CHIME/FRB Outrigger Localization of the Prolific Repeating FRB 20240209A
ATel #16682; Adam E. Lanman (MIT), Vishwangi Shah (McGill), on behalf of the CHIME/FRB Collaboration
on 1 Jul 2024; 14:32 UT
Credential Certification: Adam Lanman (alanman@mit.edu)
Subjects: Radio, Fast Radio Burst
The CHIME/FRB Collaboration reports an improved position for the new, highly-active repeater FRB 20240209A, achieved through VLBI with the first of three CHIME/FRB Outrigger telescopes, KKO. The 66-km baseline formed by CHIME and KKO is capable of ~ arcsecond-scale precision along the baseline axis, which mostly constrains the RA of the source (Lanman et al., 2024). The localizations found through this analysis further support the association of this repeater with the galaxy PSO J289.8503+86.0609, as initially reported in ATEL #16670.
We have localized two bursts from FRB 20240209A, which have TNS names FRB 20240209A and FRB 20240309A (hereafter, bursts 1 and 2, respectively). Our 1 sigma likelihood contours are reported as ellipses, with semimajor and semiminor axes "a_error" and "b_error", respectively, and a position angle "theta".
FRB 20240209A (burst 1):
- RA (J2000) = 19h19m33s
- Dec (J2000) = 86°3'5"
- a_error = 33"
- b_error = 4.0"
- theta = -9.4729°
FRB 20240309A (burst 2):
- RA (J2000) = 19h19m34s
- Dec (J2000) = 86°4'5"
- a_error = 26"
- b_error = 4.0"
- theta = 2.1723°
We form station beams using the baseband data from CHIME and KKO toward the best-fit position from the baseband analysis ("baseband position"), as well as to multiple calibrator positions that were in-beam at the time. We then cross-correlated each pointing using our software correlator PyFX (Leung et al. 2024) to form visibilities, and phase-referenced the FRB visibilities to J0117+8928, a bright VLBA calibrator about 3.9° away from the baseband position. Since the bursts are inherently narrow-band, we mask out off-burst parts of the band after cross-correlating. We mask out frequencies from 500 to 800 MHz for burst 1 and from 400-630 and 690-800 MHz for burst 2. This masking increased the cross-correlation S/N to ~ 25 in the XX polarization and 28 in YY for burst 1, and 22 and 13 for burst 2.
To solve for the position, we computed geometric delays for a grid of positions throughout the baseband localization region using the CALC11 delay model, as implemented in pycalc11. Combining the baseband localization region (from ATEL #16670) with the VLBI constraints yields the localization contours shown in the attached figures. The red circle marks the location and apparent width of the likely host galaxy.
We expect ionospheric effects to be negligible on the 66-km CHIME — KKO baseline, especially for target-calibrator separations as low as 4°. Therefore, we perform a delay-mapping analysis to accurately measure the true position. We inflate b_error to 4" to account for the small signal bandwidth (see Fig 17 of Lanman et al., 2024). Full details of the VLBI analysis pipeline may be found in Leung et al. (2024) and its application to the CHIME — KKO baseline is demonstrated in Lanman et al. (2024).
Figures:
