The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales
B. Marcote1
, Z. Paragi1
, J. W. T. Hessels2,3
, A. Keimpema1
, H. J. van Langevelde1,4
, Y. Huang5,1
, C. G. Bassa2
, S. Bogdanov6
,
G. C. Bower7
, S. Burke-Spolaor8,9,10, B. J. Butler8
, R. M. Campbell1
, S. Chatterjee11, J. M. Cordes11, P. Demorest8
,
M. A. Garrett12,4,2
, T. Ghosh13, V. M. Kaspi14, C. J. Law15, T. J. W. Lazio16, M. A. McLaughlin9,10, S. M. Ransom17, C. J. Salter13,
P. Scholz18, A. Seymour13, A. Siemion15,2,19, L. G. Spitler20, S. P. Tendulkar14, and R. S. Wharton11
1 Joint Institute for VLBI ERIC, Postbus 2, NL-7990 AA Dwingeloo, The Netherlands; marcote@jive.eu 2 ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, The Netherlands; J.W.T.Hessels@uva.nl 3 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, The Netherlands 4 Sterrewacht Leiden, Leiden University, Postbus 9513, NL-2300 RA Leiden, The Netherlands
5 Department of Physics and Astronomy, Carleton College, Northfield, MN 55057, USA 6 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA 7 Academia Sinica Institute of Astronomy and Astrophysics, 645 N. A’ohoku Place, Hilo, HI 96720, USA 8 National Radio Astronomy Observatory, Socorro, NM 87801, USA 9 Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA 10 Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA 11 Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, NY 14853, USA 12 Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK 13 Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA 14 Department of Physics and McGill Space Institute, McGill University, 3600 University Street, Montreal, QC H3A 2T8, Canada 15 Department of Astronomy and Radio Astronomy Lab, University of California, Berkeley, CA 94720, USA 16 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 17 National Radio Astronomy Observatory, Charlottesville, VA 22903, USA 18 National Research Council of Canada, Herzberg Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory,
P.O. Box 248, Penticton, BC V2A 6J9, Canada 19 Radboud University, NL-6525 HP Nijmegen, The Netherlands 20 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
Received 2016 December 10; revised 2016 December 21; accepted 2016 December 22; published 2017 January 4
Abstract
The milli
FRB 121102 has been localized to ∼100 mas precision
(Chatterjee et al. 2017). The precise localization of these
bursts has led to associations with both persistent radio and
optical sources and the identification of FRB 121102ʼs host
galaxy (Chatterjee et al. 2017; Tendulkar et al. 2017). European
VLBI Network (EVN) observations, confirmed by the Very
Long Baseline Array (VLBA), have shown that the persistent
source is compact on milliarcsecond scales (Chatterjee
et al. 2017). Optical observations have identified a faint
(m 25.1 0.1 r¢ = AB mag) and extended (0.6–0.8 arcsec)
counterpart in Keck and Gemini data, located at a redshift
z = 0.19273 0.00008—i.e., at a luminosity distance of
DL » 972 Mpc, and implying an angular diameter distance
of DA » 683 Mpc (Tendulkar et al. 2017). The centroids of the
persistent optical and radio emission are offset from each other
by ∼0.2 arcsec, and the observed optical emission lines are
dominated by star formation, with an estimated star formation
rate of ~0.4 yr M -1
(Tendulkar et al. 2017). In X-rays, XMMNewton
and Chandra observations provide a 5σ upper limit in
the 0.5–10 keV band of LX 5 10 erg s < ´ 41 1 - (Chatterjee
et al. 2017).
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 that simultaneously
detect both the persistent radio source as well as four bursts
from FRB 121102, localizing both to milliarcsecond precision.
In Section 2, we present the observations and data analysis. In
Section 3, we describe the results, and in Section 4, 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 Section 5.
