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The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102
Tendulkar, S.P.; Bassa, C.G.; Cordes, J.M.; Bower, G.C.; Law, C.J.; Chatterjee, S.; Adams,
E.A.K.; Bogdanov, S.; Burke-Spolaor, S.; Butler, B.J.; Demorest, P.; Hessels, J.W.T.; Kaspi,
V.M.; Lazio, T.J.W.; Maddox, N.; Marcote, B.; McLaughlin, M.A.; Paragi, Z.; Ransom, S.M.;
Scholz, P.; Seymour, A.; Spitler, L.G.; van Langevelde, H.J.; Wharton, R.S.
DOI
10.3847/2041-8213/834/2/L7
Publication date
2017
Document Version
Author accepted manuscript
Published in
Astrophysical Journal Letters
Link to publication
Citation for published version (APA):
Tendulkar, S. P., Bassa, C. G., Cordes, J. M., Bower, G. C., Law, C. J., Chatterjee, S.,
Adams, E. A. K., Bogdanov, S., Burke-Spolaor, S., Butler, B. J., Demorest, P., Hessels, J. W.
T., Kaspi, V. M., Lazio, T. J. W., Maddox, N., Marcote, B., McLaughlin, M. A., Paragi, Z.,
Ransom, S. M., ... Wharton, R. S. (2017). The Host Galaxy and Redshift of the Repeating
Fast Radio Burst FRB 121102.
Astrophysical Journal Letters
,
834
(2), [L7].
https://doi.org/10.3847/2041-8213/834/2/L7
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THE HOST GALAXY AND REDSHIFT OF THE REPEATING FAST RADIO BURST FRB 121102
S. P. Tendulkar,
1
C. G. Bassa,
2
J. M. Cordes,
3
G. C. Bower,
4
C. J. Law,
5
S. Chatterjee,
3
E. A. K. Adams,
2
S. Bogdanov,
6
S. Burke-Spolaor,
7, 8, 9
B. J. Butler,
7
P. Demorest,
7
J. W. T. Hessels,
2, 10
V. M. Kaspi,
1
T. J. W. Lazio,
11
N. Maddox,
2
B. Marcote,
12
M. A. McLaughlin,
8, 9
Z. Paragi,
12
S. M. Ransom,
13
P. Scholz,
14
A. Seymour,
15
L. G. Spitler,
16
H. J. van Langevelde,
12, 17
and R. S. Wharton
3
1
Department of Physics and McGill Space Institute, McGill University, 3600 University St., Montreal, QC H3A 2T8, Canada
2
ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, The Netherlands
3
Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
4
Academia Sinica Institute of Astronomy and Astrophysics, 645 N. A’ohoku Place, Hilo, HI 96720, USA
5
Department of Astronomy and Radio Astronomy Lab, University of California, Berkeley, CA 94720, USA
6
Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
7
National Radio Astronomy Observatory, Socorro, NM 87801, USA
8
Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
9
Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505
10
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
11
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
12
Joint Institute for VLBI ERIC, Postbus 2, 7990 AA Dwingeloo, The Netherlands
13
National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
14
National Research Council of Canada, Herzberg Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory, P.O. Box 248,
Penticton, BC V2A 6J9, Canada
15
Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA
16
Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, Bonn, D-53121, Germany
17
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
(Received January 6, 2017; Revised January 6, 2017; Accepted January 6, 2017)
Submitted to ApJ
ABSTRACT
The precise localization of the repeating fast radio burst (FRB 121102) has provided the first unambiguous association
(chance coincidence probability p . 3 × 10
−4
) of an FRB with an optical and persistent radio counterpart. We report
on optical imaging and spectroscopy of the counterpart and find that it is an extended (0.
00
6 − 0.
00
8) object displaying
prominent Balmer and [O III] emission lines. Based on the spectrum and emission line ratios, we classify the counterpart
as a low-metallicity, star-forming, m
r
0
= 25.1 AB mag dwarf galaxy at a redshift of z = 0.19273(8), corresponding to
a luminosity distance of 972 Mpc. From the angular size, the redshift, and luminosity, we estimate the host galaxy
to have a diameter . 4 kpc and a stellar mass of M
∗
∼ 4 − 7 × 10
7
M
, assuming a mass-to-light ratio between
2 to 3 M
L
−1
. Based on the Hα flux, we estimate the star formation rate of the host to be 0.4 M
yr
−1
and a
substantial host dispersion measure depth . 324 pc cm
−3
. The net dispersion measure contribution of the host galaxy
to FRB 121102 is likely to be lower than this value depending on geometrical factors. We show that the persistent
radio source at FRB 121102’s location reported by Marcote et al. (2017) is offset from the galaxy’s center of light
by ∼200 mas and the host galaxy does not show optical signatures for AGN activity. If FRB 121102 is typical of
the wider FRB population and if future interferometric localizations preferentially find them in dwarf galaxies with
Corresponding author: S. P. Tendulkar, C. G. Bassa
shriharsh@physics.mcgill.ca; bassa@astron.nl
arXiv:1701.01100v2 [astro-ph.HE] 5 Jan 2017
2 Tendulkar et al.
low metallicities and prominent emission lines, they would share such a preference with long gamma ray bursts and
superluminous supernovae.
