Multi-wavelength Counterparts Research Articles

Discovery of Giant Radio Sources from TGSS Alternative Data Release 1: Radio, Optical, and Infrared Properties

Giant radio sources (GRSs) are the single largest astrophysical objects known in the Universe that have grown to megaparsec scales (≥0.7 Mpc). GRSs are much rarer compared with normal-sized radio galaxies. Still, the reason for the formation of their gigantic sizes is under debate. We systematically search for GRSs from the TIFR Giant Metrewave Radio Telescope Sky Survey Alternative Data Release 1 at 150 MHz. We have newly identified 34 GRSs from this study. We have also studied the multiwavelength properties (radio, optical, and infrared) of these GRSs. We have used the likelihood ratio method to identify highly reliable multiwavelength counterparts of GRSs from Pan-STARRS (optical) and Wide-field Infrared Survey Explorer (mid-IR) data. We have classified GRSs based on their accretion mode of the central black holes using optical and mid-IR data. For all sources, we also discuss the principal characteristic parameters (redshift distribution, angular and projected linear size, total integrated radio flux density, spectral index, and radio power). We show the radio evolution track and the location of the GRSs in the P–D diagram. Using a radio–optical luminosity diagram, we identify GRSs in the Fanaroff–Riley classification. Only two GRGs in our sample reside close to the centers of galaxy clusters.

Discovery of the Optical and Radio Counterpart to the Fast X-Ray Transient EP 240315a

Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first identified ≳10 yr ago. Since then, nearly 40 events have been discovered, although almost all of these have been recovered from archival Chandra and XMM-Newton data. To date, optical sky surveys and follow-up searches have not revealed any multiwavelength counterparts. The Einstein Probe, launched in 2024 January, has started surveying the sky in the soft X-ray regime (0.5–4 keV) and will rapidly increase the sample of FXTs discovered in real time. Here we report the first discovery of both an optical and radio counterpart to a distant FXT, the fourth source publicly released by the Einstein Probe. We discovered a fast-fading optical transient within the 3′ localization radius of EP 240315a with the all-sky optical survey ATLAS, and our follow-up Gemini spectrum provides a redshift, z = 4.859 ± 0.002. Furthermore, we uncovered a radio counterpart in the S band (3.0 GHz) with the MeerKAT radio interferometer. The optical (rest-frame UV) and radio luminosities indicate that the FXT most likely originates from either a long gamma-ray burst or a relativistic tidal disruption event. This may be a fortuitous early mission detection by the Einstein Probe or may signpost a mode of discovery for high-redshift, high-energy transients through soft X-ray surveys, combined with locating multiwavelength counterparts.

A γ-Ray-emitting Collisional Ring Galaxy System in Our Galactic Neighborhood

The astrophysical γ-ray photons carry the signatures of the violent phenomena happening on various astronomical scales in our Universe. This includes supernova remnants, pulsars, and pulsar wind nebulae in the Galactic environment and extragalactic relativistic jets associated with active galactic nuclei (AGN). However, ∼30% of the γ-ray sources detected with the Fermi Large Area Telescope lack multiwavelength counterpart association, precluding us from characterizing their origin. Here we report, for the first time, the association of a collisional ring galaxy system in our Galactic neighborhood (distance ∼10 Mpc), formed as a consequence of a smaller “bullet” galaxy piercing through a larger galaxy, as the multifrequency counterpart of an unassociated γ-ray source 4FGL J1647.5−5724. The system, also known as “Kathryn’s Wheel,” contains two dwarf irregular galaxies and an edge-on, late-type, spiral galaxy surrounded by a ring of star-forming knots. We utilized observations taken from the Neil Gehrels Swift observatory, Rapid ASKAP Continuum Survey, SuperCOSMOS Hα Survey, Dark Energy Survey, and VIsible MultiObject Spectrograph at Very Large Telescope to ascertain the association with 4FGL J1647.5−5724 and to explore the connection between the star-forming activities and the observed γ-ray emission. We found that star formation alone cannot explain the observed γ-ray emission, and additional contribution likely from the pulsars/supernova remnants or buried AGN is required. We conclude that arcsecond/subarcsecond-scale observations of this extraordinary γ-ray-emitting galaxy collision will be needed to resolve the environment and explore the origin of cosmic rays.

