Abstract
The repeating fast radio burst source FRB 121102 has been shown to have an exceptionally high and variable Faraday rotation measure (RM), which must be imparted within its host galaxy, ...likely by or within its local environment. In the redshifted (
z
= 0.193) source reference frame, the RM decreased from 1.46 × 10
5
rad m
−2
to 1.33 × 10
5
rad m
−2
between 2017 January and August, showing day-timescale variations of ∼200 rad m
−2
. Here we present 16 FRB 121102 RMs from burst detections with the Arecibo 305 m radio telescope, the Effelsberg 100 m, and the Karl G. Jansky Very Large Array, providing a record of FRB 121102’s RM over a 2.5 yr time span. Our observations show a decreasing trend in RM, although the trend is not linear, dropping by an average of 15% year
−1
and is ∼ 9.7 × 10
4
rad m
−2
at the most recent epoch of 2019 August. Erratic, short-term RM variations of ∼10
3
rad m
−2
week
−1
were also observed between MJDs 58215–58247. A decades-old neutron star embedded within a still-compact supernova remnant or a neutron star near a massive black hole and its accretion torus have been proposed to explain the high RMs. We compare the observed RMs to theoretical models describing the RM evolution for FRBs originating within a supernova remnant. FRB 121102’s age is unknown, and we find that the models agree for source ages of ∼6–17 yr at the time of the first available RM measurements in 2017. We also draw comparisons to the decreasing RM of the Galactic center magnetar, PSR J1745−2900.
Abstract
We present a search for optical bursts from the repeating fast radio burst FRB 121102 using simultaneous observations with the high-speed optical camera ULTRASPEC on the 2.4-m Thai National ...Telescope and radio observations with the 100-m Effelsberg Radio Telescope. A total of 13 radio bursts were detected, but we found no evidence for corresponding optical bursts in our 70.7-ms frames. The 5σ upper limit to the optical flux density during our observations is 0.33 mJy at 767 nm. This gives an upper limit for the optical burst fluence of 0.046 Jy ms, which constrains the broad-band spectral index of the burst emission to α ≤ −0.2. Two of the radio pulses are separated by just 34 ms, which may represent an upper limit on a possible underlying periodicity (a rotation period typical of pulsars), or these pulses may have come from a single emission window that is a small fraction of a possible period.
ABSTRACT
Detections from the repeating fast radio burst FRB 121102 are clustered in time, noticeable even in the earliest repeat bursts. Recently, it was argued that the source activity is periodic, ...suggesting that the clustering reflected a not-yet-identified periodicity. We performed an extensive multiwavelength campaign with the Effelsberg telescope, the Green Bank telescope, and the Arecibo Observatory to shadow the Gran Telescope Canaria (optical), NuSTAR (X-ray) and INTEGRAL (γ-ray). We detected 36 bursts with Effelsberg, one with a pulse width of 39 ms, the widest burst ever detected from FRB 121102. With one burst detected during simultaneous NuSTAR observations, we place a 5σ upper limit of 5 × 1047 erg on the 3–79 keV energy of an X-ray burst counterpart. We tested the periodicity hypothesis using 165 h of Effelsberg observations and find a periodicity of 161 ± 5 d. We predict the source to be active from 2020 July 9 to October 14 and subsequently from 2020 December 17 to 2021 March 24. We compare the wait times between consecutive bursts within a single observation to Weibull and Poisson distributions. We conclude that the strong clustering was indeed a consequence of a periodic activity and show that if the few events with millisecond separation are excluded, the arrival times are Poisson distributed. We model the bursts’ cumulative energy distribution with energies from ∼1038–1039 erg and find that it is well described by a power law with slope of γ = −1.1 ± 0.2. We propose that a single power law might be a poor descriptor of the data over many orders of magnitude.
We present 41 bursts from the first repeating fast radio burst (FRB) discovered (FRB 121102). A deep search has allowed us to probe unprecedentedly low burst energies during two consecutive ...observations (separated by one day) using the Arecibo telescope at 1.4 GHz. The bursts are generally detected in less than one-third of the 580 MHz observing bandwidth, demonstrating that narrowband FRB signals may be more common than previously thought. We show that the bursts are likely faint versions of previously reported multi-component bursts. There is a striking lack of bursts detected below 1.35 GHz and simultaneous Very Large Array observations at 3 GHz did not detect any of the 41 bursts, but did detect one that was not seen with Arecibo, suggesting preferred radio emission frequencies that vary with epoch. A power-law approximation of the cumulative distribution of burst energies yields an index −1.8 0.3, which is much steeper than the previously reported value of ∼−0.7. The discrepancy may be evidence for a more complex energy distribution. We place constraints on the possibility that the associated persistent radio source is generated by the emission of many faint bursts (∼700 ms−1). We do not see a connection between burst fluence and wait time. The distribution of wait times follows a log-normal distribution centered around ∼200 s; however, some bursts have wait times below 1 s and as short as 26 ms, which is consistent with previous reports of a bimodal distribution. We caution against exclusively integrating over the full observing band during FRB searches, because this can lower signal to noise.
Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities ...that are orders of magnitude larger than those of all known short-duration radio transients. So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts and tidal disruption events. However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.
