Stingray 2 (ASKAP J0245-5642)

• Stingray 2 is a diffuse, extragalactic radio source with a near-circular body and one-sided tail, likely representing an evolved head–tail radio galaxy.
• It exhibits a steep spectral index (α ≈ -1.77) and low surface brightness, indicating aging synchrotron electrons and complex environmental interactions.
• The source’s identification is complicated by dual usage of the name, overlapping with a near-field nanosecond burst, necessitating high-resolution follow-up observations.

Searching arXiv for the cited papers to ground the article in the relevant literature. Stingray 2 is the designation applied in the literature to an unusual ASKAP radio detection discussed in two distinct observational contexts. In "Stingrays in the radio sky: Two unusual diffuse radio relic sources in the direction of the Magellanic Stream" (Smeaton et al., 13 Aug 2025), Stingray 2 denotes the extended, low surface-brightness diffuse source ASKAP J0245−5642, discovered in ASKAP EMU imaging at 944 MHz and characterized by a near-circular body connected to a one-sided tail. Separately, "A nanosecond-duration radio pulse originating from the defunct Relay 2 satellite" (James et al., 13 Jun 2025) states that, within ASKAP’s internal transient taxonomy, a near-field satellite-origin nanosecond burst was also nicknamed "Stingray 2" and corresponded to the catalog label ASKAP J0245−5642 used by the ASKAP fast-transient program. The diffuse-source usage is the one explicitly associated with the source morphology, continuum spectrum, and Magellanic Stream line of sight (Smeaton et al., 13 Aug 2025), whereas the transient usage refers to a short-duration burst localized to the defunct Relay 2 satellite (James et al., 13 Jun 2025). This dual usage has made nomenclature an intrinsic part of the subject.

1. Identification and nomenclatural ambiguity

In the diffuse-source study, Stingray 2 is designated ASKAP J0245−5642 and is described as one of two unusual, low surface-brightness radio sources discovered in ASKAP EMU imaging toward the Magellanic Stream (Smeaton et al., 13 Aug 2025). It consists of a near-circular radio body connected to a one-sided tail and exhibits a very steep non-thermal spectrum. In the GLEAM catalog, the tail region is listed as GLEAM J024626−564007. The circular head region is centered at RA $= 02{:}45{:}54.7$, Dec $= -56{:}42{:}06.0$, and the tail region at RA $= 02{:}46{:}26.9$, Dec $= -56{:}40{:}15.2$; these centers were defined by eye using regions that encompass the diffuse emission, and formal positional uncertainties are not provided, but are of order a few arcseconds relative to the $15''$ EMU beam (Smeaton et al., 13 Aug 2025).

The transient paper explicitly states that the burst localized to the defunct NASA Relay 2 satellite is the event informally nicknamed "Stingray 2," and corresponds to the catalog label ASKAP J0245−5642 used by the ASKAP fast-transient program (James et al., 13 Jun 2025). In that paper, the authors refer to it simply as the Relay 2 burst; alternate identifiers used include "Relay 2" and NORAD ID 737. Its apparent imaged position under the far-field assumption is given as RA $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$, while the DEC value is not included in the excerpt provided here (James et al., 13 Jun 2025).

This suggests that "Stingray 2" is not a uniquely stable astronomical name across ASKAP subprograms. A plausible implication is that the encyclopedia treatment of Stingray 2 must distinguish the diffuse synchrotron source from the near-field transient event rather than assume that both papers describe the same astrophysical object.

2. Discovery as a diffuse radio source

The diffuse Stingray 2 was discovered in the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe survey at a central frequency $\nu = 943.5$ MHz, hereafter 944 MHz, with bandwidth 288 MHz (Smeaton et al., 13 Aug 2025). Stingray 2 was observed in scheduling block SB49990 with total integration time $\approx 10$ h. Stokes I images are used, and Stokes V shows no detection. Low-frequency measurements came from the GaLactic and Extragalactic All-sky MWA Survey at $\nu = 88$, 118, 155, and 200 MHz; the 200 MHz band has 60 MHz bandwidth and the others 30 MHz. H I data were taken from HI4PI, specifically the Parkes GASS contribution. Ancillary catalogs and images inspected include DESI Legacy Survey DR10, WISE/AllWISE, 2MASS, Gaia, and eROSITA X-ray (Smeaton et al., 13 Aug 2025).

For the convolved ASKAP EMU Stokes I image, the synthesized beam is $15''$, with rms noise $= -56{:}42{:}06.0$0 near Stingray 2; the diffuse structure is significantly detected in integrated flux, with no compact core at the center of the circle (Smeaton et al., 13 Aug 2025). The MWA GLEAM beams and rms values are $= -56{:}42{:}06.0$1 with rms $= -56{:}42{:}06.0$2 at 88 MHz, $= -56{:}42{:}06.0$3 with rms $= -56{:}42{:}06.0$4–$= -56{:}42{:}06.0$5 at 118 MHz, $= -56{:}42{:}06.0$6 with rms $= -56{:}42{:}06.0$7 at 155 MHz, and $= -56{:}42{:}06.0$8 with rms $= -56{:}42{:}06.0$9 at 200 MHz (Smeaton et al., 13 Aug 2025).