Multiwavelength Catalog of 10,000 4XMM-DR13 Sources with Known Classifications

We present a collection of ∼10,000 X-ray sources from the 4th XMM-Newton Serendipitous Source Catalog (4XMM-DR13) with literature-verified classifications and multi-wavelength (MW) counterparts. We describe the process by which MW properties are obtained and an interactive online visualization tool we developed.

ERASSU J060839.5–704014: A double degenerate ultra-compact binary in the direction of the LMC

Context. During four all-sky surveys (eRASS1–4), eROSITA, the soft X-ray instrument aboard Spektrum-Roentgen-Gamma (SRG) detected a new supersoft X-ray source, eRASSU J060839.5−704014, in the direction of the Large Magellanic Cloud (LMC). Aims. We arranged follow-up observations in the X-ray and optical wavelengths and further searched in archival observations to reveal the nature of the object. Methods. Using X-ray observations with XMM-Newton we investigated the temporal and spectral behaviour of the source. Results. We discover pulsations at 374 s with a pulse profile consistent with 100% modulation. We identify two other periodicities in the eROSITA data, which we establish as aliases due to the sampling of the eROSITA light curve. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The timing and spectral characteristics of the source are consistent with a double degenerate ultra-compact binary system in the foreground of the LMC. eRASSU J060839.5−704014 belongs to a rare class of AM CVns, which are important to study in the context of progenitors of SN Ia and for persistent gravitational wave detection. Conclusions. We identify eRASSU J060839.5−704014 as a new double degenerate ultra-compact binary located in the foreground of the LMC.

Correction to: MIGHTEE: Multi-wavelength counterparts in the COSMOS field

Correction to: MIGHTEE: Multi-wavelength counterparts in the COSMOS field

A Markedly Expanded Sample of Candidate X-Ray-emitting Isolated Neutron Stars

The current sample of 12 radio-quiet isolated neutron stars that emit strongly in X-rays (XINSs) is both small and heterogeneous, limiting its usefulness for understanding the physics of neutron star atmospheres and cooling rates and for constraining the equation of state of neutron degenerate matter. Utilizing the ROSAT 1RXS and 2RXS data sets, in conjunction with the Sloan Digital Sky Survey Data Release 17 and other companion multiwavelength surveys, we have extended previous searches for blank-field X-ray source candidate XINSs, ultimately recovering two known XINSs while identifying 46 new, unstudied candidate fields devoid of likely multiwavelength counterparts. In this publication, we describe our selection approach and provide detailed information regarding our sample of new candidate XINSs. Future opportunities to verify or to refute these X-ray sources as isolated neutron stars by obtaining more accurate X-ray source positions, quality X-ray spectra, or deeper optical imaging are also discussed.

MIGHTEE: Multi-wavelength counterparts in the COSMOS field

ABSTRACT In this paper, we combine the Early Science radio continuum data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey, with optical and near-infrared data and release the cross-matched catalogues. The radio data used in this work covers 0.86 deg2 of the COSMOS field, reaches a thermal noise of 1.7 μJy beam−1 and contains 6102 radio components. We visually inspect and cross-match the radio sample with optical and near-infrared data from the Hyper Suprime-Cam (HSC) and UltraVISTA surveys. This allows the properties of active galactic nuclei and star-forming populations of galaxies to be probed out to z ≈ 5. Additionally, we use the likelihood ratio method to automatically cross-match the radio and optical catalogues and compare this to the visually cross-matched catalogue. We find that 94 per cent of our radio source catalogue can be matched with this method, with a reliability of 95 per cent. We proceed to show that visual classification will still remain an essential process for the cross-matching of complex and extended radio sources. In the near future, the MIGHTEE survey will be expanded in area to cover a total of ∼20 deg2; thus the combination of automated and visual identification will be critical. We compare the redshift distribution of SFG and AGN to the SKADS and T-RECS simulations and find more AGN than predicted at z ∼ 1.

Swift Deep Galactic Plane Survey classification of Swift J170800−402551.8 as a candidate intermediate polar cataclysmic variable

ABSTRACT Here, we present the results of our multiwavelength campaign aimed at classifying Swift J170800−402551.8 as part of the Swift Deep Galactic Plane Survey (DGPS). We utilized Target of Opportunity (ToO) observations with the Neil Gehrels Swift Observatory, Chandra X-ray Observatory, Neutron star Interior Composition Explorer (NICER), X-ray Multi-Mirror Mission (XMM–Newton), Nuclear Spectroscopic Telescope Array (NuSTAR), and the Southern African Large Telescope (SALT), as well as multiwavelength archival observations from Gaia, VST Photometric Hα Survey, and VISTA Variables in the Via Lactea. The source displays a periodicity of 784 s in our XMM–Newton observation. The X-ray spectrum (XMM–Newton and NuSTAR) can be described by thermal bremsstrahlung radiation with a temperature of kT ≈ 30 keV. The phase-folded X-ray light curve displays a double-peaked, energy-dependent pulse profile. We used Chandra to precisely localize the source, allowing us to identify and study the multiwavelength counterpart. Spectroscopy with SALT identified a Balmer H α line, and potential He i lines, from the optical counterpart. The faintness of the counterpart (r ≈ 21 AB mag) favours a low-mass donor star. Based on these criteria, we classify Swift J170800−402551.8 as a candidate intermediate polar cataclysmic variable, where the spin period of the white dwarf is 784 s.

Simultaneous and panchromatic observations of the fast radio burst FRB 20180916B

Aims. Fast radio bursts are bright radio transients whose origins are not yet understood. The search for a multi-wavelength counterpart of those events can set a tight constraint on the emission mechanism and the progenitor source. Methods. We conducted a multi-wavelength observational campaign on FRB 20180916B between October 2020 and August 2021 over eight activity cycles of the source. Observations were carried out in the radio band by the SRT both at 336 and 1547 MHz and the uGMRT at 400 MHz. Simultaneous observations were conducted by the optical telescopes Asiago (Galileo and Copernico), CMO SAI MSU, CAHA 2.2 m, RTT-150 and TNG, and X/γ-ray detectors on board the AGILE, Insight–HXMT, INTEGRAL, and Swift satellites. Results. We present the detection of 14 new radio bursts detected with the SRT at 336 MHz and seven new bursts with the uGMRT from this source. We provide the deepest prompt upper limits in the optical band for FRB 20180916B to date. In fact, the TNG/SiFAP2 observation simultaneous to a burst detection by uGMRT gives an upper limit Eoptical/Eradio < 1.3 × 102. Another burst detected by the SRT at 336 MHz was also co-observed by Insight–HXMT. The non-detection in the X-rays yields an upper limit (1 − 30 keV band) of EX − ray/Eradio in the range of (0.9 − 1.3) × 107, depending on the model that is considered for the X-ray emission.

Radio variable and transient sources on minute time-scales in the ASKAP pilot surveys

ABSTRACT We present results from a radio survey for variable and transient sources on 15-min time-scales, using the Australian SKA Pathfinder (ASKAP) pilot surveys. The pilot surveys consist of 505 h of observations conducted at around 1 GHz observing frequency, with a total sky coverage of 1476 deg2. Each observation was tracked for approximately 8 – 10 h, with a typical rms sensitivity of ∼30 μJy beam−1 and an angular resolution of ∼12 arcsec. The variability search was conducted within each 8 – 10 h observation on a 15-min time-scale. We detected 38 variable and transient sources. Seven of them are known pulsars, including an eclipsing millisecond pulsar, PSR J2039−5617. Another eight sources are stars, only one of which has been previously identified as a radio star. For the remaining 23 objects, 22 are associated with active galactic nuclei or galaxies (including the five intra-hour variables that have been reported previously), and their variations are caused by discrete, local plasma screens. The remaining source has no multiwavelength counterparts and is therefore yet to be identified. This is the first large-scale radio survey for variables and transient sources on minute time-scales at a sub-mJy sensitivity level. We expect to discover ∼1 highly variable source per day using the same technique on the full ASKAP surveys.

Discovery of periodicities in two highly variable intermediate polars towards the Galactic centre