2. Observations and Data Analysis
We have observed FRB 121102 using the EVN at 1.7 and
5 GHz central frequencies (with a maximum bandwidth of
128 MHz in both cases) in eight observing sessions that span
2016 February 1 to September 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.
We simultaneously acquired both EVN VLBI data products
(buffered baseband data and real-time correlations) as well as
wideband, high-time-resolution data from Arecibo as a standalone
telescope. The Arecibo single-dish data provide poor
angular resolution (∼3 arcmin at 1.7 GHz), but unparalleled
sensitivity in order to search for faint millisecond bursts. By
first detecting bursts in the Arecibo single-dish data, we could
then zoom in on specific times in the multi-telescope EVN data
set where we could perform high-angular-resolution imaging of
the bursts themselves.
2.1. Arecibo Single-dish Data
For the 1.7 GHz observations, Arecibo single-dish observations
used the Puerto-Rican Ultimate Pulsar Processing
Instrument (PUPPI) in combination with the L-band Wide
receiver, which provided ~600 MHz of usable bandwidth
between 1150 and 1730 MHz. The PUPPI data were coherently
dedispersed to a DM 557 pc cm = -3, as previously done by
Scholz et al. (2016). Coherent dedispersion removes the
dispersive smearing of the burst width within each spectral
channel. The time resolution of the data was 10.24 μs, and we
recorded full Stokes parameters. At 5 GHz, the Arecibo singledish
observations were recorded with the Mock Spectrometers
Table 1
Properties of the Persistent Radio Source and Detected FRB 121102 Bursts from the Arecibo+EVN Observations
Session Epoch ν Da Dd Sn x
(YYYY Month DD) (GHz) (mas) (mas) (μJy) (Jy ms1/2
)
RP024B 2016 Feb 10 1.7 1.5 ± 2 −2 ± 3 200 ± 20 L
RP024C 2016 Feb 11 1.7 −4 ± 2 −5 ± 3 175 ± 14 L
RP024D 2016 May 24 1.7 1 ± 3 −5 ± 4 220 ± 40 L
RP024E 2016 May 25 1.7 1 ± 3 2 ± 4 180 ± 40 L
RP026B 2016 Sep 20 1.7 1.9 ± 1.8 −0.4 ± 2.3 168 ± 11 L
RP026C 2016 Sep 21 5.0 0.0 ± 0.6 0.0 ± 0.7 123 ± 14 L
(YYYY Month DD hh:mm:ss.sss) (Jy)
Burst #1 2016 Sep 20 09:52:31.634 1.7 −14 ± 3 −1.4 ± 1.8 0.46 ± 0.02 ∼0.8
Burst #2 2016 Sep 20 10:02:44.716 1.7 −3.3 ± 2.5 4.3 ± 1.6 3.72 ± 0.12 ∼5
Burst #3 2016 Sep 20 10:03:29.590 1.7 −10 ± 5 0.8 ±3 0.22 ± 0.03 ∼0.4
Burst #4 2016 Sep 20 10:50:57.695 1.7 3 ± 6 6 ± 4 0.17 ± 0.03 ∼0.2
Avg. burst pos. 2016 Sep 20 1.7 −5 ± 4 3.5 ± 2.2 L L
Note. All positions are referred to the 5 GHz detection of the persistent source (RP026C epoch): J2000 5 31 58. 70159 h m s a = , dJ2000 = ¢ 33 8 52. 5501. The observations
conducted on 2016 February 1 (RP024A) and 2016 September 19 (RP026A) did not produce useful data and are not included here (see the main text). The arrival
times of the bursts are UTC topocentric at Arecibo at the top of the observing band (1690.49 MHz). All these bursts had gate widths of 2–3 ms, and the quoted flux
densities are averages over these time windows. We note that the larger error on the flux density of burst #2 is due to the fact that the image is dynamic-range limited
because of the burstʼs brightness. The last row shows the average position obtained from the four bursts weighted by the detection statistic x = F w (fluence
divided by the square root of the burst width).
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The Astrophysical Journal Letters, 834:L8 (9pp), 2017 January 10 Marcote et al.
in combination with the C-band receiver, which together
provided spectral coverage from 4484 to 5554 MHz. The Mock
data were recorded in seven partially overlapping subbands of
172 MHz, with 5.376 MHz channels and 65.476 μs time
resolution. In addition to the PUPPI and Mock data, the
autocorrelations of the Arecibo data from the VLBI recording
were also available (these are restricted to only 64 MHz of
bandwidth; see below).
