arXiv subjects

C. A. Braga

Publications and source records attributed to C. A. Braga.

3 recordsLinked to original sources

arXiv2026

Chromatic activity window of periodic fast radio bursts: FRB 20121102A and FRB 20180916B

Two fast radio bursts, FRB 20121102A and FRB 20180916B, show periodic activity with cycles of 159.3 and 16.33 days, respectively. These cycles consist of active and inactive windows, with the peak activity centred within the active phase. For FRB 20180916B, studies reported a frequency-dependent or chromatic behaviour, for which the activity window starts earlier and becomes narrower at higher frequencies. The activity across frequencies is typically modelled with a power law. We developed a simple model that combines the phase and frequency dependence of the activity windows of FRB 20121102A and FRB 20180916B. Our goal was to perform a chromaticity study for FRB 20121102A that incorporates model improvements to account for the cyclic nature of its activity window and to compare the chromatic behaviour of the two periodic FRBs. We standardised the detections from the 425 observing epochs for FRB 20121102A and the 214 epochs for FRB20180916B to account for differences in the radio telescope sensitivity. To the normalised detection-rate phase distribution, we fitted a von Mises distribution and extracted the peak activity phase and activity width. These quantities as a function of frequency were then modelled as power laws to construct the chromatic model. The activity window starts earlier at higher frequencies for the two sources. The activity window of FRB 20121102A broadens at higher frequencies, however, and that of FRB 20180916B broadens at lower frequencies. Interestingly, it appears to remain active during at least 80% of the cycle at C band. The observed chromatic behaviour of FRB 20180916B is consistent with previous findings. For FRB 20121102A, a chromaticity in its activity window is also seen, but the source appears to be active for longer at higher frequencies, which is different from the behaviour of FRB 20180916B.

astro-ph.HE↗

arXiv2025

Observing radio transients with Phased ALMA: Pulses from the Galactic Centre magnetar

Radio transients, such as pulsars and Fast Radio Bursts (FRBs), are primarily detected at centimetre radio wavelengths, where higher luminosities are found. However, observations of sources in dense environments are heavily affected by propagation effects which may hinder a detection. Millimetre wave observations bypass this complication but require the largest radio telescopes to compensate for the lower flux densities. When used in phased mode, the ALMA radio telescope provides an equivalent dish size of 84m, being the most sensitive instrument at mm/sub mm. With its high time resolution it offers a unique opportunity to study radio transients in an unexplored window. We study the Galactic Centre (GC) magnetar, PSR J1745$-$2900, as a laboratory for magnetars in complex magneto-turbulent environments and to link with FRBs. We showcase the potential of ALMA in phased mode to observe radio transients and to achieve, for some sources, the first ever detections outside the cm wave range. We studied the GC magnetar using ALMA archival data of Sgr A* at Band 3 from the 2017 GMVA campaign. We searched in intensity and classified the pulses based on their circular and linear polarisation properties and arrival phase. We detected eight pulses with energies in the range of 10$^{29}$ erg. We constructed its cumulative energy distribution and we fit a power law, where the event rate scales with energy as $R \propto E^γ$. The result is an exponent of $γ= -2.4 \pm 0.1$. With the $γ-$value and the system properties of phased ALMA, we estimate that over 160 known pulsars could be detected by ALMA. For repeating FRBs, observing during their peak activity window could lead to several detections. We expect that ALMA's lower frequency bands with polarisation capabilities, will serve as a pioneer on mm wave searches for pulsars and to study complex environments involving radio transients.

astro-ph.HE↗

arXiv2025

FRB 20121102A monitoring: Updated periodicity in the L band

FRB 20121102A was the first fast radio burst to be observed to repeat. Since then, thousands of bursts have been detected by multiple radio telescopes around the world. Previous work has shown an indication of a cyclic activity level with a periodicity of around 160 days. Knowing when the source repeats is essential for planning multi-wavelength monitoring to constrain the emission extent and progenitor source. We report the monitoring of FRB 20121102A using the 100-m Effelsberg radio telescope in the L-band and update the periodicity of the cyclic activity level. We used the Lomb-Scargle periodogram on a sample of 284 observing epochs, of which 42% correspond to detections and 58% to non-detections. Our dataset is composed of the seven epochs of our monitoring plus publicly available data. We investigated two methods: i) a binary model, describing the observing epochs with 1 if there are detections and with 0 for non-detections, and ii) a normalised rates model that considers the inferred detections rates. We report no detections in 12.5-hour observations down to a fluence of 0.29 Jy ms. The best period we find for the cyclic activity window is $159.3 \pm 0.8$ days for the binary model and $159.3 \pm 0.3$ days for the normalised rates model. We show the activity phase to be 53%. The normalised rates show clear Gaussian-like behaviour for the activity level, in that the number of detections peak at the centre of the activity window. The periodicity found through both methods is consistent for the L and S-band datasets implying it is intrinsic to the source. The activity phase in the S band however, shows an indication of it ending before the L-band activity phase, supporting the idea of a chromatic dependence of the activity window. The sample in the C-band is not large enough to further confirm this result.

astro-ph.HE↗