FRB 121102 is the only known repeating fast radio burst source. Here we analyze a wide-frequency-range (1-8 GHz) sample of high signal-to-noise, coherently dedispersed bursts detected using the ...Arecibo and Green Bank telescopes. These bursts reveal complex time-frequency structures that include subbursts with finite bandwidths. The frequency-dependent burst structure complicates the determination of a dispersion measure (DM); we argue that it is appropriate to use a DM metric that maximizes frequency-averaged pulse structure, as opposed to peak signal-to-noise, and find DM = 560.57 0.07 pc cm−3 at MJD 57,644. After correcting for dispersive delay, we find that the subbursts have characteristic frequencies that typically drift lower at later times in the total burst envelope. In the 1.1-1.7 GHz band, the ∼0.5-1 ms subbursts have typical bandwidths ranging from 100 to 400 MHz, and a characteristic drift rate of ∼200 MHz ms−1 toward lower frequencies. At higher radio frequencies, the subburst bandwidths and drift rate are larger, on average. While these features could be intrinsic to the burst emission mechanism, they could also be imparted by propagation effects in the medium local to the source. Comparison of the burst DMs with previous values in the literature suggests an increase of ΔDM ∼ 1-3 pc cm−3 in 4 yr; though, this could be a stochastic variation as opposed to a secular trend. This implies changes in the local medium or an additional source of frequency-dependent delay. Overall, the results are consistent with previously proposed scenarios in which FRB 121102 is embedded in a dense nebula.
Fast radio bursts are millisecond-duration, extragalactic radio flashes of unknown physical origin. The only known repeating fast radio burst source-FRB 121102-has been localized to a star-forming ...region in a dwarf galaxy at redshift 0.193 and is spatially coincident with a compact, persistent radio source. The origin of the bursts, the nature of the persistent source and the properties of the local environment are still unclear. Here we report observations of FRB 121102 that show almost 100 per cent linearly polarized emission at a very high and variable Faraday rotation measure in the source frame (varying from +1.46 × 10
radians per square metre to +1.33 × 10
radians per square metre at epochs separated by seven months) and narrow (below 30 microseconds) temporal structure. The large and variable rotation measure demonstrates that FRB 121102 is in an extreme and dynamic magneto-ionic environment, and the short durations of the bursts suggest a neutron star origin. Such large rotation measures have hitherto been observed only in the vicinities of massive black holes (larger than about 10,000 solar masses). Indeed, the properties of the persistent radio source are compatible with those of a low-luminosity, accreting massive black hole. The bursts may therefore come from a neutron star in such an environment or could be explained by other models, such as a highly magnetized wind nebula or supernova remnant surrounding a young neutron star.
ABSTRACT
The repeating fast radio burst (FRB) source, FRB 20201124A, was found to be highly active in 2021 March and April. We observed the source with the Effelsberg 100-m radio telescope at ...1.36 GHz on 2021 April 9 and detected 20 bursts. A downward drift in frequency over time is clearly seen from the majority of bursts in our sample. A structure-maximizing dispersion measure (DM) search on the multicomponent bursts in our sample yields a DM of 411.6 ± 0.6 pc cm−3. We find that the rotation measure (RM) of the bursts varies around their weighted mean value of −601 rad m−2 with a standard deviation of 11.1 rad m−2. This RM magnitude is 10 times larger than the expected Galactic contribution along this line of sight (LoS). We estimate an LoS magnetic field strength of 4–6 µG, assuming that the entire host galaxy DM contributes to the RM. Further polarization measurements will help determine FRB 20201124A’s RM stability. The bursts are highly linearly polarized, with some showing signs of circular polarization, the first for a repeating FRB. Their polarization position angles (PAs) are flat across the burst envelopes and vary between bursts. We argue that the varying polarization fractions and PAs of FRB 20201124A are similar to known magnetospheric emission from pulsars, while the observed circular polarization, combined with the RM variability, is hard to explain with Faraday conversion. The high linear polarization fractions, flat PAs, and downward drift from FRB 20201124A bursts are similar to previous repeating sources, while the observed circular polarization is a newly seen behaviour among repeaters.
ABSTRACT We report on radio and X-ray observations of the only known repeating Fast Radio Burst (FRB) source, FRB 121102. We have detected six additional radio bursts from this source: five with the ...Green Bank Telescope at 2 GHz, and one at 1.4 GHz with the Arecibo Observatory, for a total of 17 bursts from this source. All have dispersion measures consistent with a single value (∼559 pc cm−3) that is three times the predicted maximum Galactic contribution. The 2 GHz bursts have highly variable spectra like those at 1.4 GHz, indicating that the frequency structure seen across the individual 1.4 and 2 GHz bandpasses is part of a wideband process. X-ray observations of the FRB 121102 field with the Swift and Chandra observatories show at least one possible counterpart; however, the probability of chance superposition is high. A radio imaging observation of the field with the Jansky Very Large Array at 1.6 GHz yields a 5 upper limit of 0.3 mJy on any point-source continuum emission. This upper limit, combined with archival Wide-field Infrared Survey Explorer 22 m and IPHAS H surveys, rules out the presence of an intervening Galactic H ii region. We update our estimate of the FRB detection rate in the PALFA survey to be FRBs sky−1 day−1 (95% confidence) for peak flux density at 1.4 GHz above 300 mJy. We find that the intrinsic widths of the 12 FRB 121102 bursts from Arecibo are, on average, significantly longer than the intrinsic widths of the 13 single-component FRBs detected with the Parkes telescope.
Polarized radio emission from PSR J1745−2900 has already been used to investigate the strength of the magnetic field in the Galactic center (GC), close to Sagittarius A*. Here we report how ...persistent radio emission from this magnetar, for over four years since its discovery, has revealed large changes in the observed Faraday rotation measure (RM), by up to 3500 rad m−2 (a 5% fractional change). From simultaneous analysis of the dispersion measure, we determine that these fluctuations are dominated by variations in either the projected magnetic field or the free electron content within the GC, along the changing line of sight to the rapidly moving magnetar. From a structure function analysis of RM variations, and a recent epoch of rapid change of RM, we determine a minimum scale of magneto-ionic fluctuations of size ∼2 au at the GC distance, inferring PSR J1745−2900 is just ∼0.1 pc behind an additional scattering screen.
PDF
