J.-M. Grießmeier’s research while affiliated with French National Centre for Scientific Research and other places

We evaluate the impact of Starlink satellites on low-frequency radio astronomy below 100 MHz. We focus on challenges of the data processing and on scientific goals. We conducted 40 hours of imaging observations using NenuFAR in the 30.8–78.3 MHz range. Observations included both the targeted tracking of specific satellites based on orbital predictions and the untargeted searches focused on high-elevation regions of the sky. Images in total intensity and polarimetry were obtained, and full Stokes dynamic spectra were generated for several hundred directions within the field of view. The detected signals were cross-matched with satellite orbital data to confirm the satellite associations. We performed detailed analyses of the observed spectra, polarization, and temporal characteristics to investigate the origin and properties of the detected emissions. We detected broadband emissions from Starlink satellites, predominantly between 54–66 MHz, with flux densities exceeding 500 Jy. These signals are highly polarized and unlikely to originate from ground-based radio frequency interference or reflected astronomical sources. Instead, they are likely intrinsic to the satellites, and distinct differences in the emission properties are observed for the different satellite generations. These findings highlight significant challenges to the data processing and scientific discoveries at these low frequencies and emphasize the need for effective mitigation strategies, in particular, through collaboration between astronomers and satellite operators.

This study evaluates the impact of Starlink satellites on low-frequency radio astronomy below 100 MHz, focusing on challenges on data processing and scientific goals. We conducted 40 hours of imaging observations using NenuFAR, in the 30.8-78.3 MHz range. Observations included both targeted tracking of specific satellites based on orbital predictions and untargeted searches focused on high-elevation regions of the sky. Images in total intensity and polarimetry were obtained, and full Stokes dynamic spectra were generated for several hundred directions within the Field of View. Detected signals were cross-matched with satellite orbital data to confirm satellite associations. Detailed analyses of the observed spectra, polarization, and temporal characteristics were performed to investigate the origin and properties of the detected emissions. We detected broadband emissions from Starlink satellites, predominantly between 54-66 MHz, with flux densities exceeding 500 Jy. These signals are highly polarized and unlikely to originate from ground-based RFI or reflected astronomical sources. Instead, they are likely intrinsic to the satellites, with distinct differences in emission properties observed between satellite generations. These findings highlight significant challenges to data processing and scientific discoveries at these low frequencies, emphasizing the need for effective mitigation strategies, particularly through collaboration between astronomers and satellite operators.

Radio-frequency interference (RFI) is a major systematic limitation in radio astronomy, particularly for science cases requiring high sensitivity, such as 21 cm cosmology. Traditionally, RFI is dealt with by identifying its signature in the dynamic spectra of visibility data and flagging strongly affected regions. However, for RFI sources that do not occupy narrow regions in the time-frequency space, such as persistent local RFI, modeling these sources could be essential to mitigating their impact. This paper introduces two methods for detecting and characterizing local RFI sources from radio interferometric visibilities: matched filtering and maximum a posteriori (MAP) imaging. These algorithms use the spherical wave equation to construct three-dimensional near-field image cubes of RFI intensity from the visibilities. The matched filter algorithm can generate normalized maps by cross-correlating the expected contributions from RFI sources with the observed visibilities, while the MAP method performs a regularized inversion of the visibility equation in the near field to construct image cubes in physical units as a function of frequency. We developed a full polarization simulation framework for RFI and demonstrated the methods on simulated observations of local RFI sources. The stability, speed, and errors introduced by these algorithms were investigated, and, as a demonstration, the algorithms were applied to a subset of NenuFAR observations to perform spatial, spectral, and temporal characterization of two local RFI sources. We used simulations to assess the impact of local RFI on images, the u plane, and cylindrical power spectra, and to quantify the level of bias introduced by the algorithms in order to understand their implications for the estimated 21 cm power spectrum with radio interferometers. The near-field imaging and simulation codes are publicly available in the Python library nfis

Radio-frequency interference (RFI) is a major systematic limitation in radio astronomy, particularly for science cases requiring high sensitivity, such as 21-cm cosmology. Traditionally, RFI is dealt with by identifying its signature in the dynamic spectra of visibility data and flagging strongly affected regions. However, for RFI sources that do not occupy narrow regions in the time-frequency space, such as persistent local RFI, modeling these sources could be essential to mitigating their impact. This paper introduces two methods for detecting and characterizing local RFI sources from radio interferometric visibilities: matched filtering and maximum a posteriori (MAP) imaging. These algorithms use the spherical wave equation to construct three-dimensional near-field image cubes of RFI intensity from the visibilities. The matched filter algorithm can generate normalized maps by cross-correlating the expected contributions from RFI sources with the observed visibilities, while the MAP method performs a regularized inversion of the visibility equation in the near field. We also develop a full polarization simulation framework for RFI and demonstrate the methods on simulated observations of local RFI sources. The stability, speed, and errors introduced by these algorithms are investigated, and, as a demonstration, the algorithms are applied to a subset of NenuFAR observations to perform spatial, spectral, and temporal characterization of two local RFI sources. We assess the impact of local RFI on images, the uv plane, and cylindrical power spectra through simulations and describe these effects qualitatively. We also quantify the level of errors and biases that these algorithms induce and assess their implications for the estimated 21-cm power spectrum with radio interferometers. The near-field imaging and simulation codes are made available publicly in the Python library nfis.