Aims. We performed a systematic analysis of X-ray point sources within 1.°5 of the Galactic centre using archival XMM-Newton data. While doing so, we discovered Fe Kα complex emission and pulsation in two highly variable sources, 4XMM J174917.7–283329 and 4XMM J174954.6–294336. In this work we report the findings of the X-ray spectral and timing studies. Methods. We performed detailed spectral modelling of the sources and searched for pulsation in the light curves using Fourier timing analysis. We also searched for multi-wavelength counterparts for the characterization of the sources. Results. The X-ray spectrum of 4XMM J174917.7–283329 shows the presence of complex Fe K emission in the 6–7 keV band. The equivalent widths of the 6.4 and 6.7 keV lines are 99−72+84 and 220−140+160 eV, respectively. The continuum is fitted by a partially absorbed apec model with a plasma temperature of kT = 13−2+10 keV. The inferred mass of the white dwarf (WD) is 0.9−0.2+0.3 M⊙. We detected pulsations with a period of 1212 ± 3 s and a pulsed fraction of 26 ± 6%. The light curves of 4XMM J174954.6–294336 display an asymmetric eclipse and dipping behaviour. To date, this is only the second known intermediate polar to show a total eclipse in X-rays. The spectrum of the sources is characterized by a power-law model with photon index Γ = 0.4 ± 0.2. The equivalent widths of the iron fluorescent (6.4 keV) and Fe XXV (6.7 keV) lines are 171−79+99 and 136−81+89 eV, respectively. The continuum is described by emission from optically thin plasma with a temperature of kT ∼ 35 keV. The inferred mass of the WD is 1.1−0.3+0.2 M⊙. We detect coherent pulsations from the source with a period of 1002 ± 2 s. The pulsed fraction is 66 ± 15%. Conclusions. The spectral modelling indicates the presence of intervening clouds with a high absorbing column density in front of both sources. The detected periodic modulations in the light curves are likely associated with the spin period of WDs in magnetic cataclysmic variables. The measured spin period, hard photon index, and equivalent width of the fluorescent Fe Kα line are consistent with the values found in intermediate polars. 4XMM J174954.6–294336 has already been classified as an intermediate polar, and we suggest that 4XMM J174917.7–283329 is a new intermediate polar. The X-ray eclipses in 4XMM J174954.6–294336 are most likely caused by a low-mass companion star obscuring the central X-ray source. The asymmetry in the eclipse is likely caused by a thick bulge that intercepts the line of sight during the ingress phase but not during the egress phase located behind the WD along the line of sight.

ERASSt J040515.6 − 745202, an X-ray burster in the Magellanic Bridge

Context. During the third all-sky survey (eRASS3), eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma, detected a new hard X-ray transient, eRASSt J040515.6 − 745202, in the direction of the Magellanic Bridge. Aims. We arranged follow-up observations and searched for archival data to reveal the nature of the transient. Methods. Using X-ray observations with XMM-Newton, NICER, and Swift, we investigated the temporal and spectral behaviour of the source for over about 10 days. Results. The X-ray light curve obtained from the XMM-Newton observation with an ∼28 ks exposure revealed a type-I X-ray burst with a peak bolometric luminosity of at least 1.4 × 1037 erg s−1. The burst energetics are consistent with a location of the burster at the distance of the Magellanic Bridge. The relatively long exponential decay time of the burst of ∼70 s indicates that it ignited in a H-rich environment. The non-detection of the source during the other eROSITA surveys, twelve and six months before and six months after eRASS3, suggests that the burst was discovered during a moderate outburst which reached 2.6 × 1036 erg s−1 in persistent emission. During the NICER observations, the source showed alternating flux states with the high level at a similar brightness as during the XMM-Newton observation. This behaviour is likely caused by dips as also seen during the last hour of the XMM-Newton observation. Evidence for a recurrence of the dips with a period of ∼21.8 h suggests eRASSt J040515.6 − 745202 is a low-mass X-ray binary (LMXB) system with an accretion disk seen nearly edge on. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The spectral energy distribution is consistent with radiation from an accretion disk which dominates the UV and from a cool late-type star detected in the optical to infrared wavelengths. Conclusions. After the discovery of X-ray bursts in M 31, the Magellanic Bridge is only the second location outside of the Milky Way where an X-ray burster was found. The burst uniquely identifies eRASSt J040515.6 − 745202 as an LMXB system with a neutron star. Its location in the Magellanic Bridge confirms the existence of an older stellar population which is expected if the bridge was formed by tidal interactions between the Magellanic Clouds, which stripped gas and stars from the clouds.

New Identifications and Multiwavelength Properties of Extragalactic Fermi Gamma-Ray Sources in the SPT-SZ Survey Field

