J/A+A/694/A75 uGMRT FRB 20180916B rotation measures (Bethapudi+, 2025)
Rotation measure study of FRB 20180916B with the uGMRT.
Bethapudi S., Spitler L.G., Li D.Z., Marthi V.R., Bause M., Main R.A.,
Wharton R.S.
<Astron. Astrophys. 694, A75 (2025)>
=2025A&A...694A..75B 2025A&A...694A..75B (SIMBAD/NED BibCode)
ADC_Keywords: Radio sources ; Polarization
Keywords: instrumentation: interferometers - instrumentation: polarimeters -
methods: data analysis - methods: observational -
techniques: polarimetric
Abstract:
Fast Radio Burst 20180916B is a repeating FRB whose activity window
has a 16.34 day periodicity that also shifts and varies in duration
with the observing frequency. Recent observations report that the FRB
has started to show an increasing trend in secular Rotation Measure
(RM) after only showing stochastic variability around a constant value
of -114.6rad/m2 since its discovery. RM studies let us directly
probe the magnetic field structure in the local environment of the
FRB. The trend of the variability can be used to constrain progenitor
models of the FRB. Hence, further study of the RM variability forms
the basis of this work.
We studied the local environment of FRB 20180916B. We did so by
focusing on polarization properties, namely RM, and studied how it
varies with time. The data comes from the ongoing campaigns of FRB
20180916B using the upgraded Giant Metrewave Radio Telescope (uGMRT).
The majority of the observations are in Band 4, which is centered at
650MHz with 200MHz bandwidth. Additionally, we used a few
observations where we had simultaneous coverage in Band 4 and Band 5
(centered at 1100MHz).
We apply a standard single pulse search pipeline to search for bursts.
In total, we detect 116 bursts with ∼36 hours of on-source time
spanning 1200 days, with two bursts detected during simultaneous
frequency coverage observations. We develop and apply a polarization
calibration strategy suited for our dataset. On the calibrated bursts,
we use QU-fitting to measure RM. Lastly, we also measure various other
properties such as rate, linear polarization fraction and fluence
distribution.
Of the 116 detected bursts, we could calibrate 79 of them. From which,
we observed in our early observations the RM continued to follow
linear trend as modeled by arxiv:2205.09221. However, our later
observations suggest the source switch from the linear trend to
stochastic variations around a constant value of -58.75rad/m2.
We also study cumulative rate against fluence and note that rate at
higher fluences (>1.2Jy.ms) scales as γ=-1.09(7) whereas that at
lower fluences (between 0.2 and 1.2Jy.ms) only scales as
γ=-0.51(1), meaning rate at higher fluence regime is steeper
than at lower fluence regime is steeper than at lower fluence regime.
Lastly, we qualitatively assess the two extremely large bandwidth
bursts that we detected in our simultaneous multi band observations.
Future measurements of RM variations would help place stronger
constraints on the local environment. Moreover, any periodic behavior
in the RM measurements would directly test progenitor models.
Therefore, we motivate such endeavors.
Description:
Observations were performed using uGMRT. It is a radio interferometer
situated in Khodad, near Pune, India. It consists of thirty 45-meter
parabolic dishes which can be used to perform imaging- and phased
array-mode observations. It offers observation capability from 100MHz
to 1.4GHz broken into five bands. Our observations used Band 4 (550
to 750MHz) and Band 5 (1000 to 1200MHz). Observations from GMRT
Proposal Cycle-39 (from October-2020 to March- 2021), Cycle-43 (from
October-2022 to April-2023) and Cycle-45 (from October-2023 to
April-2024), aswell as one observation from Cycle-42 (from April-2022
to October-2022), are presented in this paper.
Each individual scan is listed in Table 1 for Band 4.
Table 3 list the properties of the Band 4 bursts detected in this
work.
Objects:
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RA (2000) DE Designation(s)
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01 58 00.75 +65 43 00.3 FRB 20180916B = AT 2020hur
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File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 35 75 Band 4 observations described in this paper
table3.dat 436 100 Properties of the Band 4 bursts detected
in this work
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 23 A23 "datime" Date Start time of the scan in UTC
25- 32 F8.3 s Tobs Scan duration
34- 35 I2 --- Nbursts Number of bursts detected
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 20 F20.16 ms Width Burst width
22- 40 F19.15 MHz Bandwidth Burst bandwidth
42- 59 F18.16 Jy.ms Fluence ? Burst fluence
61- 79 F19.16 mJy Fpeak ? Peak flux
81-138 A58 --- BurstFile Burst filename
140-157 F18.12 d MJD Burst MJD in UTC (1)
159-193 A35 --- Tag Identifier tag
195-319 A125 --- Filepath Filepath
321-337 F17.13 MHz Fref ? Burst center frequency
339-358 F20.15 rad/m2 RM ? Rotation measure
360-377 F18.16 rad/m2 e_RM ? Error in RM
379-396 F18.16 --- Linear ? Linear polarization fraction
398-415 F18.16 --- e_Linear ? Error in Linear
417-436 F20.17 --- RMion Ionospheric RM contribution
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Note (1): MJD refers to topocentric UTC time of the burst when de-dispersed at
48.820pc/cm3 with 750MHz as the reference frequency.
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Acknowledgements:
Suryarao Bethapudi, shining.surya.d8(at)gmail.com
(End) Patricia Vannier [CDS] 18-Dec-2024