Aditya Parthasarathy

Hi, welcome to my corner of the internet.

Here you’ll find all about my research, software development projects, blog posts and my various side hustles. My CV can be found here.

I’m an astrophysicist. I study neutron stars and use them to search for the weakest gravitational waves in the Universe. Detecting these gravitational waves can help us understand the evolution of galaxies, the dyamics of supermassive black hole mergers and solve some of the biggest fundamental questions in physics. On July 29th, 2023, the international pulsar timing array community announced the first compelling evidence of the gravitational wave background! See this press release, where my colleagues and I from the European Pulsar Timing Array Collaboration (EPTA) discuss the importance of such a discovery.

Complimentary to radio pulsar timing arrays, I also co-developed the idea of the gamma-ray pulsar timing array (GPTA). See , this press release, when my colleague Dr. Matthew Kerr and I discuss the potential of the GPTA.

I recently was awarded a Starting Grant by the European Research Council of around 1.6 million Euros to start my own research group to advance pulsar timing array and gravitaitonal wave science. See here for some details and here is the project webpage.

My research also involves software development and data science. I am the lead developer of the data analysis pipeline used for the MeerKAT radio telescope in South Africa. This is one of the most sensitive radio telescopes in the world. I spent a few weeks in Cape Town commissioning this telescope. See here for some photos from this trip.

I also develop Bayesian inference software to aid the detection of weak gravitational waves. Here are some talks and tutorials I have given to students around the world on timing neutron stars, Bayesian statistics and using the relevant software packages.

For a collection of my software development projects, see here.

I am very passionate about science outreach and have given more than 60 talks to non-scientific audiences around the globe.

When I’m outdoors, I enjoy long-distance cycling and adventure sports. When indoors, I enjoy gaming.

Apart from astrophysics, I am passionate about neuroscience, advanced data modelling, formula one and mixology.

selected publications

arXiv e-prints

The second data release from the European Pulsar Timing Array II. Customised pulsar noise models for spatially correlated gravitational waves

Antoniadis, J., Arumugam, P., Arumugam, S., Babak, S., Bagchi, M., Bak Nielsen, A. S., Bassa, C. G., Bathula, A., Berthereau, A., Bonetti, M., Bortolas, E., Brook, P. R., Burgay, M., Caballero, R. N., Chalumeau, A., Champion, D. J., Chanlaridis, S., Chen, S., Cognard, I., Dandapat, S., Deb, D., Desai, S., Desvignes, G., Dhanda-Batra, N., Dwivedi, C., Falxa, M., Ferdman, R. D., Franchini, A., Gair, J. R., Goncharov, B., Gopakumar, A., Graikou, E., Grießmeier, J. -M., Guillemot, L., Guo, Y. J., Gupta, Y., Hisano, S., Hu, H., Iraci, F., Izquierdo-Villalba, D., Jang, J., Jawor, J., Janssen, G. H., Jessner, A., Joshi, B. C., Kareem, F., Karuppusamy, R., Keane, E. F., Keith, M. J., Kharbanda, D., Kikunaga, T., Kolhe, N., Kramer, M., Krishnakumar, M. A., Lackeos, K., Lee, K. J., Liu, K., Liu, Y., Lyne, A. G., McKee, J. W., Maan, Y., Main, R. A., Mickaliger, M. B., Niţu, I. C., Nobleson, K., Paladi, A. K., Parthasarathy, A., Perera, B. B. P., Perrodin, D., Petiteau, A., Porayko, N. K., Possenti, A., Prabu, T., Quelquejay Leclere, H., Rana, P., Samajdar, A., Sanidas, S. A., Sesana, A., Shaifullah, G., Singha, J., Speri, L., Spiewak, R., Srivastava, A., Stappers, B. W., Surnis, M., Susarla, S. C., Susobhanan, A., Takahashi, K., Tarafdar, P., Theureau, G., Tiburzi, C., van der Wateren, E., Vecchio, A., Venkatraman Krishnan, V., Verbiest, J. P. W., Wang, J., Wang, L., and Wu, Z.