Ultralight axionlike particles (ALPs) can be a viable solution to the dark matter problem. The scalar field associated with ALPs, coupled to the electromagnetic field acts as an active birefringent medium, altering the polarization properties of light through which it propagates. In particular, oscillations of the axionic field induce monochromatic variations of the plane of linearly polarized radiation of astrophysical signals. The radio emission of millisecond pulsars provides an excellent tool to search for such manifestations, given their high fractional linear polarization and negligible fluctuations of their polarization properties. We have searched for evidence of ALPs in polarimetry measurements of pulsars collected and preprocessed for the European Pulsar Timing Array (EPTA) campaign. Focusing on the twelve brightest sources in linear polarization, we searched for an astrophysical signal from axions using both frequentist and Bayesian statistical frameworks. For the frequentist analysis, which uses Lomb-Scargle periodograms at its core, no statistically significant signal has been found. The model used for the Bayesian analysis has been adjusted to accommodate multiple deterministic systematics that may be present in the data. A statistically significant signal has been found in the dataset of multiple pulsars with common frequencies between 10 − 8 and 2 × 10 − 8 Hz , which can most likely be explained by the residual Faraday rotation in the terrestrial ionosphere. Strong bounds on the coupling constant g a γ , in the same ballpark as other searches, have been obtained in the mass range between 6 × 10 − 24 and 5 × 10 − 21 eV . We conclude by discussing the problems that can limit the sensitivity of our search for ultralight axions in the polarimetry data of pulsars, and possible ways to resolve them. Published by the American Physical Society 2025

Context. Radio pulsars exhibit a plethora of complex phenomena at the single-pulse level. However, the intricacies of their radio emission remain poorly understood. Aims. We aim to elucidate the pulsar radio emission by studying several single-pulse phenomena, how they relate, and how they evolve with observing frequency. We intend to inspire models for the pulsar radio emission and fast radio bursts. Methods. We set up an observing programme called the SUSPECT project running at the Nançay Radio Observatory telescopes in France (10–85 MHz, 110–240 MHz, and 1.1–3.5 GHz) and the upgraded Giant Metrewave Radio Telescope (uGMRT) in India. This first paper focuses on high sensitivity data of PSR B1822−09 obtained with the uGMRT between 550 and 750 MHz. The pulsar has precursor (PC), main pulse (MP), and interpulse (IP) emission and exhibits mode switching. We present its single-pulse stacks, investigate its mode switching using a hidden Markov switching model, and analyse its single-pulse morphology. Results. PSR B1822−09’s pulse profile decomposes into seven components. We show that its mode switching is well described using a hidden Markov switching model operating on single-pulse profile features. The pulsar exhibits at least three stable emission modes, one of which is a newly discovered bright flaring Bf-mode. We confirm that the PC and MP switch synchronously to each other and both asynchronously to the IP, indicating information transfer between the polar caps. Additionally, we performed a fluctuation spectral analysis and discovered three fluctuation features in its quiescent Q-mode emission, one of which is well known. We conclude that the latter feature is due to longitude-stationary amplitude modulation. Finally, we visually classified the single pulses into four categories. We found extensive microstructure in the PC with a typical duration of 0.2–0.4 ms and a quasi-periodicity of 0.8 ms. There is clear evidence of mode mixing. We discovered low-intensity square-like pulses and extremely bright pulses in the MP, which suggest bursting. Conclusions. PSR B1822−09’s PC resembles magnetar radio emission, while its MP and IP are canonical radio pulsar-like. Hence, the pulsar combines both attributes, which is rare. This work introduces several new data analysis techniques to pulsar astrophysics.

Astrometry of pulsars, particularly their distances, serves as a critical input for various astrophysical experiments using pulsars. Pulsar timing is a primary approach for determining a pulsar’s position, parallax, and distance. In this paper, we explore the influence of the solar wind on astrometric measurements obtained through pulsar timing, focusing on its potential to affect the accuracy of these parameters. Using both theoretical calculation and mock-data simulations, we demonstrate a significant correlation between the pulsar position, annual parallax and the solar-wind density parameters. This correlation strongly depends on the pulsar’s ecliptic latitude. We show that fixing solar-wind density to an arbitrary value in the timing analysis can introduce significant bias in the estimated pulsar position and parallax, and its significance is highly dependent on the ecliptic latitude of the pulsar and the timing precision of the data. For pulsars with favourable ecliptic latitude and timing precision, the astrometric and solar-wind parameters can be measured jointly with other timing parameters using single-frequency data. The parameter correlation can be mitigated by using multi-frequency data, which also significantly improves the measurement precision of these parameters; this is particularly important for pulsars at a medium or high ecliptic latitude. Additionally, for a selection of pulsars we reprocess their EPTA Data Release 2 data to include modelling of solar-wind effect in the timing analysis. This delivers significant measurements of both parallax and solar-wind density, the latter of which are consistent with those obtained at low-frequency band. In the future, combining pulsar timing data at gigahertz and lower frequencies will probably deliver the most robust and precise measurements of astrometry and solar wind properties in pulsar timing.

