A luminous fast radio burst that probes the Universe at redshift 1 S. D. Ryder12 K. W. Bannister3 S. Bhandari45 A. T. Deller6 R. D. Ekers37

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A luminous fast radio burst that probes the Universe at

redshift 1

S. D. Ryder1,2, K. W. Bannister3, S. Bhandari4,5, A. T. Deller6, R. D. Ekers3,7,

M. Glowacki7, A. C. Gordon8, K. Gourdji6, C. W. James7,

C. D. Kilpatrick8,9, W. Lu10,11, L. Marnoch1,2,3,12, V. A. Moss3,

J. X. Prochaska13,14, H. Qiu15, E. M. Sadler16,3, S. Simha13,

M. W. Sammons7, D. R. Scott7, N. Tejos17, R. M. Shannon6,∗

1School of Mathematical and Physical Sciences, Macquarie University, NSW 2109, Australia

2Astrophysics and Space Technologies Research Centre, Macquarie University, Sydney, NSW 2109, Australia

3Australia Telescope National Facility, Commonwealth Science and Industrial Research Organisation (CSIRO),

Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia

4The Netherlands Institute for Radio Astronomy (ASTRON), 7991 PD Dwingeloo, The Netherlands

5Joint institute for Very Long Baseline Interferometry in Europe, 7991 PD Dwingeloo, The Netherlands

6Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn,VIC 3122, Australia

7International Centre for Radio Astronomy Research, Curtin Institute of Radio Astronomy,

Curtin University, Perth, Western Australia, Australia.

8Center for Interdisciplinary Exploration and Research in Astrophysics, Northwestern University,

Evanston, IL 60208, USA

9Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA

10Department of Astronomy University of California, Berkeley, CA 94720, USA

11 Theoretical Astrophysics Center, University of California, Berkeley, CA 94720, USA

12 Australian Research Council Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia

13 Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA

14 Kavli Institute for the Physics and Mathematics of the Universe, Kashiwa, 277-8583, Japan

15SKA Observatory, Jodrell Bank, Lower Withington, Macclesﬁeld, SK11 9FT, UK

16Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia

17Instituto de F´

ısica, Pontiﬁcia Universidad Cat´

olica de Valpara´

ıso, Casilla 4059, Valpara´

ıso, Chile

∗Corresponding author: E-mail: rshannon@swin.edu.au.

arXiv:2210.04680v2 [astro-ph.HE] 25 Oct 2023

Fast radio bursts (FRBs) are millisecond-duration pulses of radio emission

originating from extragalactic distances. Radio dispersion on each burst is im-

parted by intervening plasma mostly located in the intergalactic medium. We

observe a burst, FRB 20220610A, in a morphologically complex host galaxy

system at redshift 1.016 ±0.002. The burst redshift and dispersion are con-

sistent with passage through a substantial column of material from the inter-

galactic medium. The burst shows evidence for passage through additional

turbulent magnetized plasma, potentially associated with the host galaxy. We

use the burst energy of 2×1042 erg, to revise the maximum energy of an FRB.

Fast radio bursts (FRBs, (1, 2)) are transient radio sources that last a few milliseconds emit-

ted by extragalactic sources. Free electrons along the path between the FRB source and the

Earth impart a frequency-dependent time delay (dispersion) on the radio signal. This dispersion

can be used to measure the column density of free electrons (quantiﬁed by the dispersion mea-

sure, DM) between the FRB source and observer. FRBs localized to host galaxies at different

redshifts exhibit a positive correlation between extragalactic DM and host redshift, known as

the Macquart relation (3). This relation has been used to measure the cosmic baryon fraction

and the expansion rate of the Universe (4). This relation has been measured using identiﬁed

FRB host galaxies at relatively low redshifts, z≲0.5. Some unlocalized FRBs (with unknown

host galaxies) have DMs consistent with z > 1(5); however an FRB associated with a galaxy

at z= 0.241 had a high DM that would have implied z > 1(6). This indicates that estimates

of redshift from DM alone can be misled by plasma within the host galaxy, which also imparts

a contribution to the DM, in addition to that of the intergalactic medium.

Observations of FRB 20220610A

FRBs have been searched for (7) and localized (8) using the Australian Square Kilometre

Array Pathﬁnder (ASKAP, (9)), a radio interferometer in Western Australia comprising 36, 12-

m antennas. Each antenna is equipped with phased-array receiving systems, which provide 36

beams across the focal plane, covering approximately 30 square degrees. FRBs are searched for

in real time using the incoherent sum of the intensities of each antenna in each beam. When an

FRB is detected, voltage buffers are downloaded, correlated, calibrated, and imaged, enabling

the position of the burst to be measured to an absolute precision of typically a few tenths of an

arcsecond (10, 11).

We detected FRB 20220610A in observations around the previously known burst FRB 20220501C,

but the two bursts are not related (11). The observations were centred at a frequency of

1271.5MHz and had a time resolution of 1.18 ms. The dispersion of FRB 20220610A indi-

cated a DM of 1458.15+0.25

−0.55 pc cm−3. This is higher than all but one of the 55 FRBs previously

observed using ASKAP (4). A dedispersed dynamic spectrum of the burst is shown in Figure 1,

and properties of the burst are listed in Table 1. The burst does not show the 10-100 MHz modu-

lation in the spectrum characteristic of many lower DM, high Galactic latitude ASKAP-detected

FRBs (7).