Jun 2023

ADS LINK
arXiv e-prints

The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals

Antoniadis, J., Arumugam, P., Arumugam, S., Babak, S., Bagchi, M., Bak Nielsen, A. -S., Bassa, C. G., Bathula, A., Berthereau, A., Bonetti, M., Bortolas, E., Brook, P. R., Burgay, M., Caballero, R. N., Chalumeau, A., Champion, D. J., Chanlaridis, S., Chen, S., Cognard, I., Dandapat, S., Deb, D., Desai, S., Desvignes, G., Dhanda-Batra, N., Dwivedi, C., Falxa, M., Ferdman, R. D., Franchini, A., Gair, J. R., Goncharov, B., Gopakumar, A., Graikou, E., Grießmeier, J. -M., Guillemot, L., Guo, Y. J., Gupta, Y., Hisano, S., Hu, H., Iraci, F., Izquierdo-Villalba, D., Jang, J., Jawor, J., Janssen, G. H., Jessner, A., Joshi, B. C., Kareem, F., Karuppusamy, R., Keane, E. F., Keith, M. J., Kharbanda, D., Kikunaga, T., Kolhe, N., Kramer, M., Krishnakumar, M. A., Lackeos, K., Lee, K. J., Liu, K., Liu, Y., Lyne, A. G., McKee, J. W., Maan, Y., Main, R. A., Mickaliger, M. B., Nitu, I. C., Nobleson, K., Paladi, A. K., Parthasarathy, A., Perera, B. B. P., Perrodin, D., Petiteau, A., Porayko, N. K., Possenti, A., Prabu, T., Quelquejay Leclere, H., Rana, P., Samajdar, A., Sanidas, S. A., Sesana, A., Shaifullah, G., Singha, J., Speri, L., Spiewak, R., Srivastava, A., Stappers, B. W., Surnis, M., Susarla, S. C., Susobhanan, A., Takahashi, K., Tarafdar, P., Theureau, G., Tiburzi, C., van der Wateren, E., Vecchio, A., Venkatraman Krishnan, V., Verbiest, J. P. W., Wang, J., Wang, L., and Wu, Z.

Jun 2023

ADS LINK
Science

A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background

Kerr, M., Parthasarathy, A., and FERMI-LAT Collaboration,

Apr 2022

Abstract ADS LINK Code Poster Media Coverage

After large galaxies merge, their central supermassive black holes are expected to form binary systems. Their orbital motion should generate a gravitational wave background (GWB) at nanohertz frequencies. Searches for this background use pulsar timing arrays, which perform long-term monitoring of millisecond pulsars at radio wavelengths. We used 12.5 years of Fermi Large Area Telescope data to form a gamma-ray pulsar timing array. Results from 35 bright gamma-ray pulsars place a 95% credible limit on the GWB characteristic strain of 1.0 \texttimes 10^\ensuremath-14 at a frequency of 1 year^–1. The sensitivity is expected to scale with t_obs, the observing time span, as tobs\ensuremath-13/6. This direct measurement provides an independent probe of the GWB while offering a check on radio noise models.
MNRAS

Noise analysis in the European Pulsar Timing Array data release 2 and its implications on the gravitational-wave background search

Chalumeau, A., Babak, S., Petiteau, A., Chen, S., Samajdar, A., Caballero, R. N., Theureau, G., Guillemot, L., Desvignes, G., Parthasarathy, A., Liu, K., Shaifullah, G., Hu, H., van der Wateren, E., Antoniadis, J., Bak Nielsen, A. -S., Bassa, C. G., Berthereau, A., Burgay, M., Champion, D. J., Cognard, I., Falxa, M., Ferdman, R. D., Freire, P. C. C., Gair, J. R., Graikou, E., Guo, Y. J., Jang, J., Janssen, G. H., Karuppusamy, R., Keith, M. J., Kramer, M., Lee, K. J., Liu, X. J., Lyne, A. G., Main, R. A., McKee, J. W., Mickaliger, M. B., Perera, B. B. P., Perrodin, D., Porayko, N. K., Possenti, A., Sanidas, S. A., Sesana, A., Speri, L., Stappers, B. W., Tiburzi, C., Vecchio, A., Verbiest, J. P. W., Wang, J., Wang, L., and Xu, H.

Feb 2022

Abstract ADS LINK

The European Pulsar Timing Array (EPTA) collaboration has recently released an extended data set for six pulsars (DR2) and reported evidence for a common red noise signal. Here we present a noise analysis for each of the six pulsars. We consider several types of noise: (i) radio frequency independent, ’achromatic’, and time-correlated red noise; (ii) variations of dispersion measure and scattering; (iii) system and band noise; and (iv) deterministic signals (other than gravitational waves) that could be present in the PTA data. We perform Bayesian model selection to find the optimal combination of noise components for each pulsar. Using these custom models we revisit the presence of the common uncorrelated red noise signal previously reported in the EPTA DR2 and show that the data still supports it with a high statistical significance. Next, we confirm that there is no preference for or against the Hellings-Downs spatial correlations expected for the stochastic gravitational-wave background. The main conclusion of the EPTA DR2 paper remains unchanged despite a very significant change in the noise model of each pulsar. However, modelling the noise is essential for the robust detection of gravitational waves and its impact could be significant when analysing the next EPTA data release, which will include a larger number of pulsars and more precise measurements.
MNRAS

The thousand-pulsar-array programme on MeerKAT VII: polarisation properties of pulsars in the Magellanic Clouds

Johnston, S., Parthasarathy, A., Main, R. A., Ridley, J. P., Koribalski, B. S., Bailes, M., Buchner, S. J., Geyer, M., Karastergiou, A., Keith, M. J., Kramer, M., Serylak, M., Shannon, R. M., Spiewak, R., and Krishnan, V. Venkatraman

Feb 2022

Abstract ADS LINK

The Magellanic Clouds are the only external galaxies known to host radio pulsars. The dispersion and rotation measures of pulsars in the Clouds can aid in understanding their structure, and studies of the pulsars themselves can point to potential differences between them and their Galactic counterparts. We use the high sensitivity of the MeerKAT telescope to observe 17 pulsars in the Small and Large Magellanic Clouds in addition to five foreground (Galactic) pulsars. We provide polarisation profiles for 18 of these pulsars, improved measurements of their dispersion and rotation measures, and derive the mean parallel magnetic field along the lines of sight. The results are broadly in agreement with expectations for the structure and strength of the magnetic field in the Large and Small Magellanic Clouds. The Magellanic Cloud pulsars have profiles which are narrower than expected from the period-width relationship and we show this is due to selection effects in pulsar surveys rather than any intrinsic difference between the population of Galactic and Magellanic objects.
ApJl

The Radius of PSR J0740+6620 from NICER and XMM-Newton Data

Miller, M. C., Lamb, F. K., Dittmann, A. J., Bogdanov, S., Arzoumanian, Z., Gendreau, K. C., Guillot, S., Ho, W. C. G., Lattimer, J. M., Loewenstein, M., Morsink, S. M., Ray, P. S., Wolff, M. T., Baker, C. L., Cazeau, T., Manthripragada, S., Markwardt, C. B., Okajima, T., Pollard, S., Cognard, I., Cromartie, H. T., Fonseca, E., Guillemot, L., Kerr, M., Parthasarathy, A., Pennucci, T. T., Ransom, S., and Stairs, I.

Sep 2021

Abstract ADS LINK

PSR J0740+6620 has a gravitational mass of 2.08 \ensuremath\pm 0.07 M_\ensuremath⊙, which is the highest reliably determined mass of any neutron star. As a result, a measurement of its radius will provide unique insight into the properties of neutron star core matter at high densities. Here we report a radius measurement based on fits of rotating hot spot patterns to Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) X-ray observations. We find that the equatorial circumferential radius of PSR J0740+6620 is 13.7_-1.5^+2.6 km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030+0451, the masses of two other \raisebox-0.5ex\textasciitilde2 M_\ensuremath⊙ pulsars, and the tidal deformability constraints from two gravitational wave events, to three different frameworks for equation-of-state modeling, and find consistent results at \raisebox-0.5ex\textasciitilde1.5-5 times nuclear saturation density. For a given framework, when all measurements are included, the radius of a 1.4 M_\ensuremath⊙ neutron star is known to \ensuremath\pm4% (68% credibility) and the radius of a 2.08 M_\ensuremath⊙ neutron star is known to \ensuremath\pm5%. The full radius range that spans the \ensuremath\pm1\ensuremathσ credible intervals of all the radius estimates in the three frameworks is 12.45 \ensuremath\pm 0.65 km for a 1.4 M_\ensuremath⊙ neutron star and 12.35 \ensuremath\pm 0.75 km for a 2.08 M_\ensuremath⊙ neutron star.
MNRAS

Measurements of pulse jitter and single-pulse variability in millisecond pulsars using MeerKAT

Parthasarathy, A., Bailes, M., Shannon, R. M., van Straten, W., Osłowski, S., Johnston, S., Spiewak, R., Reardon, D. J., Kramer, M., Venkatraman Krishnan, V., Pennucci, T. T., Abbate, F., Buchner, S., Camilo, F., Champion, D. J., Geyer, M., Hugo, B., Jameson, A., Karastergiou, A., Keith, M. J., and Serylak, M.

