First detection of frequency-dependent, time-variable dispersion measures

Donner, J. Y.; Verbiest, J. P. W.; Tiburzi, C.; Osłowski, S.; Michilli, D.; Serylak, M.; Anderson, J. M.; Horneffer, A.; Krämer, M.; Grießmeier, Jean-Mathias; Künsemöller, Jörn; Hessels, J. W. T.; Hoeft, M.; Miskolczi, A.

doi:10.1051/0004-6361/201834059

Cited by 54 publications

(47 citation statements)

References 61 publications

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“…As can be seen in Table 2, this is in the range (2 − 6) × 10 −6 pc cm −3 , depending on the quality of detection. This is already an order of magnitude better than what is currently achievable at timing frequencies (3×10 −5 pc cm −3 ; Jones et al 2017), and even the most recent work of Donner et al (2019), who reach a similar level of precision. Since our analysis relied on the use of a single template (and not frequency-dependent templates that are ideal for wideband timing), there is indeed scope for considerable improvement as we accrue more data with the MWA.…”

Section: Dm Measurements and Precisionmentioning

confidence: 73%

A High Time-resolution Study of the Millisecond Pulsar J2241−5236 at Frequencies Below 300 MHz

Kaur

Bhat

Tremblay

et al. 2019

ApJ

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One of the major challenges for pulsar timing array (PTA) experiments is the mitigation of the effects of the turbulent interstellar medium (ISM) from timing data. These can potentially lead to measurable delays and/or distortions in the pulse profiles and scale strongly with the inverse of the radio frequency. Low-frequency observations are therefore highly appealing for characterizing them. However, in order to achieve the necessary time resolution to resolve profile features of short-period millisecond pulsars, phase-coherent de-dispersion is essential, especially at frequencies below 300 MHz. We present the lowest-frequency (80-220 MHz), coherently de-dispersed detections of one of the most promising pulsars for current and future PTAs, PSR J2241−5236, using our new beam-former software for the MWA's voltage capture system (VCS), which reconstructs the time series at a much higher time resolution of ∼ 1 µs by re-synthesizing the recorded voltage data at 10-kHz/100-µs native resolutions. Our data reveal a dual-precursor type feature in the pulse profile that is either faint or absent in highfrequency observations from Parkes. The resultant high-fidelity detections have enabled dispersion measure (DM) determinations with very high precision, of the order of (2-6) × 10 −6 pc cm −3 , owing to the microsecond level timing achievable for this pulsar at the MWA's low frequencies. This underscores the usefulness of low-frequency observations for probing the ISM toward PTA pulsars and informing optimal observing strategies for PTA experiments.

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Section: Dm Measurements and Precisionmentioning

confidence: 73%

A High Time-resolution Study of the Millisecond Pulsar J2241−5236 at Frequencies Below 300 MHz

Kaur

Bhat

Tremblay

et al. 2019

ApJ

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“…While a single LOFAR station such as FR606 only has a limited effective area, it allows us to take advantage of very flexible scheduling, especially for long observations or high cadence monitoring. The capability of this setup for pulsar science has already been demonstrated (Rajwade et al 2016;Mereghetti et al 2016Mereghetti et al , 2018Grießmeier et al 2018;Bondonneau et al 2018;Tiburzi et al 2019;Michilli et al 2018a,b;Hermsen et al 2018;Donner et al 2019).…”

Section: Observationsmentioning

confidence: 93%

“…In addition, the relative amplitudes of the components seem to be frequency-dependent. Pulsar B2217+47 is highly affected by time-variable scattering (Michilli et al 2018a;Donner et al 2019). This effect, convolved with the profile, produces a frequency dependent exponential tail which is clearly visible in Figure 4.…”

Section: Detected Pulsarsmentioning

confidence: 99%

A census of the pulsar population observed with the international LOFAR station FR606 at low frequencies (25–80 MHz)