The Arecibo single-dish data were analyzed using tools from
the PRESTO21 suite of pulsar software (Ransom 2001) and
searched for bursts using standard procedures (e.g., Scholz
et al. 2016). The data were first subbanded to 8´ lower
time and frequency resolution and were then dedispersed
using prepsubband to trial DMs between 487 and
627 pc cm−3 in order to search for pulses that peak in signalto-noise
ratio (S/N) at the expected DM of FRB 121102. This
is required to separate astrophysical bursts from radio
frequency interference (RFI). For each candidate burst found
using single_pulse_search.py (and grouping common
events across DM trials), the astrophysical nature was
confirmed by producing a frequency versus time diagram to
show that the signal is (relatively) broadband compared to the
narrowband RFI signals that can sometimes masquerade as
dispersed pulses.
2.2. 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 fiber 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-timeresolution
correlations afterward 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
away from FRB 121102). In the first five sessions (conducted
in February and May), we scheduled phase-referencing cycles
of 8 minutes on the target and 1 minute 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 minute cycles with
3.5 minutes on the target and 1.5 minutes 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 February 1) was used
to explore different calibration approaches, whereas the 2016
September 19 session was unusable because the largest EVN
stations were unavailable and the data could not be properly
calibrated without them. An extragalactic ∼2 mJy compact
source (VLA2 in Kulkarni et al. 2015) was identified in the
same primary beam as FRB 121102 (with coordinates aJ2000 =
5 31 53. 92244, 33 10 20. 0739 h m s dJ2000 = ¢ ). 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 AIPS22 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 autocorrelations 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
dedispersed 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 data sets were measured using
Difmap and CASA23 by fitting a circular Gaussian component
to the detected source in the uv-plane.
3. Results
3.1. Burst Detections
The EVN observations detect the compact and persistent
source found by Chatterjee et al. (2017) with a synthesized
beam size (FWHM) of » ´ 21 mas 2 mas at 1.7 GHz and
» ´ 4 mas 1 mas at 5 GHz, with position angle »-55 in
both cases.
On 2016 September 20, we detected four individual bursts in
the Arecibo single-dish PUPPI data that overlap with EVN data
acquisition (Table 1). No bursts were detected in the Arecibo
PUPPI (1.7 GHz) or Mock (5 GHz) data from other sessions in
which there are simultaneous EVN observations that can be
used for imaging the bursts. We formed images from the
calibrated visibility data for each burst and measured their
positions with respect to the persistent radio source. Figure 1
shows these positions together with the persistent source at 1.7
and 5.0 GHz. The nominal positions measured for the four
bursts are spread 15 mas around the position of the persistent
source, and we discuss this scatter in Section 3.2.
21 Available at https://github.com/scottransom/presto.
22 The Astronomical Image Processing System (AIPS) is a software package
produced and maintained by the National Radio Astronomy Observatory
(NRAO). 23 The Common Astronomy Software Applications (CASA) is software
produced and maintained by the NRAO.
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The Astrophysical Journal Letters, 834:L8 (9pp), 2017 January 10 Marcote et al.
Figures 2 and 3 show data corresponding to the strongest
burst (burst #2)—in the time domain and in the image plane,
respectively. We have characterized the bursts using the
detection statistic x = F w (fluence divided by the square
root of the burst width; e.g., Cordes & McLaughlin 2003; Trott
et al. 2013). When matched filtering is done to detect a pulse
(as we have done, starting with the single-dish PUPPI data),
then the S/N of the detection statistic, i.e., the output of the
correlation, is proportional to ξ. Localization of the source in an
image (whether in the image or in the uv-domain) will tend to
have the same scaling if the uv-data are calculated with a tight
gate (time window) around the pulse so that it also scales as w.
Using only fluence as a detection statistic is not appropriate
because a high-fluence, very wide burst can still be buried in
the noise, whereas a narrower burst with equivalent fluence is
more easily discriminated from noise. Burst #2 was roughly an
order of magnitude brighter than the other three bursts and
shows a detection statistic ξ that is also a factor of >6 higher
than the other bursts. This brightest burst is separated by only
~7 mas from the centroid of the persistent source at the same
epoch and is positionally consistent at the ∼2σ level. We thus
find no convincing evidence that there is a significant offset
between the source of the bursts and the persistent source.
