On the nature of FRB 150418: clues from European VLBI Network and e-MERLIN observations

Introduction

Fast radio bursts (FRBs) are transient episodes characterised by short (sub-ms) duration and large dispersion measure (DM). After the initial discovery by Lorimer et al. 2007, several new such events have been discovered (Thornton et al. 2013; Champion et al. 2016, e.g.,), triggering debate about their nature. It is possible that they are due to young, highly magnetized neutron stars, as suggested for the repeating FRB 121102 (Spitler et al. 2016), or to cataclysmic events.

Both Galactic and extragalactic origins have been proposed. An extragalactic origin is preferred based on the large DM, \lower 2.15277pt\hbox{\;\mathrel{\mathop{\kern 0.0pt\sim}\limits^{>}}\;}(0.5-1)\times 10^{3} cm-3 pc typically found; however, only a precise localisation and a measurement of the redshift could be conclusive. For this reason, the reported localisation of FRB 150418 to the elliptical galaxy, WISE J071634.59$-$190039.2 (hereafter, WISE J0716$-$19) by Keane et al. 2016 has attracted interest in the community. The precise redshift determination for this FRB has immediate implications for the system’s energetics, thus possible FRB progenitors (Liu et al. 2016; Zhang 2016), as well as applications to probe fundamental physics (Bonetti et al. 2016; Tingay & Kaplan 2016).

The proposed identification of FRB 150418 with WISE J0716$-$19 was based on the prompt detection (beginning 2 hrs after the FRB discovery) with the Australia Telescope Compact Array (ATCA) of a fading radio source within one beam of the 21-cm Parkes multi-beam receiver. Optical photometric and spectroscopic follow-up observations with the Subaru telescope identified an elliptical galaxy at $z=0.492\pm 0.008$ consistent with the radio source within the $\sim 1\arcsec$ positional uncertainty. In more detail, the radio transient emission was observed in only the first two epochs of ATCA follow-up separated by 6-days (flux densities $\sim 0.2$ mJy at 5.5 GHz) with three subsequent detections of essentially steady emission ($\sim 0.1$ mJy at 5.5 GHz) attributed to the emission from the host galaxy.

This association of the prompt ms-duration emission from the FRB with the variable ATCA source has been questioned. Williams & Berger 2016a argued instead that WISE J0716$-$19 is consistent with being a random active galactic nucleus (AGN) found within the Parkes beam based on the known rate of variable (rather than transient) radio sources, that the steady radio emission component implies a large luminosity more typical of an AGN, and that the radio light curve is inconsistent with the evolution of a standard afterglow.

A final confirmation of the AGN scenario, plus a relevant contribution from refractive interstellar scintillation, can be obtained from high angular resolution Very Long Baseline Interferometry (VLBI) observations. In this Letter, we thus report on the results of European VLBI Network (EVN) e-MERLIN observations of WISE J0716$-$19. In the following, we describe the observations in Sect. 2, present the results in Sect. 3, and discuss them in Sect. 4.

Observations and data reduction

We observed WISE J0716$-$19 four times between 2016 March 16 and June 2 (Table 1) with a subset of the EVN. The participating stations were Effelsberg, Hartebeesthoek, Jodrell Bank (Mark2), Medicina, Noto, Onsala, Torun, Yebes, and a single Westerbork telescope. We observed at 5.0 GHz, with eight 16-MHz-wide baseband channels, in dual polarization, and with 2-bit sampling. The data were electronically transferred over fibre links to the SFXC correlator at JIVE, where they were correlated in real time with the so-called e-VLBI technique.

We carried out all observations in phase-reference mode, with 2.5 min scans on the target source bracketed by 1.5 min scans on the nearby ($0.9^{\circ}$ offset) calibrator J0718–1813. Each observation lasted for $\sim 5.5$ hours, with on-source time of $\sim 2.4$ hours. We calibrated visibility amplitudes based on the a-priori gain curves and measured system temperatures at each station. Parallactic angle corrections were applied and we determined instrumental single band delays using a scan on a strong calibrator. We then determined phase, rates, and residual delays for the phase calibrator. Since the calibrator has a double component structure, we imaged it with hybrid mapping procedure, and then repeated the fringe fitting process using the obtained image as the input model. The resultant solutions were applied back to the phase reference source, the target, and the additional check source J0712–1847. Bandpass solutions were then determined combining all the data for the calibrator. Finally, we carried out one cycle of phase-only and one of phase-and-amplitude self-calibration for the phase reference source, and transferred the solutions to the target. A parallel analysis of the check source based on either direct fringe fitting of its visibility data or phase self-calibration indicated that coherence losses affected the detected ranging between $20\%$ and $40\%$ of the real flux density; this is not surprising given the low elevation of the target.

