The nature of fast radio bursts (FRBs), which occur on millisecond timescales in the radio band, is well-understood. Among their unknown observational properties are their broadband spectra, and persistent and transient multiwavelength counterparts. Well-localized FRBs provide the opportunity to address these issues in archival observations. We performed searches for 15–150 keV hard X-ray bursts on timescales as short as 1 ms in the direction of the repeating FRB 121102 (with a spacial resolution of a few arcminutes) in the archival Swift/BAT data between 2016 October and 2017 September. We found no significant (5σ) hard X-ray bursts in the direction of the repeating FRB. We derived an upper limit of the hard X-ray (15–150 keV) flux of any X-ray bursts on a 1 ms timescale of around 1.01 × 10−7 erg cm−2 s−1, if assuming a photoindex of 2 for potential X-ray flares in the X-ray band. A plausible scenario for the repeating FRB as being associated with a magnetar giant flare is still far below the upper limit.
We report a further investigation of the optical low frequency quasi periodic oscillations (LF QPOs) detected in the black hole transient MAXI J1820+070 in the 2018 observations with the YFOSC mounted on Lijang 2.4-m telescope (LJT). Besides, we make use of the Insight-HXMT/HE observations to measure the properties of the quasi-simultaneous X-ray LF QPOs of MAXI~J1820+070 on the same day. We compared the centroid frequency, the full width at half maximum (FWHM) and the fractional RMS of the LF QPOs in both wavelength ranges. We found that the centroid frequency of the optical QPO is at a frequency of 51.58 mHz, which is consistent with that of the X-ray LF QPO detected on the same day within 1 mHz. We also found that the FWHM of the optical LF QPO is significantly smaller than that of the X-ray LF QPO, indicating that the optical QPO has a higher coherence. The quasi-simultaneous optical and the X-ray LF QPO at a centroid frequency of about 52 mHz suggests that the actual mechanisms of these LF QPOs should work in the vicinity of a radius of about 80--117 gravitational radii (Rg=GM/c2, M is the mass of the BH) from the black hole if the frequency is a proxy of the orbital frequency in the accretion flow. Furthermore, the apparent higher coherence of the optical QPO favors that it is a more original signal.
The X-ray variability in the soft X-ray spectral state of black hole binaries is primarily characterized by a power-law noise (PLN), which is thought to originate from the propagation of the modulation in the mass accretion rate of a standard accretion disk flow. Such a PLN has also been revealed in the disk spectral component in the hard and the intermediate states in several black hole binaries. Here we present an investigation of the Rossi X-ray Timing Explorer (RXTE) observations of four black hole transients in which soft spectral states were observed twenty times or more. We show that in the soft spectral state, the PLN index varied in a large range between –1.64 and –0.62, and the fractional rms variability calculated in the 0.01 – 20 Hz frequency range reached as large as 7.67% and as low as 0.83%. Remarkably, we have found evidence of an inclination dependence of the maximal fractional rms variability, the averaged fractional rms variability and the fractional rms variability of the median in the sample based on current knowledge of inclination of black hole binaries. An inclination dependence has only been predicted in early magnetohydrodynamic simulations of isothermal disks limited to a high-frequency regime. In theory, the noise index is related to the physics of inward propagation of disk fluctuations, while the fractional rms amplitude reflects the intrinsic properties of the magnetohydrodynamic nature of the accretion flow. Our results therefore suggest that X-ray variability in the soft state can be used to put constraints on the properties of the accretion flow as well as the inclination of the accretion disk.
The nature of fast radio bursts (FRBs) is currently unknown. Repeating FRBs offer better observation opportunities than nonrepeating FRBs because their simultaneous multiwavelength counterparts might be identified. The magnetar flare model of FRBs is one of the most promising models that predict high-energy emission in addition to radio burst emission. To investigate such a possibility, we have searched for simultaneous and quasi-simultaneous short-term hard X-ray bursts in all Swift/BAT event mode data, which covered the periods when FRB detections were reported in the repeating FRB 121102, by making use of BAT’s arcminute-level spatial resolution and wide field of view. We did not find any significant hard X-ray bursts that occurred simultaneously with those radio bursts. We also investigated potential short X-ray bursts that occurred quasi-simultaneously with those radio bursts (occurrence time differs in the range from hundreds of seconds to thousands of seconds) and concluded that even the best candidates are consistent with background fluctuations. Therefore, our investigation concluded that there were no hard X-ray bursts detectable with Swift/BAT that occurred simultaneously or quasi-simultaneously with those FRBs in the repeating FRB 121102.
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