Since burst #2ʼs detection statistic, ξ, is significantly larger
than for any of the other three bursts, its apparent position is
least affected by noise in the image plane, as we explain in the
following section, Section 3.2. As such, in principle, it provides
the most accurate position of all four detected bursts and the
strongest constraint on the maximum offset between bursts and
the compact, persistent radio source.
3.2. 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
Figure 1. EVN image of the persistent source at 1.7 GHz (white contours)
together with the localization of the strongest burst (red cross), the other three
observed bursts (gray crosses), and the position obtained after averaging all
four bursts detected on 2016 September 20 (black cross). Contours start at a 2σ
noise level of 10 μJy and increase by factors of 21 2. Dashed contours represent
negative levels. The color scale shows the image at 5.0 GHz from 2016
September 21. The synthesized beam at 5.0 GHz is represented by the gray
ellipse at the bottom left of the figure and for 1.7 GHz at the bottom right. The
lengths of the crosses represent the 1σ uncertainty in each direction. Crosses for
each individual burst reflect only the statistical errors derived from their S/N
and the beam size. The size of the cross for the mean position is determined
from the spread of the individual burst locations, weighted by ξ (see the text),
and is consistent with the centroid of the persistent source to within <2s.
Figure 2. Top: dynamic spectrum of the strongest burst detected on 2016
September 20 (burst #2 in Table 1) from Arecibo autocorrelations, showing
the dispersive sweep across the observing band. Bottom: coherently
dedispersed and band-integrated profiles of the same burst as observed in the
cross-correlations for Arecibo–Effelsberg (Ar-Ef), Arecibo–Medicina (Ar-Mc),
and Effelsberg–Onsala (Ef-O8) after only applying a priori calibration. The
measured peak brightnesses are 11.9, 10.7, and 10.9 Jy, respectively, where the
error is typically 10%–20% for a priori calibration. The rms on each baseline is
12, 80, and 300 mJy, respectively.
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The Astrophysical Journal Letters, 834:L8 (9pp), 2017 January 10 Marcote et al.
only the residual delays (delay mapping; Y. Huang et al. 2017,
in preparation). With this method we have obtained an
approximate position of 5 31 58. 698 , J2000
h m s 0.006
0.004 a d = = - J2000
+
( )
33 8 52. 586 0.044
0.040 ¢ -
+
( ), where the quoted errors are at the 3σ
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
3σ level.
Next, we carried out an empirical analysis of single-burst
EVN astrometry by imaging 406 pulses recorded from the
pulsar B0525+21, which was used as a test source in the 2016
February 11 session. PSR B0525+21 has typical pulse widths
of roughly 200 ms and peak flux densities of ∼70–900 mJy.
This corresponds to a range of measured detection statistics
x ~ 0.5–27 Jy ms1/2
, compared to the range x ~ 0.2–
5 Jy ms1/2 measured for the four detected FRB 121102 bursts.
Figure 4 shows the obtained positions for the different
PSR B0525+21 pulses along with the synthesized beam
FWHM for comparison. This demonstrates that the positional
accuracy of the bursts increases for larger ξ. It shows that
pulses with x 5 Jy ms1/2 are typically offset by less than the
beam FWHM, whereas for x ~ 0.5–1 Jy ms1/2 the scatter can
be closer to 10 mas. This matches well with what we have
observed in the four detected FRB 121102 bursts (Table 1;
Figure 1).
While burst #2 is thus expected to provide the most accurate
position, a more conservative way to estimate the positional
error on the burst source is to consider the scatter in all four
detections, which is ∼10 mas around the average position. In
Figure 1, we show the average position from the four observed
bursts, weighted by their detection statistic ξ (Table 1). This
average position (separated ~8 mas with respect to the
persistent source) shows that the average burst position and
the persistent source position are coincident within 2s. We
therefore claim no significant positional offset between the
persistent radio source and the source of the FRB 121102
bursts.
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 12 mas (40 pc) at the 95%
confidence level (or 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.
