Publications by Aris Karastergiou

A self-lensing binary massive black hole interpretation of quasi-periodic eruptions

Monthly Notices of the Royal Astronomical Society Oxford University Press 503 (2021) 1703-1716

A Ingram, S Motta, S Aigrain, A Karastergiou

Binary supermassive black hole (SMBH) systems result from galaxy mergers, and will eventually coalesce due to gravitational wave (GW) emission if the binary separation can be reduced to . 0.1 pc by other mechanisms. Here, we explore a gravitational self-lensing binary SMBH model for the sharp (duration ⇠ 1 hr), quasi-regular X-ray flares – dubbed quasiperiodic eruptions – recently observed from two low mass active galactic nuclei: GSN 069 and RX J1301.9+2747. In our model, the binary is observed ⇠edge-on, such that each SMBH gravitationally lenses light from the accretion disc surrounding the other SMBH twice per orbital period. The model can reproduce the flare spacings if the current eccentricity of RX J1301.9+2747 is n0 & 0.16, implying a merger within ⇠ 1000 yrs. However, we cannot reproduce the observed flare profiles with our current calculations. Model flares with the correct amplitude are ⇠ 2/5 the observed duration, and model flares with the correct duration are ⇠ 2/5 the observed amplitude. Our modelling yields three distinct behaviours of self-lensing binary systems that can be searched for in current and future X-ray and optical time-domain surveys: i) periodic lensing flares, ii) partial eclipses (caused by occultation of the background mini-disc by the foreground mini-disc), and iii) partial eclipses with a very sharp in-eclipse lensing flare. Discovery of such features would constitute very strong evidence for the presence of a supermassive binary, and monitoring of the flare spacings will provide a measurement of periastron precession.

Initial results from a realtime FRB search with the GBT

Monthly Notices of the Royal Astronomical Society Oxford University Press 497 (2020) 352-360

D Agarwal, D Lorimer, MP Surnis, X Pei, A Karastergiou, G Golpayegani, D Werthimer, J Cobb, MA McLaughlin, S White, W Armour, DHE MacMahon, APV Siemion, G Foster

We present the data analysis pipeline, commissioning observations, and initial results from the GREENBURST fast radio burst (FRB) detection system on the Robert C. Byrd Green Bank Telescope (GBT) previously described by Surnis et al., which uses the 21-cm receiver observing commensally with other projects. The pipeline makes use of a state-of-the-art deep learning classifier to winnow down the very large number of false-positive single-pulse candidates that mostly result from radio frequency interference. In our observations, totalling 156.5 d so far, we have detected individual pulses from 20 known radio pulsars that provide an excellent verification of the system performance. We also demonstrate, through blind injection analyses, that our pipeline is complete down to a signal-to-noise threshold of 12. Depending on the observing mode, this translates into peak flux sensitivities in the range 0.14–0.89 Jy. Although no FRBs have been detected to date, we have used our results to update the analysis of Lawrence et al. to constrain the FRB all-sky rate to be 1150+200−180 per day above a peak flux density of 1 Jy. We also constrain the source count index α = 0.84 ± 0.06, which indicates that the source count distribution is substantially flatter than expected from a Euclidean distribution of standard candles (where α = 1.5). We discuss this result in the context of the FRB redshift and luminosity distributions. Finally, we make predictions for detection rates with GREENBURST, as well as other ongoing and planned FRB experiments.

The Galactic population and properties of young, highly energetic pulsars


S Johnston, DA Smith, A Karastergiou, M Kramer

Pulsar polarimetry with the Parkes ultra-wideband receiver


L Oswald, A Karastergiou, S Johnston

FPGA architecture to search for accelerated pulsars with SKA

2020 33rd General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2020 (2020)

P Thiagaraj, B Stappers, A Ghalame, L Levin, A Karastergiou, J Roy, M Mickaliger, M Keith

This paper presents a design which uses an FPGA (Field Programmable Gate Array) to search for radio pulsars in binary systems for the Square Kilometre Array (SKA). The search pipeline executes Doppler de-acceleration algorithm in real-time to identify binary pulsar spin periods. It involves a very large computation and the algorithm implementation for a real-time execution with power efficiency for the SKA is a challenging task. The design follows a Fourier domain acceleration search method, which is essentially a matched filtering technique, where the complex spectra of the dedispersed time-series are convolved with a set of Doppler demodulation templates. Convolution outputs are systematically summed over a number of harmonics and signatures of periodic signals are detected and passed on for subsequent downstream processing. This design is developed for modern FPGA technology and a prototype of the design is implemented and tested on a commercial FPGA accelerator platform. Salient details of the work are presented in the paper.

