Constraining the transient high-energy activity of FRB 180916.J0158+65 with Insight–HXMT follow-up observations

Abstract

Context. A link has finally been established between magnetars and fast radio burst (FRB) sources. Within this context, a major issue that remains unresolved pertains to whether sources of extragalactic FRBs exhibit X/γ-ray outbursts and whether this is correlated with radio activity. If so, the subsequent goal is to identify these sources. Aims. We aim to constrain possible X/γ-ray burst activity from one of the nearest extragalactic FRB sources currently known. This is to be done over a broad energy range by looking for bursts over a range of timescales and energies that are compatible with those of powerful flares from extragalactic magnetars. Methods. We followed up on the observation of the as-yet nearest extragalactic FRB source, located at a mere 149 Mpc distance, namely, the periodic repeater FRB 180916.J0158+65. This took place during the active phase between 4 and 7 February 2020, using the Insight–Hard X-ray Modulation Telescope (Insight–HXMT). By taking advantage of the combination of broad-band wavelengths, a large effective area, and several independent detectors at our disposal, we searched for bursts over a set of timescales from 1 ms to 1.024 s with a sensitive algorithm that had been previously characterised and optimised. Moreover, through simulations, we studied the sensitivity of our technique in the released energy-duration phase space for a set of synthetic flares and assuming a range of different energy spectra. Results. We constrain the possible occurrence of flares in the 1−100 keV energy band to E < 1046 erg for durations Δ t < 0.1 s over several tens of ks exposure. Conclusions. We can rule out the occurrence of giant flares similar to the ones that were observed in the few cases of Galactic magnetars. The absence of reported radio activity during our observations prevents us from making any determinations regarding the possibility of simultaneous high-energy emission.

Limits on the Hard X-Ray Emission From the Periodic Fast Radio Burst FRB 180916.J0158+65

“…Observations with the AGILE telescope (Tavani et al 2021), working in the energy range 400 keV-100 MeV, could constrain the bursting X-ray energy of FRB 180916 to ∼10 46 erg, which is smaller than that observed from giant flares from Galactic magnetars. Similar upper limits of ∼10 46 erg on the bursting energy of FRB 180916 were obtained using Insight-HMXT in the energy range 1-100 keV (Guidorzi et al 2020). On the soft X-ray range 0.3-10 keV strong limits on persistent luminosity (<2 × 10 40 erg s −1 ) and prompt emission fluence (<5 × 10 −10 erg cm −2 ) could be obtained during the bursting phases of FRB 180916 using observations from Chandra (Scholz et al 2020).…”

Multiwavelength Observations of Fast Radio Bursts

“…Searches for FRBs optical or high-energy counterparts were conducted during standard "triggered" follow-up observations (e.g., [68,69,110,111]) as well as during simultaneous observations with wide-field telescopes (e.g., [112][113][114][115]) or targeting the two "golden" repeaters (e.g., [26,49,[116][117][118][119]). Various radio, optical, X-ray, and γ-ray bands coordinated observations have targeted again the two repeaters and periodic FRB 20121102A [33,35,42,49,116] and FRB 20180916B [22][23][24][25]120,121].…”

“…Dashed and solid lines are 5σ upper limits derived on the integration time of a single FRB and stacked ones. Chandra and Fermi limits were derived simultaneously with FRBs, whereas no FRB was reported from FRB 20180916B for the Insight-HXMT one, which is given at 90% confidence and assumes an integration time of 0.1 s (from [26]) (adapted with permission from Scholz, P., et al; published by IOP Publishing, 2020 [22]).…”

“…The unique combination of three instruments with a timing accuracy of <10 µs, along with the unprecedentedly large geometric area in this energy range (5100 cm 2 ) makes it an optimal instrument to catch short faint bursts. Using a sophisticated set of triggering algorithms expressly devised and tailored to the instrumental background of each detector, upper limits on burst activity in the 1-100 keV energy band were obtained, constraining the released energy as a function of timescale, from 1 ms to 1 s: in particular, E < 10 46 erg for duration 0.1 s during several tens of ks exposure [26] (see the black dashed thick limit in Figure 5).…”

“…As new MWL observational data are being published on the transient emission from the Galactic magnetars SGR J1935+2154 [12][13][14][15][16][17][18], 1E 1547.0-5408 [19], XTE J1810-197 [20], Swift J1818.0-1607 [21], similarities with the FRB phenomena become more and more striking, and then the possible common physical processes involved [8]. On the other hand MWL campaigns on FRB 20180916B can rule out the occurrence of magnetar giant flares (MGF) (E < 10 45−47 erg) either simultaneous to a few radio bursts, or in general during some of the radio-burst active phases [22][23][24][25][26] and constrain the possible associated persistent X-ray luminosity to <2 × 10 40 erg s −1 [22], which is still decades above the observed persistent luminosity of magnetars. In addition, the possible existence of a population of extragalactic magnetars that are equally or even more active than their Galactic siblings and that can emit even more energetic flares [27,28], as was also the recent case of a MGF from NGC 253 (Sculptor Galaxy) at 3.5 Mpc [29][30][31], adds to the case of monitoring the high-energy activity of nearby rFRBs.…”