3.3. Measured Properties
Fitting the uv-plane data with a circular Gaussian component
shows that both the bursts and persistent radio source appear to
be slightly extended. We measure a source size of ~ 2 1 mas
at 1.7 GHz in the detected bursts in the uv-plane. In the
persistent source, we measure a similar value of 2–4 mas at
1.7 GHz in all sessions, whereas at 5.0 GHz we measure an
angular size of ∼0.2–0.4 mas. Measurements in the image
plane (after deconvolving the synthesized beam) result in
similar values. The measured source sizes for the persistent
source are consistent with the ones obtained in Chatterjee
et al. (2017).
Figure 3. Top: amplitudes and phases of the obtained visibilities for the
strongest burst observed on 2016 September 20 (burst #2 in Table 1) as a
function of the uv-distance. Bottom: dirty (left) and cleaned (right) image for
the same burst. The cleaned image has been obtained by fitting the uv-data with
a circular Gaussian component. The synthesized beam is shown by the gray
ellipse at the bottom right of the figure. The coordinates are relative to the
position of the persistent source obtained in the same epoch.
Figure 4. Pulse localizations from the pulsar B0525+21 observed on 2016
February 11 at 1.7 GHz. A total of 406 pulses were imaged. Systematic
uncertainties inversely proportional to the detection statistic ξ are observed. We
note that only pulses with x 5 Jy ms1/2 are robustly localized to within the
FWHM, and for pulses with x ~ 0.5–1 Jy ms1/2 the scatter can be closer to
10 mas.
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The Astrophysical Journal Letters, 834:L8 (9pp), 2017 January 10 Marcote et al.
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; J. W. T. Hessels et al. 2017, in preparation).
Furthermore, as can be seen in the dynamic spectrum of the
burst, there is fine-scale frequency structure (∼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.
The different epochs at which the persistent radio source was
observed allow us to obtain the light curve of the compact
source. Figure 5 shows the flux densities measured for the five
sessions at 1.7 GHz, which are compatible with an average flux
density of S1.7 = 177 18 Jy m . The only session at 5.0 GHz
shows a compact source with a flux density of
S5 = 123 14 Jy m . Assuming that the source exhibited a
similar flux density at 1.7 GHz compared with the day before,
we infer a two-point spectral index a =- 0.27 0.24, where
Sn µ na, for the source. Table 1 summarizes the obtained
results.
4. Discussion
Chatterjee et al. (2017) have shown that the persistent radio
source is associated with an optical counterpart, which
Tendulkar et al. (2017) show is a low-metallicity, star-forming
dwarf galaxy at a redshift of z = 0.19273 0.00008. In the
following, we use the luminosity and angular diameter distance
of DL » 972 Mpc and DA » 683 Mpc, respectively, determined
by Tendulkar et al. (2017). We show that the VLBI data
alone provide further support to the extragalactic origin of both
radio sources. Furthermore, we argue that the bursts and the
persistent radio source must be physically related because of
their close proximity to each other. We assume such a direct
physical link in the following discussion.
4.1. Persistent Source and Burst Properties
The results from all the EVN observations conducted at
1.7 GHz show a compact source with a persistent emission of
~180 Jy m , which is consistent with the flux densities inferred
at ~ ´ 100 larger angular scales with the VLA (Chatterjee
et al. 2017). No significant, short-term changes in the flux
density are observed after the arrival of the bursts or otherwise
(Figure 5). The average flux density of the persistent source
implies a radio luminosity of L1.7 3 10 erg s » ´ 38 1 - . The
single measured flux density at 5.0 GHz corresponds to a
similar luminosity of L5.0 7 10 erg s » ´ 38 1 - (nLn, with a
bandwidth of 128 MHz at both frequencies). Additionally, the
5.0 GHz data allow us to set a constraint on the brightness
temperature of the persistent source of Tb 5 10 K ´ 7 .