The MeerKAT telescope as a pulsar facility: System verification and early science results from MeerTime

Publications of the Astronomical Society of Australia (2020)

M Bailes, A Jameson, F Abbate, ED Barr, NDR Bhat, L Bondonneau, M Burgay, SJ Buchner, F Camilo, DJ Champion, I Cognard, PB Demorest, PCC Freire, T Gautam, M Geyer, JM Griessmeier, L Guillemot, H Hu, F Jankowski, S Johnston, A Karastergiou, R Karuppusamy, D Kaur, MJ Keith, M Kramer, J Van Leeuwen, ME Lower, Y Maan, MA McLaughlin, BW Meyers, S Osłowski, LS Oswald, A Parthasarathy, T Pennucci, B Posselt, A Possenti, SM Ransom, DJ Reardon, A Ridolfi, CTG Schollar, M Serylak, G Shaifullah, M Shamohammadi, RM Shannon, C Sobey, X Song, R Spiewak, IH Stairs, BW Stappers, W Van Straten, A Szary, G Theureau, V Venkatraman Krishnan, P Weltevrede, N Wex, TD Abbott, GB Adams, JP Burger, RRG Gamatham, M Gouws, DM Horn, B Hugo, AF Joubert, JR Manley, K McAlpine, SS Passmoor, A Peens-Hough, ZR Ramudzuli, A Rust, S Salie, LC Schwardt, R Siebrits, G Van Tonder, V Van Tonder, MG Welz

Copyright © Astronomical Society of Australia 2020; published by Cambridge University Press. We describe system verification tests and early science results from the pulsar processor (PTUSE) developed for the newly commissioned 64-dish SARAO MeerKAT radio telescope in South Africa. MeerKAT is a high-gain low-system temperature radio array that currently operates at 580-1 670 MHz and can produce tied-array beams suitable for pulsar observations. This paper presents results from the MeerTime Large Survey Project and commissioning tests with PTUSE. Highlights include observations of the double pulsar, pulse profiles from 34 millisecond pulsars (MSPs) from a single 2.5-h observation of the Globular cluster Terzan 5, the rotation measure of Ter5O, a 420-sigma giant pulse from the Large Magellanic Cloud pulsar PSR , and nulling identified in the slow pulsar PSR J0633-2015. One of the key design specifications for MeerKAT was absolute timing errors of less than 5 ns using their novel precise time system. Our timing of two bright MSPs confirm that MeerKAT delivers exceptional timing. PSR exhibits a jitter limit of whilst timing of PSR over almost 11 months yields an rms residual of 66 ns with only 4 min integrations. Our results confirm that the MeerKAT is an exceptional pulsar telescope. The array can be split into four separate sub-arrays to time over 1 000 pulsars per day and the future deployment of S-band (1 750-3 500 MHz) receivers will further enhance its capabilities.

Possible periodic activity in the repeating FRB 121102


KM Rajwade, MB Mickaliger, BW Stappers, V Morello, D Agarwal, CG Bassa, RP Breton, M Caleb, A Karastergiou, EF Keane, DR Lorimer

Limits on absorption from a 332-MHz survey for fast radio bursts


KM Rajwade, MB Mickaliger, BW Stappers, CG Bassa, RP Breton, A Karastergiou, EF Keane

The Thousand-Pulsar-Array programme on MeerKAT - I. Science objectives and first results

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY Oxford University Press (OUP) 493 (2020) 3608-3615

S Johnston, A Karastergiou, M Keith, X Song, P Weltevrede, F Abbate, M Bailes, S Buchner, F Camilo, M Geyer, B Hugo, A Jameson, M Kramer, A Parthasarathy, D Reardon, A Ridolfi, M Serylak, R Shannon, R Spiewak, W van Straten, VV Krishnan, F Jankowski, B Meyers, L Oswald, B Posselt

© 2020 The Author(s). We report here on initial results from the Thousand-Pulsar-Array (TPA) programme, part of the Large Survey Project 'MeerTime' on the MeerKAT telescope. The interferometer is used in the tied-array mode in the band from 856 to 1712 MHz, and the wide band coupled with the large collecting area and low receiver temperature make it an excellent telescope for the study of radio pulsars. The TPA is a 5 year project, which aims at to observing (a) more than 1000 pulsars to obtain high-fidelity pulse profiles, (b) some 500 of these pulsars over multiple epochs, and (c) long sequences of single-pulse trains from several hundred pulsars. The scientific outcomes from the programme will include the determination of pulsar geometries, the location of the radio emission within the pulsarmagnetosphere, the connection between the magnetosphere and the crust and core of the star, tighter constraints on the nature of the radio emission itself, as well as interstellar medium studies. First, results presented here include updated dispersion measures, 26 pulsars with Faraday rotation measures derived for the first time, and a description of interesting emission phenomena observed thus far.

Understanding the radio beam of PSR J1136+1551 through its single pulses

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 310-324

L Oswald, A Karastergiou, S Johnston

The frequency widening of pulsar profiles is commonly attributed to lower frequencies being produced at greater heights above the surface of the pulsar; so-called radius-to-frequency mapping (RFM). The observer’s view of pulsar emission is a 1D cut through a 3D magnetosphere: we can only see that emission which points along our line of sight. However, by comparing the frequency evolution of many single pulses positioned at different phases, we can build up an understanding of the shape of the active emission region. We use single pulses observed with the Giant Metrewave Radio Telescope to investigate the emission region of PSR J1136+1551 and test RFM. Assuming that emission is produced tangential to the magnetic field lines and that each emission frequency corresponds to a single height, we simulate the single pulse profile evolution resulting from the canonical conal beam model and a fan beam model. Comparing the results of these simulations with the observations, we conclude that the emission region of PSR J1136+1551 is better described by the fan beam model. The diversity of profile widening behaviour observed for the single pulses can be explained by orthogonally polarized modes propagating along differing frequency-dependent paths in the magnetosphere.

Linking long- and short-term emission variability in pulsars


PR Brook, A Karastergiou, S Johnston

GREENBURST: A commensal Fast Radio Burst search back-end for the Green Bank Telescope

Publications of the Astronomical Society of Australia Cambridge University Press 36 (2019) e032

MP Surnis, D Agarwal, Lorimer, X Pei, G Foster, A Karastergiou, G Golpayegani, RJ Maddalena, S White, W Armour, J Cobb, MA McLaughlin, DHE Macmahon, APV Siemion, D Werthimer, CJ Williams

We describe the design and deployment of GREENBURST, a commensal Fast Radio Burst (FRB) search system at the Green Bank Telescope. GREENBURST uses the dedicated L-band receiver tap to search over the 960$-$1920 MHz frequency range for pulses with dispersion measures out to $10^4$ pc cm$^{-3}$. Due to its unique design, GREENBURST will obtain data even when the L-band receiver is not being used for scheduled observing. This makes it a sensitive single pixel detector capable of reaching deeper in the radio sky. While single pulses from Galactic pulsars and rotating radio transients will be detectable in our observations, and will form part of the database we archive, the primary goal is to detect and study FRBs. Based on recent determinations of the all-sky rate, we predict that the system will detect approximately one FRB for every 2$-$3 months of continuous operation. The high sensitivity of GREENBURST means that it will also be able to probe the slope of the FRB source function, which is currently uncertain in this observing band.

A fast radio burst with frequency-dependent polarization detected during Breakthrough Listen observations


DC Price, G Foster, M Geyer, W van Straten, V Gajjar, G Hellbourg, A Karastergiou, EF Keane, APV Siemion, I Arcavi, R Bhat, M Caleb, S-W Chang, S Croft, D DeBoer, I de Pater, J Drew, JE Enriquez, W Farah, N Gizani, JA Green, H Isaacson, J Hickish, A Jameson, M Lebofsky, DHE MacMahon, A Moller, CA Onken, E Petroff, D Werthimer, C Wolf, SP Worden, YG Zhang

The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries


S Sanidas, S Cooper, CG Bassa, JWT Hessels, VI Kondratiev, D Michilli, BW Stappers, CM Tan, J van Leeuwen, L Cerrigone, RA Fallows, M Iacobelli, E Orru, RF Pizzo, A Shulevski, MC Toribio, S ter Veen, P Zucca, L Bondonneau, J-M Griessmeier, A Karastergiou, M Kramer, C Sobey

The period-width relationship for radio pulsars revisited


S Johnston, A Karastergiou

Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3D Galactic halo magnetic field


C Sobey, AV Bilous, J-M Griessmeier, JWT Hessels, A Karastergiou, EF Keane, VI Kondratiev, M Kramer, D Michilli, A Noutsos, M Pilia, EJ Polzin, BW Stappers, CM Tan, J van Leeuwen, JPW Verbiest, P Weltevrede, G Heald, MIR Alves, E Carretti, T Ensslin, M Haverkorn, M Iacobelli, W Reich, C Van Eck

A GPU implementation of the correlation technique for real-time Fourier domain pulsar acceleration searches

Astrophysical Journal Supplement Series American Astronomical Society 239 (2018) 28

S Dimoudi, K Adamek, P Thiagaraj, SM Ransom, A Karastergiou, W Armour

The study of binary pulsars enables tests of general relativity. Orbital motion in binary systems causes the apparent pulsar spin frequency to drift, reducing the sensitivity of periodicity searches. Acceleration searches are methods that account for the effect of orbital acceleration. Existing methods are currently computationally expensive, and the vast amount of data that will be produced by next-generation instruments such as the Square Kilometre Array necessitates real-time acceleration searches, which in turn requires the use of high-performance computing (HPC) platforms. We present our implementation of the correlation technique for the Fourier Domain Acceleration Search (FDAS) algorithm on Graphics Processor Units (GPUs). The correlation technique is applied as a convolution with multiple finite impulse response (FIR) filters in the Fourier domain. Two approaches are compared: the first uses the NVIDIA cuFFT library for applying Fast Fourier transforms (FFTs) on the GPU, and the second contains a custom FFT implementation in GPU shared memory. We find that the FFT shared-memory implementation performs between 1.5 and 3.2 times faster than our cuFFT-based application for smaller but sufficient filter sizes. It is also 4–6 times faster than the existing GPU and OpenMP implementations of FDAS. This work is part of the AstroAccelerate project, a many-core accelerated time-domain signal-processing library for radio astronomy.

Verifying and reporting Fast Radio Bursts


G Foster, A Karastergiou, M Geyer, M Surnis, G Golpayegani, K Lee, D Lorimer, DC Price, K Rajwade

The NANOGrav 11-year Data Set: Pulse Profile Variability


PR Brook, A Karastergiou, MA McLaughlin, MT Lam, Z Arzoumanian, S Chatterjee, JM Cordes, K Crowter, M DeCesar, PB Demorest, T Dolch, JA Ellis, RD Ferdman, E Ferrara, E Fonseca, PA Gentile, G Jones, ML Jones, TJW Lazio, L Levin, DR Lorimer, RS Lynch, C Ng, DJ Nice, TT Pennucci, SM Ransom, PS Ray, R Spiewak, IH Stairs, DR Stinebring, K Stovall, JK Swiggum, WW Zhu

LOFAR Discovery of a 23.5 s Radio Pulsar


CM Tan, CG Bassa, S Cooper, TJ Dijkema, P Esposito, JWT Hessels, VI Kondratiev, M Kramer, D Michilli, S Sanidas, TW Shimwell, BW Stappers, J van Leeuwen, I Cognard, J-M Griessmeier, A Karastergiou, EF Keane, C Sobey, P Weltevrede