Swift Observations of the Repeating FRB 20240209A

ATel #17380; F. Verrecchia, M. Perri (INAF/OAR & ASI/SSDC), M. Tavani (INAF/IAPS & Univ. Roma Tor Vergata), M. Pilia (INAF/OA-Cagliari), C. Casentini (INAF/IAPS), D. Pelliciari (Univ. of Bologna, INAF/IRA), C. Pittori (INAF/OAR & ASI/SSDC), G. Bernardi, G. Bianchi, G. Naldi, G. Pupillo (INAF/IRA), A. Geminardi, P. Esposito (IUSS), on behalf of a large Collaboration.
on 9 Sep 2025; 10:09 UT
Credential Certification: Francesco Verrecchia (francesco.verrecchia@ssdc.asi.it)

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We report of the observations campaign with Swift XRT and UVOT instruments of the repeating Fast Radio Burst source FRB 20240209A (J2000 R.A., Dec.= 289.8875, 86.0644 deg) during a time interval included in the expected activity phase, centered on July 17, based on a four months periodicity evaluated using data of previous activity phases reported by Pal et al. 2025 (ApJ, 983, L15).

No radio burst has been reported, except five bursts by CHIME/FRB published on the public online service (https://www.chime-frb.ca/repeaters/FRB20240209A).

Our Swift X-ray observations are part of a coordinated multiwavelength campaign of this FRB including optical and X-ray observations and simultaneous radio observations by the italian Northern Cross radio telescope in Medicina, the 32 m dish in Medicina and the Sardinia Radio Telescope.

We observed the location of FRB 20240A209A daily from July 27th to August 2nd, 2025 in the following time intervals:

July 27th, within 12:38 - 20:43 UT, for 1641 s of effective exposure;
July 28th, within 19:44 - 21:36 UT, for 1652 s of effective exposure;
July 29th, within 19:03 - 20:47 UT, for 956 s of effective exposure;
July 30th, within 16:52 - 21:47 UT, for 1260 s of effective exposure;
July 31th, within 02:14 - 16:26 UT, for 1580 s of effective exposure;
Aug 1th, within 18:35 - 20:23 UT, for 1463 s of effective exposure;
Aug 2nd, within 16:25 - 22:48 UT, for 1343 s of effective exposure;

We obtained XRT data in Windowed Timing (WT) mode: a preliminary analysis indicates that no uncatalogued X-ray source compatible with the FRB position was detected in all observations. For the Galactic HI column density value in the source direction (n_H = 5.59 x 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775) and assuming a single power-law spectrum of photon index 2, we obtain the following 3-sigma flux upper limits (ULs) in the range 0.3-10 keV:

3.6 x 10^-13 erg/cm²/s for the first observation on July 27th;
2.7 x 10^-13 erg/cm²/s for the observations on July 28th;
5.0 x 10^-13 erg/cm²/s for the observations on July 29th;
5.4 x 10^-13 erg/cm²/s for the observations on July 30th;
7.1 x 10^-13 erg/cm²/s for the observations on July 31th;
4.2 x 10^-13 erg/cm²/s for the observations on August 1st;
3.7 x 10^-13 erg/cm²/s for the observations on August 2nd;

UVOT data where acquired in imaging and event mode, with "filter of the day" and exposures similar to the XRT ones. No detection on the full exposure images were obtained, and we extracted flux ULs, dereddened according to mean galactic extinction law (Fitzpatrick , E.L. 1999, PASP, 111, 63) and the E(B-V) value for the source position (0.0100; Schlafly, E.F., & Finkbeiner, D.P. 2011, ApJ, 737, 103). The following flux ULs all for U filter were estimated:

1.2 x 10^-13 erg/cm^2/s for July 27th;
8.8 x 10^-14 erg/cm^2/s for July 28th;
1.4 x 10^-13 erg/cm^2/s for July 29th;
1.2 x 10^-13 erg/cm^2/s for July 30th;
1.2 x 10^-13 erg/cm^2/s for July 31st;
1.6 x 10^-13 erg/cm^2/s for August 1st;
1.7 x 10^-13 erg/cm^2/s for August 2nd;

Moreover, we checked UVOT event data but no short timescale detection in a region compatible with the FRB position was found. Further observations with Swift will follow.

We encourage multifrequency observations of FRB 202400209A. The Swift mission is part of NASA's medium explorer (MIDEX) program, led by NASA with participation of Italy and the UK. We thank the Swift Team for making these observations possible, in particular B. Cenko and K.L. Page (Swift Observatory Duty Scientist).