-like transient, consistent with the position of GRB 201015A until 23 and 47 days post-burst at 1.5 and 5 GHz, respectively. The source was detected also in both optical (1.4 and 2.2 days post-burst) and X-ray (8.4 and 13.6 days post-burst) observations. The multi-wavelength afterglow light curves can be explained with the standard model for a GRB seen on-axis, which expands and decelerates into a medium with a homogeneous density, while a circumburst medium with a wind-like profile is disfavoured. Notwithstanding the high resolution provided by the VLBI, we could not pinpoint any expansion or centroid displacement of the outflow. If the GRB is seen at the viewing angle which maximises the apparent velocity, we estimate that the Lorentz factor for the possible proper motion is $Γ_α$ < 40 in right ascension and $Γ_δ$ < 61 in declination. On the other hand, if the GRB is seen on-axis, the size of the afterglow is <5 pc and <16 pc at 25 and 47 days. Finally, the early peak in the optical light curve suggests the presence of a reverse shock component before 0.01 days from the burst.

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arXiv2022

Jet-Cocoon Geometry in the Optically Dark, Very High Energy Gamma-ray Burst 201216C

We present the results of a radio observing campaign on GRB 201216C, combined with publicly available optical and X-ray data. The detection of very high energy (VHE, >100GeV) emission by MAGIC makes this the fifth VHE GRB at time of publication. Comparison between the optical and X-ray light curves show that GRB 201216C is a dark GRB, i.e. the optical emission is significantly absorbed and is fainter than expected from the X-ray detections. Our e-MERLIN data also shows evidence of diffractive interstellar scintillation. We can study the column density along the line-of-sight to the GRB in both the host galaxy, from the damped optical light curve, and the Milky Way, via scintillation studies. We find that the afterglow is best modelled using a jet-cocoon geometry within a stellar wind environment. Fitting the data with a multi-component model we estimate that the optical, X-ray and higher-frequency radio data before ~25days originates from an ultra-relativistic jet with an isotropic equivalent kinetic energy of (0.6-10)x10^52erg and an opening angle of ~1-9deg. The lower-frequency radio emission detected by MeerKAT, from day 28 onwards, is produced by the cocoon with a kinetic energy that is between two and seven orders of magnitude lower (0.02-50)x10^48erg. The energies of the two components are comparable to those derived in simulations of such scenarios.

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arXiv2022

A persistent ultraviolet outflow from an accreting neutron star binary transient

All disc-accreting astrophysical objects produce powerful outflows. In binaries containing neutron stars (NS) or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated "soft states". By contrast,optical wind-formed lines have recently been detected in "hard states", when a hot corona dominates the luminosity. The relationship between these signatures is unknown, and no erupting system has revealed wind-formed lines between the X-ray and optical bands yet, despite the many strong resonance transitions in this ultraviolet (UV) region. Here, we show that the transient NS binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state. This represents the first evidence for a warm, moderately ionized outflow component in this state. Simultaneously observed optical lines also display transient blue-shifted absorption. Decomposing the UV data into constant and variable components, the blue-shifted absorption is associated with the former. This implies that the outflow is not connect to the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or stratified outflow from the outer disc. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations and helps to explain the shorter-than-expected outbursts duration.

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arXiv2021

MeerKAT radio detection of the Galactic black hole candidate Swift J1842.5-1124 during its 2020 outburst

Swift J1842.5$-$1124 is a transient Galactic black hole X-ray binary candidate, which underwent a new outburst in May 2020. We performed multi-epoch MeerKAT radio observations under the ThunderKAT Large Survey Programme, coordinated with quasi-simultaneous Swift/XRT X-ray observations during the outburst, which lasted nearly a month. We were able to make the first-ever radio detection of this black hole binary with the highest flux density of 229$\pm$31 $μ$Jy when the source was in the hard state, after non-detection in the radio band in the soft state which occurred immediately after its emergence during the new X-ray outburst. Therefore, its radio and X-ray properties are consistent with the disk-jet coupling picture established in other black hole X-ray binaries. We place the source's quasi-simultaneous X-ray and radio measurements on the radio/X-ray luminosity correlation plane; two quasi-simultaneous radio/X-ray measurements separated by 11 days were obtained, which span $\sim$ 2 dex in the X-ray luminosity. If the source follows the black hole track in the radio/X-ray correlation plane during the outburst, it would lie at a distance beyond $\sim$ 5 kpc.