Considering the measured radio luminosities and the current
5σ X-ray upper limit in the 0.5–10 keV band of
5 ´ 10 erg s cm - -- 15 1 2 (Chatterjee et al. 2017; which implies
LX 5 10 erg s < ´ 41 1 - ), we infer a ratio between the 5.0 GHz
radio and X-ray luminosities of log 2.4 RX > - , where
RX X = nL L n ( ) (– ) 5 GHz 2 10 keV as defined by Terashima
& Wilson (2003). The strongest observed burst exhibits a
luminosity of 6 10 erg s ~ ´ 42 1 - at 1.7 GHz in the 2 ms
integrated data. These values imply an energy of
10 4 erg 40 ~ DW ( ) p , where DW is the emission solid angle.
With the EVN sessions spanning a period of approximately 7
months, we derived a constraint on the proper motion of the
persistent source of 6.4 1.4 mas yr 1 - < < ma - and
2.8 6.2 mas yr 1 - < < md - at a 3σ confidence level. These
values have been obtained after removing the offsets measured
in the in-beam calibrator source (VLA2; see Section 2). Since
most of our short observations were not ideal for astrometry,
this is a preliminary result, to be further improved on by
follow-up observations, which will also have the advantage of
spanning a longer period of time. Nonetheless, these results
already rule out the presence of parallax 3 mas at a 3σ
confidence level, setting a distance for the persistent source
0.3 kpc.
The compactness of the source at 5.0 GHz allows us to set an
upper limit on its projected physical size of 0.7 pc
(1.4 10 au ´ 5 , given the distance of the source). The angular
size measured for the source at 1.7 and 5.0 GHz (∼2 and
∼0.2 mas, respectively) is consistent with the angular broadening
expected in the direction to FRB 121102 for extragalactic
sources due to local scattering (multi-path propagation)
of the signal by the intervening Galactic material between the
source and the observer (predicted by the NE2001 model to be
~2 mas at 1.7 GHz; Cordes & Lazio 2002). Angular broadening
scales as 2 µn- , and thus we would expect a size of
~0.4 mas at 5.0 GHz, also roughly consistent with the
measured value at that frequency.
The obtained angular sizes are thus likely to be produced by
angular broadening and not by the fact that we are resolving the
Figure 5. Top: light curve of the persistent source at 1.7 GHz during all the
EVN epochs. The horizontal line represents the average flux density value and
its 1σ standard deviation. Bottom: light curve of the source within the 2016
September 20 epoch (last epoch in the top figure). The vertical red lines
represent the arrival times of the four detected bursts. We do not detect
brightening of the persistent source on these timescales after the bursts.
6
The Astrophysical Journal Letters, 834:L8 (9pp), 2017 January 10 Marcote et al.
source. The angular broadening measured in the bursts
(∼2 mas) supports this statement as this broadening must be
produced extrinsically to the bursts, given that their millisecond
duration implies that the emitting region must be smaller than
the light-crossing time, i.e., 1000 km, and thus must appear to
be point-like. A caveat here is that the measured source size
depends strongly on the gain of the telescope providing the
longest baselines (Natarajan et al. 2017), in our case Arecibo at
1.7 GHz. We exclude the possibility that the Arecibo baselines
have a lower amplitude (due to a large gain error), because the
measured persistent source size agrees with that obtained with
the VLBA independently (Chatterjee et al. 2017). At 5.0 GHz
the presence of three telescopes with similar baseline lengths
(Arecibo, Hartebeesthoek, and Tianma) also assures the
consistency of the measured source size.
The intrinsic source size of the persistent source could be as
small as 7 as 1
5 4 m n- , the limit implied by synchrotron selfabsorption
for a frequency of optical depth unity n1 ~ 1 GHz
(Harris et al. 1970). Or if Tb 10 12 K by synchrotron selfCompton
radiation, the size is 20 as 1 m n- . These angles are
too small to resolve with VLBI but could be probed with
interstellar scintillations.
We have constrained the projected separation between the
source of the bursts and the persistent radio source to be
40 pc. Such a close proximity strongly suggests that there is a
direct physical link between the bursts and the persistent
source, as we now discuss in more detail.
