The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

I Introduction

In the past few years, significant efforts have been made to detect and localize millisecond transient signals using the EVN (Paragi 2016). This was made possible by the recently commissioned EVN Software Correlator (SFXC; Keimpema et al. 2015) at the Joint Institute for VLBI ERIC (JIVE; Dwingeloo, the Netherlands). Here we present joint Arecibo and EVN observations of FRB 121102 which simultaneously detect both the persistent radio source as well as four bursts from FRB 121102, localizing both to milliarcsecond precision. In §II we present the observations and data analysis. In §III we describe the results and in §IV we discuss the properties of the persistent source and its co-localization with the source of the bursts. A discussion of the constraints that these data place on the physical scenarios is also provided. Finally, we present our conclusions in §V.

II Observations and Data Analysis

We have observed FRB 121102 using the EVN at 1.7 GHz and 5 GHz central frequencies (with a maximum bandwidth of 128 MHz in both cases) in 8 observing sessions that span 2016 Feb 1 to Sep 21 (Table 1). These observations included the 305-m William E. Gordon Telescope at the Arecibo Observatory (which provides raw sensitivity for high signal-to-noise burst detection) and the following regular EVN stations: Effelsberg, Hartebeesthoek, Lovell Telescope or Mk2 in Jodrell Bank, Medicina, Noto, Onsala, Tianma, Toruń, Westerbork (single dish), and Yebes. Of these antennas, Hartebeesthoek, Noto, Tianma, and Yebes only participated in the single 5-GHz session.

II.2 Arecibo+EVN Interferometric Data

EVN data were acquired in real time using the e-EVN setup, in which the data are transferred to the central processing center at JIVE via high-speed fibre networks and correlated using the SFXC software correlator. The high data rate of VLBI observations requires visibilities to be typically averaged to 2-s intervals during correlation, which is sufficient to study persistent compact sources near the correlation phase center, like the persistent radio counterpart to FRB 121102. However, we also buffered the baseband EVN data to produce high-time-resolution correlations afterwards for specific times when bursts have been identified in the Arecibo single-dish data.

We used J0529+3209 as phase calibrator in all sessions ($1.1^{\circ}$ away from FRB 121102). In the first five sessions (conducted in Feb and May) we scheduled phase referencing cycles of 8 min on the target and 1 min on the phase calibrator. Whereas this setup maximized the on-source time for burst searches, it provided less accurate astrometry due to poorer phase solutions. The pulsar B0525+21 was also observed in one of these sessions following the same strategy (phase referenced using J0521+2112), in order to perform an empirical analysis of the derived astrometry in interferometric single-burst imaging. In the following three sessions in September, however, we conducted 5-min cycles with 3.5 min on the target and 1.5 min on the phase calibrator, improving the phase referencing, and hence providing more accurate astrometry. Two sessions failed to produce useful calibrated data on the faint target, and are not listed in Table 1. The first session (2016 Feb 1) was used to explore different calibration approaches, whereas the 2016 Sep 19 session was unusable because the largest EVN stations were unavailable and the data could not be properly calibrated without them. An extragalactic $\sim 2$ mJy compact source (Kulkarni et al. 2015, VLA2 in), was identified in the same primary beam as FRB 121102 (with coordinates $\alpha_{\rm J2000}=5^{\rm h}31^{\rm m}53.92244^{\rm s},\ \delta_{\rm J2000}=33^{\circ}10^{\prime}20.0739^{\prime\prime}$). This source has been used to acquire relative astrometry of FRB 121102 during all the sessions and to provide a proper motion constraint.

The 2-s integrated data were calibrated using standard VLBI procedures within AIPS The Astronomical Image Processing System, AIPS, is a software package produced and maintained by the National Radio Astronomy Observatory (NRAO). and ParselTongue (Kettenis et al. 2006), including a priori amplitude calibration using system temperatures and gain curves for each antenna, antenna-based delay correction and bandpass calibration. The phases were corrected by fringe-fitting the calibrators. The phase calibrator J0529+3209 was then imaged and self-calibrated using the Caltech Difmap package (Shepherd et al. 1994). These corrections were interpolated and applied to FRB 121102, which was finally imaged in Difmap.

The arrival times of the bursts were first identified using Arecibo single-dish data, and then slightly refined for application to the EVN data. First using coherently dedispersed Arecibo auto-correlations from the EVN data, we performed a so-called gate search by creating a large number of short integrations inside a 50-ms window around the nominal Arecibo single-dish arrival times. A pulse profile was then created for each of the bursts by plotting the total power in the cross-correlations as a function of time. We then used this pulse profile to determine the exact time window for which the correlation function was accumulated, i.e. the ‘gate’. We de-dispersed and correlated the EVN data to produce visibilities for windows covering only the times of detected bursts. We applied the previously described calibration to the single-pulse data and imaged them. The final images were produced using a Briggs robust weighting of zero (Briggs 1995) as it produced the most consistent results (balance between the longest baselines to Arecibo and the shorter, intra-European baselines). Images with natural or uniform weighting did not produce satisfactory results due to the sparse $uv$-coverage. The flux densities and positions for all datasets were measured using Difmap and CASA The Common Astronomy Software Applications, CASA, is software produced and maintained by the NRAO. by fitting a circular Gaussian component to the detected source in the $uv$-plane.