Keywords: stars: neutron – stars: magnetars – galaxies: distances and redshifts – galaxies: dwarf –
galaxies: ISM
The host of FRB 121102 3
1. INTRODUCTION
Fast radio bursts (FRBs) are bright (∼Jy) and short
(∼ms) bursts of radio emission that have dispersion
measures (DMs) in excess of the line of sight DM con-
tribution expected from the electron distribution of our
Galaxy. To date 18 FRBs have been reported — most
of them detected at the Parkes telescope (Lorimer et al.
2007; Thornton et al. 2013; Burke-Spolaor & Bannister
2014; Keane et al. 2012; Ravi et al. 2015; Petroff et al.
2015; Keane et al. 2016; Champion et al. 2016; Ravi
et al. 2016) and one each at the Arecibo (Spitler et al.
2014) and Green Bank telescopes (Masui et al. 2015).
A plethora of source models have been proposed to
explain the properties of FRBs (see e.g. Katz 2016, for
a brief review). According to the models, the excess
DM for FRBs may be intrinsic to the source, placing it
within the Galaxy; it may arise mostly from the inter-
galactic medium, placing a source of FRBs at cosmolog-
ical distances (z ∼ 0.2 − 1) or it may arise from the host
galaxy, placing a source of FRBs at extragalactic, but
not necessarily cosmological, distances (∼ 100 Mpc).
Since the only evidence to claim an extragalactic ori-
gin for FRBs has been the anomalously high DM, some
models also attempted to explain the excess DM as a
part of the model, thus allowing FRBs to be Galactic.
All FRBs observed to date have been detected with sin-
gle dish radio telescopes, for which the localization is of
order arcminutes, insufficient to obtain an unambiguous
association with any object. To date, no independent in-
formation about their redshift, environment, and source
could be obtained due to the lack of an accurate localiza-
tion of FRBs. Keane et al. (2016) attempted to identify
the host of FRB 150418 on the basis of a fading radio
source in the field that was localized to a z = 0.492
galaxy. However, later work identified the radio source
as a variable active galactic nucleus (AGN) that may
not be related to the source (Williams & Berger 2016;
Bassa et al. 2016; Giroletti et al. 2016; Johnston et al.
2017).
Repeated radio bursts were observed from the loca-
tion of the Arecibo-detected FRB 121102 (Spitler et al.
2016; Scholz et al. 2016), with the same DM as the first
detection, indicating a common source. As discussed by
Spitler et al. (2016), it is unclear whether the repeti-
tion makes FRB 121102 unique among known FRBs, or
whether radio telescopes other than Arecibo lack the
sensitivity to readily detect repeat bursts from other
known FRBs.
Chatterjee et al. (2017) used the Karl G. Jansky Very
Large Array (VLA) to directly localize the repeated
bursts from FRB 121102 with 100-mas precision and re-
ported an unresolved, persistent radio source and an ex-
tended optical counterpart at the location with a chance
coincidence probability of ≈ 3 × 10
−4
— the first unam-
biguous identification of multi-wavelength counterparts
to FRBs. Independently, Marcote et al. (2017) used the
European VLBI Network (EVN) to localize the bursts
and the persistent source and showed that both are co-
located within ∼ 12 milliarcseconds.
Here we report the imaging and spectroscopic follow-
up of the optical counterpart to FRB 121102 using the
8-m Gemini North telescope.
2. OBSERVATIONS AND DATA ANALYSIS
The location of FRB 121102 was observed with the
Gemini Multi-Object Spectrograph (GMOS) instrument
at the 8-m Gemini North telescope atop Mauna Kea,
Hawai’i. Imaging observations were obtained with SDSS
r
0
, i
0
and z
0
filters on 2016 October 24, 25, and November
2, under photometric and clear conditions with 0.
00
58 to
0.
00
66 seeing. Exposure times of 250 s were used in the
r
0
filter and of 300 s in the i
0
and z
0
filters with total
exposures of 1250 s in r
0
, 1000 s in i
0
and 1500 s in z
0
. The
detectors were read out with 2 × 2 binning, providing a
pixel scale of 0.
00
146 pix
−1
. The images were corrected for
a bias offset, as measured from the overscan regions, flat
fielded using sky flats and then registered and co-added.