The fourth Fermi Large Area Telescope catalog (4FGL) contains 5064 γ-ray sources detected at high significance, but 26% of them still lack associations at other wavelengths. The SPT-SZ survey, conducted between 2008 and 2011 with the South Pole Telescope (SPT), covers 2500 deg2 of the southern sky in three millimeter-wavelength (mm) bands and was used to construct a catalog of nearly 5000 emissive sources. In this study, we introduce a new cross-matching scheme to search for multiwavelength counterparts of extragalactic γ-ray sources using a mm catalog. We apply a Poissonian probability to evaluate the rate of spurious false associations and compare the multiwavelength associations from the radio, mm, near-infrared, and X-ray with 4FGL γ-ray sources. In the SPT-SZ survey field, 85% of 4FGL sources are associated with mm counterparts. These mm sources include 94% of previously associated 4FGL sources and 56% of previously unassociated 4FGL sources. The latter group contains 40 4FGL sources for which SPT has provided the first identified counterparts. Nearly all of the SPT-associated 4FGL sources can be described as flat-spectrum radio quasars or blazars. We find that the mm band is the most efficient wavelength for detecting γ-ray blazars when considering both completeness and purity. We also demonstrate that the mm band correlates better to the γ-ray band than the radio or X-ray bands. With the next generation of CMB experiments, this technique can be extended to greater sensitivities and more sky area to further complete the identifications of the remaining unknown γ-ray blazars.

Extragalactic fast X-ray transient candidates discovered by Chandra (2000–2014)

Context. Extragalactic fast X-ray transients (FXRTs) are short flashes of X-ray photons of unknown origin that last a few seconds to hours. Aims. Our ignorance about their physical mechanisms and progenitor systems is due in part to the lack of clear multiwavelength counterparts in most cases, because FXRTs have only been identified serendipitously. Methods. We develop a systematic search for FXRTs in the Chandra Source Catalog (Data Release 2.0; 169.6 Ms over 592.4 deg2, using only observations with |b|> 10° and before 2015), using a straightforward X-ray flare search algorithm and incorporating various multiwavelength constraints to rule out Galactic contamination and characterize the candidates. Results. We report the detection of 14 FXRT candidates from a parent sample of 214 701 sources. Candidates have peak 0.5–7 keV fluxes between 1 × 10−13 and 2 × 10−10 erg cm−2 s−1 and T90 values from 4 to 48 ks. The sample can be subdivided into two groups: six “nearby” FXRTs that occurred within d ≲ 100 Mpc and eight “distant” FXRTs with likely redshifts ≳0.1. Three distant FXRT candidates exhibit light curves with a plateau (≈1–3 ks duration) followed by a power-law decay and X-ray spectral softening, similar to what was observed for the previously reported FXRT CDF-S XT2, a proposed magnetar-powered binary neutron star merger event. After applying completeness corrections, we calculate event rates for the nearby and distant samples of 53.7−15.1+22.6 and 28.2−6.9+9.8 deg−2 yr−1, respectively. Conclusions. This novel sample of Chandra-detected extragalactic FXRT candidates, although modest in size, breaks new ground in terms of characterizing the diverse properties, nature, and possible progenitors of these enigmatic events.

Optical microflares in LS I +61 303 and the search for their multiwavelength counterpart

Context. Stellar sources of gamma rays are one of the front lines in modern astrophysics whose understanding can benefit from observational tools not originally designed for their study. Aims. We take advantage of the high precision photometric capabilities of present-day space facilities to obtain a new perspective on the optical behavior of the X-ray and gamma-ray binary LS I +61 303. Previously unknown phenomena whose effects manifest with amplitudes below 0.01 magnitude can now be clearly observed and studied. Methods. Our work is mainly based on the analysis of optical and gamma-ray archival data and uses the tools recommended by the different collaborations that provide these valuable observational resources (in particular, the TESS and Fermi orbiting observatories). In addition, complementary ground-based optical spectroscopy has also been conducted. Results. We report the discovery of small-amplitude optical flares on timescales of a day in the LS I +61 303 light curve. Different alternative scenarios to explain their origin are tentatively proposed.

Multiwavelength properties of 850-μm selected sources from the North Ecliptic Pole SCUBA-2 survey