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arXiv2021

MeerKAT discovery of radio emission from the Vela X-1 bow shock

Vela X-1 is a runaway X-ray binary system hosting a massive donor star, whose strong stellar wind creates a bow shock as it interacts with the interstellar medium. This bow shock has previously been detected in H$α$ and IR, but, similar to all but one bow shock from a massive runaway star (BD+43$^{\rm o}$3654), has escaped detection in other wavebands. We report on the discovery of $1.3$ GHz radio emission from the Vela X-1 bow shock with the MeerKAT telescope. The MeerKAT observations reveal how the radio emission closely traces the H$α$ line emission, both in the bow shock and in the larger-scale diffuse structures known from existing H$α$ surveys. The Vela X-1 bow shock is the first stellar-wind-driven radio bow shock detected around an X-ray binary. In the absence of a radio spectral index measurement, we explore other avenues to constrain the radio emission mechanism. We find that thermal/free-free emission can account for the radio and H$α$ properties, for a combination of electron temperature and density consistent with earlier estimates of ISM density and the shock enhancement. In this explanation, the presence of a local ISM over-density is essential for the detection of radio emission. Alternatively, we consider a non-thermal/synchrotron scenario, evaluating the magnetic field and broad-band spectrum of the shock. However, we find that exceptionally high fractions ($\gtrsim 13$%) of the kinetic wind power would need to be injected into the relativistic electron population to explain the radio emission. Assuming lower fractions implies a hybrid scenario, dominated by free-free radio emission. Finally, we speculate about the detectability of radio bow shocks and whether it requires exceptional ISM or stellar wind properties.

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arXiv2021

Classification of Multiwavelength Transients with Machine Learning

With the advent of powerful telescopes such as the Square Kilometer Array and the Vera C. Rubin Observatory, we are entering an era of multiwavelength transient astronomy that will lead to a dramatic increase in data volume. Machine learning techniques are well suited to address this data challenge and rapidly classify newly detected transients. We present a multiwavelength classification algorithm consisting of three steps: (1) interpolation and augmentation of the data using Gaussian processes; (2) feature extraction using wavelets; and (3) classification with random forests. Augmentation provides improved performance at test time by balancing the classes and adding diversity into the training set. In the first application of machine learning to the classification of real radio transient data, we apply our technique to the Green Bank Interferometer and other radio light curves. We find we are able to accurately classify most of the 11 classes of radio variables and transients after just eight hours of observations, achieving an overall test accuracy of 78 percent. We fully investigate the impact of the small sample size of 82 publicly available light curves and use data augmentation techniques to mitigate the effect. We also show that on a significantly larger simulated representative training set that the algorithm achieves an overall accuracy of 97 percent, illustrating that the method is likely to provide excellent performance on future surveys. Finally, we demonstrate the effectiveness of simultaneous multiwavelength observations by showing how incorporating just one optical data point into the analysis improves the accuracy of the worst performing class by 19 percent.

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arXiv2021

Observations of a radio-bright, X-ray obscured GRS 1915+105

The Galactic black hole transient GRS1915+105 is famous for its markedly variable X-ray and radio behaviour, and for being the archetypal galactic source of relativistic jets. It entered an X-ray outburst in 1992 and has been active ever since. Since 2018 GRS1915+105 has declined into an extended low-flux X-ray plateau, occasionally interrupted by multi-wavelength flares. Here we report the radio and X-ray properties of GRS1915+105 collected in this new phase, and compare the recent data to historic observations. We find that while the X-ray emission remained unprecedentedly low for most of the time following the decline in 2018, the radio emission shows a clear mode change half way through the extended X-ray plateau in 2019 June: from low flux (~3mJy) and limited variability, to marked flaring with fluxes two orders of magnitude larger. GRS1915+105 appears to have entered a low-luminosity canonical hard state, and then transitioned to an unusual accretion phase, characterised by heavy X-ray absorption/obscuration. Hence, we argue that a local absorber hides from the observer the accretion processes feeding the variable jet responsible for the radio flaring. The radio-X-ray correlation suggests that the current low X-ray flux state may be a signature of a super-Eddington state akin to the X-ray binaries SS433 or V404 Cyg.