The 2 s dispersive sweep of the burst across the instrument bandwidth and 2.4 s latency in

the detection system resulted in only the lowest 88 MHz of the burst being captured in the 3.1s-

duration voltage buffer. This was sufﬁcient to localize the burst to a precision of 0.5 arcsec. We

used the voltage data to reconstruct the high time resolution and polarimetric properties of the

burst (11). After correcting for dispersive smearing, the burst shows an exponentially decreasing

tail (Fig. 1D), which is consistent with scatter broadening due to turbulence in intervening

plasma (12). We measure the pulse broadening time to be 0.511 ±0.012 ms at a reference

frequency of 1147.5MHz, assuming a ν−4frequency dependence.

Ordered magnetic ﬁelds in astrophysical plasmas add additional, polarization-dependent

dispersion. This manifests as wavelength-dependent variation in the linear polarization position

angle, referred to as Faraday rotation (13). The burst exhibits Faraday rotation, with a rotation

measure (RM) of 215 ±2rad m−2. After correcting for this Faraday rotation, we ﬁnd the burst

had a linear polarization fraction of 96±1%. The high fractional linear polarization allows us to

place a 67% upper limit on the Faraday dispersion (σRM <0.6rad m−2) induced by ﬂuctuations

in rotation measure in intervening turbulent plasma. Higher levels of Faraday dispersion have

been detected for other FRBs (14). The burst also shows modest fractional circular polarization

of 10±1%. While instrumental artifacts can induce spurious circular polarization, we do not see

any correlation between Stokes polarization parameters Uand Vin the spectrum, which would

be expected for an instrumental effect (11). The FRB was located approximately 4arcmin from

the beam center, which makes off-axis leakage effects less likely (15). Circular polarization

has been observed in some FRBs and could either be intrinsic to the burst (16) or result from

propagation through relativistic plasma in the immediate source environment (17).

Host-galaxy properties

We performed follow-up ground-based optical and infrared observations with the Very

Large Telescope (VLT) and the W. M. Keck Observatory to identify and characterize the host

galaxy of FRB 20220610A (11). The images (Fig. 2A-C) show an object coincident with the

source that has an extended, multi-component morphology. We label the optical source that

overlaps the radio position of the FRB as component (a), and two adjacent sources as compo-

nents (b) and (c) (Fig. 2A). We use a Bayesian method to assess the chance of coincidence

between transients and host galaxies (18), ﬁnding greater than 99.99% conﬁdence that the FRB

is associated with component (a).

We performed broad band optical and infrared spectroscopy of components (a), (b) and (c)

(Fig. 2D-E) (11). We identify two emission lines in the spectra as the [O II] 3726 and 3729 ˚

doublet, most prominently in component (b) (Fig. S2). From this we measure the redshift of

each component, ﬁnding they are all consistent with z= 1.016 ±0.002.

We estimate the total mass of the three components combined to be 1010 solar masses, with

a star formation rate of ∼0.42 solar masses per year (11). These values, in addition to the host

metallicity and star formation history are consistent with those of nearby FRB hosts (19,20), but

the source morphology is markedly different. Properties of the host galaxy are listed in Table 1.

The presence of two bright components (a) and (c) separated by 2.0 arcseconds (which

corresponds to a distance of 16 kpc at that redshift), and the diffuse feature (b) between them,

is consistent with two galaxies interacting or merging, or a compact galaxy group. It is also

possible that the morphology is due to internal structure within a single galaxy; at these redshifts

about half of all galaxies have clumpy morphologies (21). We regard the latter possibility as

unlikely, due to the large spatial separation between the components. Only component (a) is

detected in the near-infrared (Ks-band) image (Fig. 2C), indicating it hosts an older stellar

population than the other components. Component (a) is also displaced from the centroid of the

total optical light in g- and R-bands, contrary to what would be expected if it was the nuclear

bulge of a single galaxy.

Extending the Macquart Relation

We used the measured properties of FRB 20220610A to probe the Macquart relation to

z∼1, by comparing with predictions for its DM based on previous ﬁts to the relation at

z≤0.522. Figure 3 shows the relationship between DM and redshift for the FRBs detected by

ASKAP (4). We restrict our analysis to the ASKAP sample (4) to minimize observing-system-

dependent selection effects. We do not re-ﬁt the Macquart relation. Doing so in an unbiased

way would require analyzing the entire updated FRB sample from ASKAP.

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AluminousfastradioburstthatprobestheUniverseatredshift1S.D.Ryder1,2,K.W.Bannister3,S.Bhandari4,5,A.T.Deller6,R.D.Ekers3,7,M.Glowacki7,A.C.Gordon8,K.Gourdji6,C.W.James7,C.D.Kilpatrick8,9,W.Lu10,11,L.Marnoch1,2,3,12,V.A.Moss3,J.X.Prochaska13,14,H.Qiu15,E.M.Sadler16,3,S.Simha13,M.W.Sammons7,D.R.Scott7,...

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A luminous fast radio burst that probes the Universe at redshift 1 S. D. Ryder12 K. W. Bannister3 S. Bhandari45 A. T. Deller6 R. D. Ekers37

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