Mar 2021

Abstract ADS LINK

Using the state-of-the-art SKA precursor, the MeerKAT radio telescope, we explore the limits to precision pulsar timing of millisecond pulsars achievable due to pulse stochasticity (jitter). We report new jitter measurements in 15 of the 29 pulsars in our sample and find that the levels of jitter can vary dramatically between them. For some, like the 2.2 ms pulsar PSR J2241-5236, we measure an implied jitter of just \raisebox-0.5ex\textasciitilde4 ns h^-1, while others, like the 3.9 ms PSR J0636-3044, are limited to \raisebox-0.5ex\textasciitilde100 ns h^-1. While it is well known that jitter plays a central role to limiting the precision measurements of arrival times for high signal-to-noise ratio observations, its role in the measurement of dispersion measure (DM) has not been reported, particularly in broad-band observations. Using the exceptional sensitivity of MeerKAT, we explored this on the bright millisecond pulsar PSR J0437-4715 by exploring the DM of literally every pulse. We found that the derived single-pulse DMs vary by typically 0.0085 cm^-3 pc from the mean, and that the best DM estimate is limited by the differential pulse jitter across the band. We postulate that all millisecond pulsars will have their own limit on DM precision which can only be overcome with longer integrations. Using high- time resolution filterbank data of 9 \ensuremathμs, we also present a statistical analysis of single-pulse phenomenology. Finally, we discuss optimization strategies for the MeerKAT pulsar timing program and its role in the context of the International Pulsar Timing Array.
MNRAS

Timing of young radio pulsars - II. Braking indices and their interpretation

Parthasarathy, A., Johnston, S., Shannon, R. M., Lentati, L., Bailes, M., Dai, S., Kerr, M., Manchester, R. N., Osłowski, S., Sobey, C., van Straten, W., and Weltevrede, P.

May 2020

Abstract ADS LINK

In Paper I of this series, we detected a significant value of the braking index (n) for 19 young, high- \dotE radio pulsars using \ensuremath∼10 yr of timing observations from the 64-m Parkes radio telescope. Here, we investigate this result in more detail using a Bayesian pulsar timing framework to model timing noise and to perform selection to distinguish between models containing exponential glitch recovery and braking index signatures. We show that consistent values of n are maintained with the addition of substantial archival data, even in the presence of glitches. We provide strong arguments that our measurements are unlikely due to exponential recovery signals from unseen glitches even though glitches play a key role in the evolution of a pulsar’s spin frequency. We conclude that, at least over decadal time-scales, the value of n can be significantly larger than the canonical 3 and discuss the implications for the evolution of pulsars.
MNRAS

Timing of young radio pulsars - I. Timing noise, periodic modulation, and proper motion

Parthasarathy, A., Shannon, R. M., Johnston, S., Lentati, L., Bailes, M., Dai, S., Kerr, M., Manchester, R. N., Osłowski, S., Sobey, C., van Straten, W., and Weltevrede, P.

Nov 2019

Abstract ADS LINK

The smooth spin-down of young pulsars is perturbed by two non- deterministic phenomenon, glitches, and timing noise. Although the timing noise provides insights into nuclear and plasma physics at extreme densities, it acts as a barrier to high- precision pulsar timing experiments. An improved methodology based on the Bayesian inference is developed to simultaneously model the stochastic and deterministic parameters for a sample of 85 high-\textbackslashdot{E} radio pulsars observed for \ensuremath∼10 yr with the 64-m Parkes radio telescope. Timing noise is known to be a red process and we develop a parametrization based on the red-noise amplitude (A_red) and spectral index (\ensuremathβ). We measure the median A_red to be -10.4\^{+1.8}_{-1.7} yr^3/2 and \ensuremathβ to be -5.2\^{+3.0}_{-3.8} and show that the strength of timing noise scales proportionally to \ensuremathν ^1|\textbackslashdot{\ensuremathν }|\^{-0.6\ensuremath\pm 0.1}, where \ensuremathν is the spin frequency of the pulsar and \textbackslashdot{\ensuremathν } is its spin-down rate. Finally, we measure significant braking indices for 19 pulsars and proper motions for 2 pulsars, and discuss the presence of periodic modulation in the arrival times of 5 pulsars.
ASCL

Molsoft: Molonglo Telescope Observing Software

Jankowski, Fabian, Parthasarathy, Aditya, Farah, Wael, and Flynn, Chris

Aug 2019

ADS LINK