In strict simultaneity with the latter three EVN epochs (same start and end times), we observed the source with e-MERLIN, using six, five, and five stations in each experiment. We observed at 5.0 GHz, with four 128-MHz-wide channels, in dual polarisation. The same phase reference source was used as in the EVN run. The maximum elevation of the source was $18^{\circ}$, which resulted in an elongated restoring beam (axial ratio $\sim 10$, in p.a. = $\sim 10^{\circ}$). Detailed information is reported in Table 1.

Results

In Fig. 1, we show our EVN 5.0 GHz images around the position of the VLBA and e-MERLIN detections reported by 2. The main image shows a $0.3\arcsec\times 0.25\arcsec$ field-of-view based on averaging the images from all epochs. The insets show 60 mas $\times$ 60 mas image stamps of the central region from the four individual epochs.

In each of the individual epochs, the source is detected with significances above $6\sigma$. In Cols. 4, 5, and 6 of Table 1, we report the image peak brightness, the noise, and the component flux density measured with AIPS task JMFIT. The associated uncertainties were calculated as the quadratic sum of a $1\sigma$ r.m.s. statistical contribution and a 10% absolute calibration uncertainty; this provides the uncertainty on the relative calibration from epoch to epoch, while the overall scaling due to coherence losses remains unaccounted for. Within these uncertainties, the source flux density is consistent with being constant among epochs; the best fit coordinates are also consistent to 1/10 of the restoring beam, or less.

No significant variability is present from one epoch to the other. We explored the presence of variability on shorter time scales dividing each observation in three bins of $\sim 2$-hr duration each. The values of the target and of the calibrator peak brightness in each bin are reported in Table 2, normalised to the mean of each epoch. The source is generally detected in every subsets of dataset, albeit at lower significance (notice that the relative errors are comparatively large, as the noise in each bin is $\sim\sqrt{3}\times$ higher than in the full dataset). In 8/12 cases, the peak brightness is consistent with the mean value. In the last epoch, two bins are about $2\sigma$ away from the mean. The significance of these variations is difficult to establish because the $(u,v)$-plane coverage is different in every bin, as was the elevation of the source. The calibrator peak brightness is more stable, yet it generally increases in the second and third subsets of each observation, as the restoring beam rotates closer to the main p.a. of the double structure of the source. While this indicates that no systematic effects due to calibration are present, it also shows the difficulty in establishing variability on short time scales with the present data.

Finally, we detect a point-like source in every e-MERLIN observation, at a significance level of just above $3\sigma$. The coordinates are consistent with those obtained with the EVN, yet less well determined due to the elongated beam. The peak brightness in two epochs is also consistent with that detected by the EVN. In the final epoch, the peak is higher, although we caution that the weak signal-to-noise ratio and the low elevation indicate that this might not be a highly significant discrepancy.

Discussion

The presence of an AGN within WISE J0716$-$19 was already implicit in the results presented by 7. They reported an upper limit to the H$\upalpha$ luminosity associated with a star-formation rate of $\leq 0.2M_{\odot}\,{\rm yr}^{-1}$. Based on Condon 1992, this value corresponds to a radio luminosity $L_{\rm 5.0\,GHz}\leq 10^{21}$ W Hz-1, about two orders of magnitude lower than that observed at the ATCA quiescence level. Our observations now provide a firm proof of the presence of a compact radio source at the centre of WISE J0716$-$19, with a bolometric radio luminosity of $\nu L_{\nu}=5.6\times 10^{39}$ erg s-1 and not variable, within the uncertainties. At first sight, this result supports the association proposed by 7 between FRB 150418 and the subsequent episode of variable radio emission.

This requires us to explore the time, rather than the spatial, domain. It is possible, although unlikely, that the discrepancy is a chance coincidence: a K-S test on the distribution of the VLA and VLBI flux densities provides a probability that the two are drawn from the same distribution as low as 0.011. There is one significant factor to take into account: due to the different sensitivity of the two instruments, VLA data are obtained on much shorter time scales (typically, 30 minutes) than the EVN’s (many hours). We can thus hypothesise that the parsec-scale source varies on short ($<$ hr) time scales, so that the VLA-based light curve resolves the variations, while they are averaged out by the longer EVN observations.

The above scenario does also present some challenges. Intrinsic sub-hour time-scale variability from AGNs requires extremely large brightness temperature, exceeding the inverse Compton catastrophe limit. On the other hand, WISE J0716$-$19 is located at low Galactic latitude ($b=-3^{\circ}.2$), indicating that radio waves are subject to significant refractive scintillation in the ionized interstellar medium of the Milky Way. Akiyama & Johnson 2016 have argued that few-day time-scale variability of WISE J0716$-$19 could indeed be extrinsic, if the source has a $T_{\rm b}\gtrsim 10^{9}{\rm K}$, which is consistent with our result. Scintillation has so far been studied mostly in blazars and little is known about the variability properties of weak sources; however, very rapid variations in WISE J0716$-$19 would at least be in agreement with the trend of increased variability found for lower flux density sources (Lovell et al. 2008).

References