4.2. Possible Origins of FRB 121102
The data presented here, in addition to the results presented
by Chatterjee et al. (2017) and Tendulkar et al. (2017), allow us
to constrain the possible physical scenarios for the origin of
FRB 121102. While the fact that the bursts are located within
40 pc of the persistent radio source strongly suggests a direct
physical link, the persistent radio source and the source of the
FRB 121102 bursts do not necessarily have to be the same
object. We primarily consider two classes of models that could
explain FRB 121102 and its multiwavelength counterparts: a
highly energetic, extragalactic neutron star in a young supernova
remnant (SNR) or an AGN (or analogously a black hole
related system with a jet).
4.2.1. Young Neutron Star and Nebula
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.
The measured luminosity for the persistent radio source
cannot be explained by a single SNR or a pulsar wind nebula
similar to those discovered thus far in our Galaxy. A direct
comparison with one of the brightest SNRs known, Cas A
(300 years old; Baars et al. 1977; Reed et al. 1995), shows that
we would expect an emission which is ∼4 orders of magnitude
fainter at the given distance of DL » 972 Mpc. In the case of
the Crab Nebula, the expected flux density would be even
fainter (~0.5 nJy) if placed in the host galaxy of FRB 121102.
Compact star-forming regions, such as seen in Arp 220, have
collections of SNRs that have a luminosity and Tb consistent
with the persistent radio source. However, neither the SFR of
~240 1000 yr M -1 – nor the size of the region of 250–360 pc
of Arp 220 (Anantharamaiah et al. 2000) agrees with the
properties of the persistent source associated with FRB 121102
(0.4 yr M -1
and 0.7 pc).
Murase et al. (2016) and Piro (2016) discuss the properties of
a young (<1000 years) SNR that is powered by the spin-down
power of a neutron star or white dwarf. The SNR expands into
the surrounding medium and evacuates an ionized region that
can be seen as a luminous synchrotron nebula. This model is
also constrained by the observation that radio bursts of
FRB 121102 are not absorbed by the nebula and that its DM
has not evolved significantly over the last few years.
Considering all these effects, in this scenario, FRB 121102 is
likely to be between 100 and 1000 years old, and the persistent
radio source is powered by the spin down of a rapidly rotating
pulsar or magnetar. In this case, the previously shown
persistent radio source variability (Chatterjee et al. 2017; where
the higher cadence of observations allowed variability to be
studied in more detail) could be induced by scintillation, which
is consistent with the compact (sub-milliarcsecond) SNR size at
this age. Lunnan et al. (2014) and Perley et al. (2016) show that
superluminous supernovae (SLSNe) are typically hosted by
low-metallicity, low-mass galaxies and are possibly powered
by millisecond magnetars. Additionally, it is shown that SLSNe
and long gamma-ray bursts could share similar environments.
We note that these conditions are in agreement with the optical
galaxy associated with FRB 121102 (Tendulkar et al. 2017).
4.2.2. AGN/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 2017). 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.
The persistent radio source is offset by ∼0.2 arcsec (0.7 kpc)
from the apparent center of the optical emission of the dwarf
galaxy (Tendulkar et al. 2017). Therefore, it is not completely
clear whether the radio source can be associated with the
galactic nucleus or not, but an offset AGN is plausible as
reported in other galaxies (Barth et al. 2008). If the persistent
source is indeed an AGN in the right accretion state, we can
infer the mass of the black hole assuming the Fundamental
Plane of black hole activity (Merloni et al. 2003; Falcke
et al. 2004; Körding et al. 2006; Miller-Jones et al. 2012;
Plotkin et al. 2012). Given the measured radio luminosity and
the upper limit on the X-ray value, we estimate a lower limit on
the mass of the putative black hole of 2 10 ´ 9 M. This value
would be hard to reconcile with the fact that the stellar mass of
the host galaxy is likely at least an order of magnitude less than
that, and its optical spectrum shows no signatures of AGN
activity (Tendulkar et al. 2017).
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
7
The Astrophysical Journal Letters, 834:L8 (9pp), 2017 January 10 Marcote et al.
Fundamental Plane outliers (i.e., radio-loud, with RX ~ -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 RX
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.
5. Conclusions
The bursts of FRB 121102 have recently been associated
with a persistent and compact radio source (Chatterjee
et al. 2017) and a low-metallicity star-forming dwarf galaxy
at a redshift of z = 0.19273 0.00008 (Tendulkar et al.