ABSTRACT We present the multiwavelength counterparts of 850-$\mu$m selected submillimetre sources over a 2-deg2 field centred on the North Ecliptic Pole. In order to overcome the large beam size (15 arcsec) of the 850-$\mu$m images, deep optical to near-infrared (NIR) photometric data and arcsecond-resolution 20-cm images are used to identify counterparts of submillimetre sources. Among 647 sources, we identify 514 reliable counterparts for 449 sources (69 per cent in number), based either on probabilities of chance associations calculated from positional offsets or offsets combined with the optical-to-NIR colours. In the radio imaging, the fraction of 850-$\mu$m sources having multiple counterparts is 7 per cent. The photometric redshift, infrared luminosity, stellar mass, star formation rate (SFR), and the active galactic nucleus (AGN) contribution to the total infrared luminosity of the identified counterparts are investigated through spectral energy distribution fitting. The SMGs are infrared-luminous galaxies at an average 〈z〉 = 2.5 with log10(LIR/L⊙) = 11.5–13.5, with a mean stellar mass of log10(Mstar/M⊙) = 10.90 and SFR of $\mathrm{log}_{10} (\mathrm{SFR/M_\odot \, yr^{-1}})=2.34$. The submillimetre galaxies (SMGs) show twice as large SFR as galaxies on the star-forming main sequence, and about 40 per cent of the SMGs are classified as objects with bursty star formation. At z ≥ 4, the contribution of AGN luminosity to total luminosity for most SMGs is larger than 30 per cent. The FIR-to-radio correlation coefficient of SMGs is consistent with that of main-sequence galaxies at z ≃ 2.

AT2020hur: A Possible Optical Counterpart of FRB 180916B

The physical origin of fast radio bursts (FRBs) remains unclear. Finding multiwavelength counterparts of FRBs can provide a breakthrough for understanding their nature. In this work, we perform a systematic search for astronomical transients whose positions are consistent with FRBs. We find an unclassified optical transient AT2020hur (α = 01h58m00.ˢ750 ± 1″, ) that is spatially coincident with the repeating FRB 180916B (α = 01h58m00.ˢ7502 ± 2.3 mas, mas; Marcote et al. 2020). The chance possibility of the AT2020hur–FRB 180916B association is about 0.04%, which corresponds to a significance of 3.5σ. We develop a giant flare (GF) afterglow model to fit AT2020hur. Although the GF afterglow model can interpret the observations of AT2020hur, the derived kinetic energy of such a GF is at least three orders of magnitude larger than that of a typical GF, and a lot of fine-tuning and coincidences are required for this model. Another possible explanation is that AT2020hur might consist of two or more optical flares originating from the FRB source, e.g., fast optical bursts produced by the inverse Compton scattering of FRB emission. Besides, AT2020hur is located in one of the activity windows of FRB 180916B, which provides independent support for the association. This coincidence may be due to the optical counterparts being subject to the same periodic modulation as FRB 180916B, as implied by the prompt FRB counterparts. Future simultaneous observations of FRBs and their optical counterparts may help to reveal their physical origin.

21 new long-term variables in the GX 339−4 field: two years of MeerKAT monitoring

ABSTRACT We present 21 new long-term variable radio sources found commensally in 2 yr of weekly MeerKAT monitoring of the low-mass X-ray binary GX 339−4. The new sources are vary on time-scales of weeks to months and have a variety of light-curve shapes and spectral index properties. Three of the new variable sources are coincident with multiwavelength counterparts; and one of these is coincident with an optical source in deep MeerLICHT images. For most sources, we cannot eliminate refractive scintillation of active galactic nuclei as the cause of the variability. These new variable sources represent 2.2 ± 0.5 per cent of the unresolved sources in the field, which is consistent with the 1–2 per cent variability found in past radio variability surveys. However, we expect to find short-term variable sources in the field and these 21 new long-term variable sources. We present the radio light curves and spectral index variability of the new variable sources, as well as the absolute astrometry and matches to coincident sources at other wavelengths.

Radio and X-Ray Observations of the Luminous Fast Blue Optical Transient AT 2020xnd

Abstract We present deep X-ray and radio observations of the fast blue optical transient (FBOT) AT 2020xnd/ZTF 20acigmel at z = 0.2433 from 13 days to 269 days after explosion. AT 2020xnd belongs to the category of optically luminous FBOTs with similarities to the archetypal event AT 2018cow. AT 2020xnd shows luminous radio emission reaching L ν ≈ 8 × 1029 erg s−1 Hz−1 at 20 GHz and 75 days post-explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at L X ≈ 6 × 1042 erg s−1. Interpreting the radio emission in the context of synchrotron radiation from the explosion’s shock interaction with the environment, we find that AT 2020xnd launched a high-velocity outflow (v ∼ 0.1c–0.2c) propagating into a dense circumstellar medium (effective M ̇ ≈ 10 − 3 M ⊙ yr−1 for an assumed wind velocity of v w = 1000 km s−1). Similar to AT 2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT 2020xnd a high-redshift analog to AT 2018cow, and establish AT 2020xnd as the fourth member of the class of optically luminous FBOTs with luminous multiwavelength counterparts.