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arXiv2021

Fast infrared variability from the black-hole candidate MAXI J1535$-$571 and tight constraints on the modelling

We present the results regarding the analysis of the fast X-ray/infrared (IR) variability of the black-hole transient MAXI J1535$-$571. The data studied in this work consist of two strictly simultaneous observations performed with XMM-Newton (X-rays: 0.7$-$10 keV), VLT/HAWK-I ($K_{\rm s}$ band, 2.2 $μ$m) and VLT/VISIR ($M$ and $PAH2$_$2$ bands, 4.85 and 11.88 $μ$m respectively). The cross-correlation function between the X-ray and near-IR light curves shows a strong asymmetric anti-correlation dip at positive lags. We detect a near-IR QPO (2.5 $σ$) at $2.07\pm0.09$ Hz simultaneously with an X-ray QPO at approximately the same frequency ($f_0=2.25\pm0.05$). From the cross-spectral analysis a lag consistent with zero was measured between the two oscillations. We also measure a significant correlation between the average near-IR and mid-IR fluxes during the second night, but find no correlation on short timescales. We discuss these results in terms of the two main scenarios for fast IR variability (hot inflow and jet powered by internal shocks). In both cases, our preliminary modelling suggests the presence of a misalignment between disk and jet.

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arXiv2020

Measuring the distance to the black hole candidate X-ray binary MAXI J1348-630 using HI absorption

We present HI absorption spectra of the black hole candidate X-ray binary (XRB) MAXI J1348-630 using the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT. The ASKAP HI spectrum shows a maximum negative radial velocity (with respect to the local standard of rest) of $-31\pm4$ km s$^{-1}$ for MAXI J1348-630, as compared to $-50\pm4$ km s$^{-1}$ for a stacked spectrum of several nearby extragalactic sources. This implies a most probable distance of $2.2^{+0.5}_{-0.6}$ kpc for MAXI J1348-630, and a strong upper limit of the tangent point distance at $5.3\pm0.1$ kpc. Our preferred distance implies that MAXI J1348-630 reached $17\pm10$ % of the Eddington luminosity at the peak of its outburst, and that the source transited from the soft to the hard X-ray spectral state at $2.5\pm1.5$ % of the Eddington luminosity. The MeerKAT HI spectrum of MAXI J1348-630 (obtained from the older, low-resolution 4k mode) is consistent with the re-binned ASKAP spectrum, highlighting the potential of the eventual capabilities of MeerKAT for XRB spectral line studies.

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arXiv2020

Rapid compact jet quenching in the Galactic black hole candidate X-ray binary MAXI J1535-571

We present results from six epochs of quasi-simultaneous radio, (sub-)millimetre, infrared, optical, and X-ray observations of the black hole X-ray binary MAXI~J1535$-$571. These observations show that as the source transitioned through the hard-intermediate X-ray state towards the soft intermediate X-ray state, the jet underwent dramatic and rapid changes. We observed the frequency of the jet spectral break, which corresponds to the most compact region in the jet where particle acceleration begins (higher frequencies indicate closer to the black hole), evolve from the IR band into the radio band (decreasing by $\approx$3 orders of magnitude) in less than a day. During one observational epoch, we found evidence of the jet spectral break evolving in frequency through the radio band. Estimating the magnetic field and size of the particle acceleration region shows that the rapid fading of the high-energy jet emission was not consistent with radiative cooling; instead the particle acceleration region seems to be moving away from the black hole on approximately dynamical timescales. This result suggests that the compact jet quenching is not caused by local changes to the particle acceleration, rather we are observing the acceleration region of the jet travelling away from the black hole with the jet flow. Spectral analysis of the X-ray emission show a gradual softening in the few days before the dramatic jet changes, followed by a more rapid softening $\sim$1--2\,days after the onset of the jet quenching.

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