III Results

III.2 Astrometric Accuracy

The astrometric accuracy of full-track (horizon-to-horizon observations) EVN phase-referencing is usually limited by systematic errors due to the poorly modeled troposphere, ionosphere and other factors. These errors are less than a milliarcsecond in ideal cases (Pradel et al. 2006), but in practice they can be a few milliarcseconds. Given the short duration of the bursts (a few milliseconds), our interferometric EVN data only contain a limited number of visibilities for each burst, which results in a limited $uv$-coverage and thus very strong, nearly equal-power sidelobes in the image plane (see Figure 3, bottom panel). In this case we are no longer limited only by the low-level systematics described above. The errors in the visibilities, either systematic or due to thermal noise, may lead to large and non-Gaussian uncertainties in the position, especially for low S/N, because the response function has many sidelobes. It is not straightforward to derive the astrometric errors for data with just a few-milliseconds integration. Therefore, we conducted the following procedure to verify the validity of the observed positions and to estimate the errors.

First, we independently estimated the approximate position of the strongest burst by fringe-fitting the burst data and using only the residual delays (delay mapping; Huang et al. 2017). With this method we have obtained an approximate position of $\alpha_{\rm J2000}=5^{\rm h}31^{\rm m}58.698^{\rm s}(_{-0.006}^{+0.004}),\ \delta_{\rm J2000}=33^{\circ}8^{\prime}52.586^{\prime\prime}(_{-0.044}^{+0.040})$, where the quoted errors are at the 3-$\sigma$ level. This method provides additional confidence that the image-plane detection of the bursts is genuine, since the positions obtained with the two methods are consistent at the 3-$\sigma$ level.

Finally, we place limits on the angular separation between the source of the bursts and the persistent radio source by sampling from Gaussian distributions with centers and widths given by the source positions and uncertainties listed in Table 1 and deriving a numerical distribution of offsets. Using the average burst position compared to that of the persistent source, this results in a separation of $\lesssim 12$ mas ($\lesssim 40$ pc) at the 95% confidence level (or $\lesssim 50$ pc at 99.5% confidence level). Although the positional uncertainties on individual bursts are likely underestimated and non-Gaussian, as discussed previously, the effect of this should be mitigated somewhat by using the average burst position, which includes an uncertainty determined by the scatter in the separate burst detections, as also seen in Figure 4 for B0525+21. We note that nearly identical separation limits are obtained if we consider instead the position of only the strongest burst, Burst #2.

III.3 Measured Properties

Notably, Burst #2 shows two pulse components, separated by approximately 1 ms (Figure 2). Complex profile structure is commonly seen in the brightest FRB 121102 bursts observed to date, many of which show two or more pulse components (Spitler et al. 2016, Hessels et al., in prep.). Furthermore, as can be seen in the dynamic spectrum of the burst, there is fine-scale frequency structure ($\sim$ MHz) in the intensity, which in principle could be due to scintillation or self noise. This will be investigated in more detail in a forthcoming paper.

IV Discussion

IV.2 Possible Origins of FRB 121102

As previously shown by Spitler et al. 2016, the repeatability of FRB 121102 rules out an origin in a cataclysmic event that destroyed the progenitor source, e.g. the collapse of a supramassive neutron star (Falcke & Rezzolla 2014). The repetition and energetics of the bursts from FRB 121102 have been used to argue that it comes from a young neutron star or magnetar (Cordes & Wasserman 2016; Lyutikov et al. 2016; Popov & Pshirkov 2016). At birth, the rapid spin of such (potentially highly magnetized) objects can power a luminous nebula from the region evacuated by its SNR.

IV.2.2 Active galactic nucleus / accreting black hole

Models have been proposed in which the bursts are due to strong plasma turbulence excited by the relativistic jet of an AGN (Romero et al. 2016) or due to synchrotron maser activity from an AGN (Ghisellini 2016). It is also conceivable to have an extremely young and energetic pulsar and/or magnetar near to an AGN (Pen & Connor 2015; Cordes & Wasserman 2016) – either interacting or not.

Alternatively, we could be witnessing a radio-loud, but otherwise low-luminosity AGN powered by a much less massive black hole that accretes at a very low rate. This population is poorly known, but EVN observations of the brightest low-luminosity AGNs (LLAGNs) in a sample of Fundamental Plane outliers (i.e. radio-loud, with $R_{\rm X}\sim-2$) show that some of these have extended jets/lobes and the radio excess may come from strong interaction with the surrounding gas in the galaxy; others appear very compact like our persistent radio source, and the reason for their high $R_{\rm X}$ remains a mystery (Paragi et al. 2012). We note that there are other recent examples of LLAGNs identified based on their VLBI properties coupled with low-levels of X-ray emission and no signs of nuclear activity from the optical emission lines (Park et al. 2016).

Other possible associations, like a single X-ray binary (such as Cyg X-3; Merloni et al. 2003; Reines et al. 2011) or an ultraluminous X-ray nebula (such as S 26 and/or IC 342 X-1; Soria et al. 2010; Cseh et al. 2012), do not fit to the measured flux density of the persistent radio emission and/or the observed size by several orders of magnitude.

V Conclusions

References