The images were astrometrically calibrated against
the Gaia DR1 Catalog (Gaia Collaboration et al. 2016).
To limit the effects of distortion, the central 2.
0
2 × 2.
0
2
subsection of the images were used. Each of the r
0
, i
0
,
and z
0
images were matched with 35 – 50 unblended
stars yielding an astrometric calibration with 7 – 9 mas
root-mean-square (rms) position residuals in each coor-
dinate after iteratively removing ∼ 4 − 5 outliers. The
error in the mean astrometric position with respect to
the Gaia frame is thus ∼ 1 − 2 mas.
We used the Source Extractor (Bertin & Arnouts
1996) software to detect and extract sources in the coad-
ded images. The r
0
and i
0
images were photometri-
cally calibrated with respect to the IPHAS DR2 cat-
alog (Barentsen et al. 2014) using Vega-AB magnitude
conversions stated therein. We measure isophotal in-
tegrated magnitudes of m
r
0
= 25.1 ± 0.1 AB mag and
m
i
0
= 23.9 ± 0.1 AB mag for the optical counterpart of
FRB 121102. The error value includes the photometric
errors and rms zero-point scatter. Ongoing observations
will provide full photometric calibration in g
0
, r
0
, i
0
, and
z
0
bands and will be reported in a subsequent publica-
tion.
Spectroscopic observations were obtained with GMOS
on 2016 November 9 and 10 with the 400 lines mm
−1
grating (R400) in combination with a 1
00
slit, covering
the wavelength range from 4650 to 8900
˚
A. A total of
4 Tendulkar et al.
Figure 1. The co-added spectrum of the host galaxy of FRB 121102, the reference object, and the sky contribution (scaled
by 10% and offset by −3 µJy). The spectra have been resampled to the instrumental resolution. Prominent emission lines are
labelled with their rest frame wavelengths. Black horizontal bars denote the wavelength ranges of the filters used for imaging.
Most of the wavelength coverage of the z
0
band is outside the coverage of this plot.
nine 1800 s exposures were taken with 2×2 binning, pro-
viding a spatial scale of 0.
00
292 pix
−1
and an instrumental
resolution of 4.66
˚
A, sampled at 1.36
˚
A pix
−1
. The con-
ditions were clear, with 0.
00
8 to 1.
00
0 seeing on the first
night, and 0.
00
9 to 1.
00
1 on the second. To aid the spec-
tral extraction of the very faint counterpart, the slit was
oriented at a position angle of 18.
◦
6, containing the coun-
terpart to FRB 121102 as well as an m
r
0
= 24.3 AB mag,
m
i
0
= 22.7 AB mag foreground star, located 2.
00
8 to the
South (shown later in Figure 3).
The low signal-to-noise of the spectral trace of the
FRB counterpart on the individual bias-corrected long-
slit spectra complicated spectral extraction through the
optimal method by Horne (1986). Instead, we used
a variant of the optimal extraction method of Hynes
(2002) by modelling the spectral trace of the reference
object by a Moffat function (Moffat 1969) to determine
the position and width of the spatial profile as a function
of wavelength. Because of the proximity of the reference
object to the FRB counterpart (20 pix), we assume that
the spatial profile as a function of wavelength is iden-
tical for both. We note that though the counterpart is
slightly resolved in the imaging observations, the worse
seeing during the spectroscopic observations (by a fac-
tor 1.2 to 1.9) means the seeing dominates the spatial
profile. The residual images validate this assumption;
no residual flux is seen once the extracted model is sub-
tracted from the image. To optimally extract the spec-
tra of the FRB counterpart, the reference object as well
as the sky background, we then simultaneously fit the
spatial profile at the location of the counterpart and at
the location of the reference object on top of a spatially
varying linear polynomial for each column in the disper-
sion direction.
Wavelength calibrations were obtained from arc lamp
exposures, modelling the dispersion location to wave-
length through 4th order polynomials, yielding rms
residuals of better than 0.2
˚
A. The individual wavelength
calibrated spectra were then combined and averaged.
The instrumental response of the spectrograph was cal-
ibrated using an observation of the spectrophotometric
standard Hiltner 600 (Hamuy et al. 1992, 1994), which
was taken on 2016 November 7 as part of the stan-
dard Gemini calibration plan with identical instrumen-
tal setup as the science observations. The flux-calibrated
spectrum of the reference object gives a spectroscopic
AB magnitude of m
i
0
≈ 22.6, about 11% higher than
derived from photometry. Given that the spectrophoto-
metric standard was observed on a different night with
worse seeing (1.
00
4), we attribute this difference to slit
losses and scale the flux of the observed spectra of the
reference object and the FRB counterpart by a factor
0.89.
3. RESULTS AND ANALYSIS
The final combined and calibrated spectrum is shown
in Figure 1. Besides continuum emission, which is