Astrometry of pulsars, particularly their distances, serves as a critical input for various astrophysical experiments using pulsars. Pulsar timing is a primary approach for determining a pulsar's position, parallax, and distance. In this paper, we explore the influence of the solar wind on astrometric measurements obtained through pulsar timing, focusing on its potential to affect the accuracy of these parameters. Using both theoretical calculation and mock-data simulations, we demonstrate a significant correlation between the pulsar position, annual parallax and the solar-wind density parameters. This correlation strongly depends on the pulsar's ecliptic latitude. We show that fixing solar-wind density to an arbitrary value in the timing analysis can introduce significant bias in the estimated pulsar position and parallax, and its significance is highly dependent on the ecliptic latitude of the pulsar and the timing precision of the data. For pulsars with favourable ecliptic latitude and timing precision, the astrometric and solar-wind parameters can be measured jointly with other timing parameters using single-frequency data. The parameter correlation can be mitigated by using multi-frequency data, which also significantly improves the measurement precision of these parameters; this is particularly important for pulsars at a medium or high ecliptic latitude. Additionally, for a selection of pulsars we reprocess their EPTA Data Release 2 data to include modelling of solar-wind effect in the timing analysis. This delivers significant measurements of both parallax and solar-wind density, the latter of which are consistent with those obtained at low-frequency band. In the future, combining pulsar timing data at gigahertz and lower frequencies will probably deliver the most robust and precise measurements of astrometry and solar wind properties in pulsar timing.

Ultra-light axion-like particles (ALPs) can be a viable solution to the dark matter problem. The scalar field associated with ALPs, coupled to the electromagnetic field, acts as an active birefringent medium, altering the polarisation properties of light through which it propagates. In particular, oscillations of the axionic field induce monochromatic variations of the plane of linearly polarised radiation of astrophysical signals. The radio emission of millisecond pulsars provides an excellent tool to search for such manifestations, given their high fractional linear polarisation and negligible fluctuations of their polarisation properties. We have searched for the evidence of ALPs in the polarimetry measurements of pulsars collected and preprocessed for the European Pulsar Timing Array (EPTA) campaign. Focusing on the twelve brightest sources in linear polarisation, we searched for an astrophysical signal from axions using both frequentist and Bayesian statistical frameworks. For the frequentist analysis, which uses Lomb-Scargle periodograms at its core, no statistically significant signal has been found. The model used for the Bayesian analysis has been adjusted to accommodate multiple deterministic systematics that may be present in the data. A statistically significant signal has been found in the dataset of multiple pulsars with common frequency between $10^{-8}$ Hz and $2\times10^{-8}$ Hz, which can most likely be explained by the residual Faraday rotation in the terrestrial ionosphere. Strong bounds on the coupling constant $g_{a\gamma}$, in the same ballpark as other searches, have been obtained in the mass range between $6\times10^{-24}$ eV and $5\times10^{-21}$ eV. We conclude by discussing problems that can limit the sensitivity of our search for ultra-light axions in the polarimetry data of pulsars, and possible ways to resolve them.

The NenuFAR Pulsar Blind Survey (NPBS) is an all-sky survey, searching for pulsars at radio frequencies below 85 MHz with the NenuFAR radio telescope. Taking into account the turnover at low frequencies in the pulsar spectra and the widening of their emission cone towards low frequencies we expect approximately 8 -- 20 not already discovered pulsars to be detectable by this survey, most of which are likely to be non-standard pulsars or pulsars in unusual parts of the $P- P$ diagram (such as, e.g. slow pulsars). According to our simulations, we expect the discovered pulsars to feature spectra with spectral indices $and low turnover frequencies$ 85$ MHz. Conversely, a non-detection would give valuable clues as to the population of pulsars in this region of the parameter space. The current first stage of the survey observes declinations above 39 in the frequency range 39 -- 76 MHz. A frequency-averaged sky coverage of 98 is reached by observing 7\,692 pointings of about 1.5 of radius in 27 min each. The observing programme started in August 2020, and is expected to be completed during 2024. Approximately a third of the data are currently being processed using a search pipeline based on PRESTO with some adaptations to low frequencies. Because of the high scatter broadening and the coarse time resolution, the NPBS searches for pulsars with periods from 30 ms to 30 s and dispersion measures (DMs) between 1 and 70 In the processed data, 24 known pulsars have been searched in order to verify the observing setup and the search pipeline. Seven of these pulsars have been detected, with DMs between 5 and 42 The related candidates have periods between 40 ms to 3.5 s, including candidates corresponding to harmonics. Of the seven, six correspond to the most intense pulsars of the set. The last detection is presumably due to a beneficial effect of the scintillation. Based on the faintest detection, the expected minimum signal-to-noise ratio for detecting a pulsar is 4.8, corresponding to a minimum flux of 6.9 mJy in the coldest regions of the sky.