No SUMSS/NVSS integrated detections are reported for Stingray 2’s diffuse emission, although compact sources within the object’s footprint are cataloged in NED/SIMBAD (Smeaton et al., 13 Aug 2025). This observational pattern places Stingray 2 among radio sources that are conspicuous through deep, low-surface-brightness imaging but not readily reducible to a compact-core identification.

3. Morphology and continuum properties

Stingray 2 is qualitatively described as a near-circular body, approximately edge-brightened and partially filled with diffuse emission, connected to an extended, one-sided tail (Smeaton et al., 13 Aug 2025). A slight drop in brightness where the tail connects to the circular body is visible, though it is less pronounced than in Stingray 1. The angular sizes defined by eye are a circle diameter of 4.8 arcmin, a tail of $= 02{:}46{:}26.9$0 arcmin with position angle $= 02{:}46{:}26.9$1 clockwise from North, and a total end-to-end length of 9.6 arcmin (Smeaton et al., 13 Aug 2025). With the $= 02{:}46{:}26.9$2 ASKAP beam, both components are well resolved; the low-frequency MWA beams smooth substructure and prevent reliable point-source subtraction. No compact radio core is detected at the center of the circular body in EMU, although several unrelated compact radio sources lie within the defined regions, including one at the tail tip (Smeaton et al., 13 Aug 2025).

All images were convolved to the largest MWA beam before extraction to ensure consistent scaling. Fluxes were measured in CARTA using polygonal or aperture regions for the circle and tail, with background estimated locally. For ASKAP EMU, point sources were removed using AeReS, with the two brightest subtracted manually to yield diffuse-only estimates; errors are $= 02{:}46{:}26.9$3 with minimum $= 02{:}46{:}26.9$4 mJy (Smeaton et al., 13 Aug 2025).

The measured GLEAM total flux densities are:

Component	88 MHz	118 MHz	155 MHz	200 MHz
Circle	$= 02{:}46{:}26.9$5 mJy	$= 02{:}46{:}26.9$6 mJy	$= 02{:}46{:}26.9$7 mJy	$= 02{:}46{:}26.9$8 mJy
Tail	$= 02{:}46{:}26.9$9 mJy	$= -56{:}40{:}15.2$0 mJy	$= -56{:}40{:}15.2$1 mJy	$= -56{:}40{:}15.2$2 mJy

At 944 MHz in the convolved ASKAP EMU image, the circle has $= -56{:}40{:}15.2$3 mJy and $= -56{:}40{:}15.2$4 mJy, with residual point sources $= -56{:}40{:}15.2$5 mJy; the tail has $= -56{:}40{:}15.2$6 mJy and $= -56{:}40{:}15.2$7 mJy, with residual point sources $= -56{:}40{:}15.2$8 mJy (Smeaton et al., 13 Aug 2025).

To correct the MWA totals for embedded compact sources, the EMU 944 MHz point-source fluxes were scaled to MWA bands with a canonical extragalactic spectral index $= -56{:}40{:}15.2$9 and subtracted from the MWA totals (Smeaton et al., 13 Aug 2025). The scaled point-source contributions for both circle and tail are 16, 13, 11, and 9 mJy at 88, 118, 155, and 200 MHz respectively, each with quoted uncertainties of 1–2 mJy. The resulting diffuse-only flux densities are:

Component	88 MHz	118 MHz	155 MHz	200 MHz
Circle diffuse	$15''$0 mJy	$15''$1 mJy	$15''$2 mJy	$15''$3 mJy
Tail diffuse	$15''$4 mJy	$15''$5 mJy	$15''$6 mJy	$15''$7 mJy

The continuum spectrum is modeled as a power law, $15''$8, with spectral index defined by $15''$9 (Smeaton et al., 13 Aug 2025). Linear least-squares fits in $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$0–$= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$1 over 88–200 MHz plus 944 MHz give $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$2 for the circle, $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$3 for the tail, and $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$4 for the total diffuse emission (Smeaton et al., 13 Aug 2025). The steepening toward the tail is consistent with spectral aging of synchrotron electrons. A single power law adequately describes the SED within the available bands; no curvature or break frequency $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$5 is fitted because of limited frequency coverage and uncertainties introduced by point-source scaling at MWA bands. Independent GLEAM catalog fits yield $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$6 for GLEAM J024626−564007, consistent with the tail result (Smeaton et al., 13 Aug 2025).