2017). The EVN data presented in this work show for the first
time that the bursts and the persistent source are co-located
with an angular separation 12 mas (40 pc given the distance
to the host galaxy). This tight constraint—roughly an order of
magnitude more precise localization compared to that achieved
with the VLA in Chatterjee et al. (2017)—strongly suggests a
direct physical link, though the persistent radio source and the
source of the FRB 121102 bursts do not necessarily have to be
the same object. Although the origin of FRBs remains
unknown, the data presented here are consistent in many
respects with either an interpretation in terms of a lowluminosity
AGN or a young SNR powered by a highly
energetic neutron star/magnetar.
We thank the directors and staff of all the EVN telescopes
for making this series of target of opportunity observations
possible. We thank the entire staff of the Arecibo Observatory,
and in particular A. Venkataraman, H. Hernandez, P. Perillat,
and J. Schmelz, for their continued support and dedication to
enabling observations like those presented here. We thank
B. Stappers and M. Mickaliger for their support with simultaneous
pulsar recording using the Lovell Telescope. We thank
E. Adams, K. Kashiyama, N. Maddox, and E. Quataert for
useful discussions on plausible scenarios as well as O. Wucknitz
and A. Deller for reviewing a draft of the Letter. We thank
F. Camilo for access to computing resources. The Arecibo
Observatory is operated by SRI International under a
cooperative agreement with the National Science Foundation
(AST-1100968), and in alliance with Ana G. Méndez-Universidad
Metropolitana, and the Universities Space Research
Association. The European VLBI Network is a joint facility of
independent European, African, Asian, and North American
radio astronomy institutes. Scientific results from data
presented in this publication are derived from the following
EVN project codes: RP024 and RP026 (PI: J. Hessels). This
research made use of Astropy, a community-developed core
Python package for Astronomy (Astropy Collaboration
et al. 2013) and APLpy, an open-source plotting package for
Python hosted at http://aplpy.github.com. B.M. acknowledges
support by the Spanish Ministerio de Economía y
Competitividad (MINECO) under grants AYA2013-47447-
C3-1-P, AYA2016-76012-C3-1-P, and MDM-2014-0369 of
ICCUB (Unidad de Excelencia “María de Maeztu”). J.W.T.H.
is an NWO Vidi Fellow. J.W.T.H. and C.G.B. gratefully
acknowledge funding from the European Research Council
under the European Unionʼs Seventh Framework Programme
(FP/2007-2013)/ERC Grant Agreement No. 337062 (DRAGNET).
Y.H. would like to acknowledge the support of the
ASTRON/JIVE International Summer Student Programme.
ASTRON is an institute of the Netherlands Organisation for
Scientific Research (NWO). The Joint Institute for VLBI ERIC,
is a European entity established by 6 countries and funded by
10 agencies to support the use of the European VLBI Network.
S.C., J.M.C., P.D., M.A.M., and S.M.R. are partially supported
by the NANOGrav Physics Frontiers Center (NSF award
1430284). Work at Cornell (J.M.C., S.C.) was supported by
NSF grants AST-1104617 and AST-1008213. M.A.M. is
supported by NSF award #1458952. L.G.S. gratefully
acknowledges financial support by the European Research
Council for the ERC Starting Grant BEACON under contract
No. 279702, and the Max Planck Society. V.M.K. holds the
Lorne Trottier Chair in Astrophysics and Cosmology and a
Canadian Research Chair in Observational Astrophysics and
received additional support from NSERC via a Discovery
Grant and Accelerator Supplement, by FQRNT via the Centre
de Recherche Astrophysique de Québec, and by the Canadian
Institute for Advanced Research. S.B.-S. is a Jansky Fellow of
the National Radio Astronomy Observatory. Part of the
research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the
National Aeronautics and Space Administration. P.S. is a
Covington Fellow at the Dominion Radio Astrophysical
Observatory. S.P.T. acknowledges support from a McGill
Astrophysics postdoctoral fellowship.
Facilities: EVN, Arecibo Observatory.
Software: AIPS, Difmap, ParselTongue, CASA, Astropy,
APLpy, PRESTO.
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