Bondonneau

Grießmeier

Theureau

et al. 2020

A&A

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Context. To date, only 69 pulsars have been identified with a detected pulsed radio emission below 100 MHz. A LOFARcore LBA census and a dedicated campaign with the Nançay LOFAR station in stand-alone mode were carried out in the years 2014−2017 in order to extend the known population in this frequency range. Aims. In this paper, we aim to extend the sample of known radio pulsars at low frequencies and to produce a catalogue in the frequency range of 25-80 MHz. This will allow future studies to probe the local Galactic pulsar population, in addition to helping explain their emission mechanism, better characterising the low-frequency turnover in their spectra, and obtaining new information about the interstellar medium through the study of dispersion, scattering, and scintillation. Methods. We observed 102 pulsars that are known to emit radio pulses below 200 MHz and with declination above −30 • . We used the the Low Band Antennas (LBA) of the LOw Frequency ARray (LOFAR) international station FR606 at the Nançay Radio Observatory in stand-alone mode, recording data between 25-80 MHz. Results. Out of our sample of 102 pulsars, we detected 64. We confirmed the existence of ten pulsars detected below 100 MHz by the LOFAR LBA census for the first time (Bilous et al. 2019) and we added two more pulsars that had never before been detected in this frequency range. We provided average pulse profiles, DM values, and mean flux densities (or upper limits in the case of non-detections). The comparison with previously published results allows us to identify a hitherto unknown spectral turnover for five pulsars, confirming the expectation that spectral turnovers are a widespread phenomenon.

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“…Another approach is to use low-frequency observations to measure the DM very precisely and use these measurements to correct the higher-frequency ToAs, which are often more sensitive and less affected by IISM effects. One potential complication of this approach is the possibility of frequency-dependent DMs (Cordes et al 2016;Donner et al 2019), which are caused by the fact that due to interstellar scattering, low-frequency observations effectively sample a larger volume of the IISM and are sensitive to IISM structures on a larger scale than at high frequencies. Therefore, dispersive delays measured at low frequency may not be representative for those experienced at high frequencies.…”

Section: Introductionmentioning

confidence: 99%

Dispersion measure variability for 36 millisecond pulsars at 150 MHz with LOFAR

Donner

Verbiest

Tiburzi

et al. 2020

A&A

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Context. Radio pulses from pulsars are affected by plasma dispersion, which results in a frequency-dependent propagation delay. Variations in the magnitude of this effect lead to an additional source of red noise in pulsar timing experiments, including pulsar timing arrays (PTAs) that aim to detect nanohertz gravitational waves. Aims. We aim to quantify the time-variable dispersion with much improved precision and characterise the spectrum of these variations. Methods. We use the pulsar timing technique to obtain highly precise dispersion measure (DM) time series. Our dataset consists of observations of 36 millisecond pulsars (MSPs), which were observed for up to 7.1 years with the LOw Frequency ARray (LOFAR) telescope at a centre frequency of ∼ 150 MHz. Seventeen of these sources were observed with a weekly cadence, while the rest were observed at monthly cadence. Results. We achieve a median DM precision of the order of 10 −5 cm −3 pc for a significant fraction of our sources. We detect significant variations of the DM in all pulsars with a median DM uncertainty of less than 2×10 −4 cm −3 pc. The noise contribution to pulsar timing experiments at higher frequencies is calculated to be at a level of 0.1-10 µs at 1.4 GHz over a timespan of a few years, which is in many cases larger than the typical timing precision of 1 µs or better that PTAs aim for. We found no evidence for a dependence of DM on radio frequency for any of the sources in our sample. Conclusions. The DM time series we obtained using LOFAR could in principle be used to correct higher-frequency data for the variations of the dispersive delay. However, there is currently the practical restriction that pulsars tend to provide either highly precise times of arrival (ToAs) at 1.4 GHz or a high DM precision at low frequencies, but not both, due to spectral properties. Combining the higher-frequency ToAs with those from LOFAR to measure the infinite-frequency ToA and DM would improve the result.

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First detection of frequency-dependent, time-variable dispersion measures

Cited by 54 publications

References 61 publications

A High Time-resolution Study of the Millisecond Pulsar J2241−5236 at Frequencies Below 300 MHz

A High Time-resolution Study of the Millisecond Pulsar J2241−5236 at Frequencies Below 300 MHz

A census of the pulsar population observed with the international LOFAR station FR606 at low frequencies (25–80 MHz)

Dispersion measure variability for 36 millisecond pulsars at 150 MHz with LOFAR

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