No spectral index map is available for Stingray 2 because WALLABY imaging does not cover this field (Smeaton et al., 13 Aug 2025). No linear polarization or rotation measure is reported, because EMU data products for this field include only Stokes I and V, and Stokes V shows no detection. The absence of polarimetry prevents constraints on ordered magnetic fields or Faraday depolarization (Smeaton et al., 13 Aug 2025).

4. Line-of-sight environment and multiwavelength context

HI4PI data with angular resolution 16.2 arcmin and rms $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$7 mK per 1.29 km s$= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$8 channel were used to extract H I absorption spectra over the Stingray 2 region (Smeaton et al., 13 Aug 2025). The source lies in the Magellanic Stream I velocity sector, $= 1\mathrm{h}34\mathrm{m}35.6\mathrm{s}$9. Stingray 2 shows strong absorption dips at Milky Way velocity $\nu = 943.5$0 with $\nu = 943.5$1 and detection significance $\nu = 943.5$2, and at Magellanic Stream velocity $\nu = 943.5$3 with $\nu = 943.5$4 and detection significance $\nu = 943.5$5 (Smeaton et al., 13 Aug 2025). These detections imply that Stingray 2 is behind both the Milky Way H I and the Magellanic Stream H I, and is therefore not physically associated with the Stream gas.

The optical and infrared context is complex. DESI Legacy Survey DR10 images show numerous galaxies in and around the radio emission (Smeaton et al., 13 Aug 2025). A cataloged galaxy cluster, J024639.5−563904, lies at the tip of the tail, with $\nu = 943.5$6, radius $\nu = 943.5$7 kpc, and mass $\nu = 943.5$8; its brightest cluster galaxy, LEDA 398369, has $\nu = 943.5$9 (Smeaton et al., 13 Aug 2025). A possible central host candidate nearer the body–tail junction is 2dFGRS TGS845Z440, also identified as WISEA J024602.67−564033.5, with $\approx 10$0; within a $\approx 10$1 diameter region, 305 galaxies are found, with 31 at $\approx 10$2 (Smeaton et al., 13 Aug 2025).

No diffuse X-ray counterpart is detected in available eROSITA images (Smeaton et al., 13 Aug 2025). Deeper Chandra or XMM observations are identified as necessary to test for cluster-related thermal gas or AGN environments.

This multiwavelength context establishes two facts of classification relevance. First, Stingray 2 is not a Galactic or Magellanic-Stream radio structure. Second, its radio morphology is embedded in a projected field containing both cluster-scale and smaller-group-scale galaxy environments, without yet yielding an unambiguous host galaxy.

5. Classification and physical interpretation

The diffuse-source paper evaluates Galactic, near-Galactic, and extragalactic scenarios against morphology, spectrum, H I absorption, environment, and counterparts (Smeaton et al., 13 Aug 2025). A runaway or circumgalactic SNR associated with the Magellanic Stream is disfavored because the steep total spectral index $\approx 10$3 is far outside the $\approx 10$4 range typical of Magellanic Cloud SNRs, and the robust Stream absorption places the source behind the Stream. A parentless PWN is also disfavored, because PWNe typically show flatter radio spectra and $\approx 10$5 is much too steep (Smeaton et al., 13 Aug 2025).

Among extragalactic scenarios, a radio AGN with FR-type jets or lobes is considered possible, because the one-sided tail plus circular plume could result from environmental interaction; however, no clear central radio core is seen in the circular body, and the abrupt transition from a narrow tail to a nearly circular lobe is unusual (Smeaton et al., 13 Aug 2025). A dying or remnant radio galaxy is regarded as plausible, particularly given the very steep indices. Galaxy pair or group interaction is also considered possibly consistent with the optical density, but the radio scale is stated to be very large for typical group-driven structures (Smeaton et al., 13 Aug 2025). A galaxy cluster halo or relic is deemed unlikely because the radio extent would exceed typical cluster halo or relic sizes at the cluster redshift, the cluster is offset toward the tail tip rather than near the geometric center of the circular body, and no eROSITA diffuse X-ray counterpart is apparent (Smeaton et al., 13 Aug 2025). An Odd Radio Circle interpretation is considered unlikely because the prominent tail is atypical of known single ORCs, and the angular size is much larger than typical ORCs. Chance alignment of two unrelated structures is possible, but the shared morphology with Stingray 1 makes two independent chance alignments less likely (Smeaton et al., 13 Aug 2025).

The most likely scenario identified by the authors is a head–tail radio galaxy (Smeaton et al., 13 Aug 2025). In that interpretation, hydrodynamic interaction with winds or shocks in a gaseous environment, such as an intracluster or intragroup medium, can sweep back one jet into a narrow tail while inflating a circular or ring-like plume at the leading end. The model is said to explain the one-sided tail, circular plume, slight offset between the head and the tail origin, and edge-brightening (Smeaton et al., 13 Aug 2025). The main caveat is the absence of an unambiguous host galaxy at the head; LEDA 398369 at the tail tip and 2dFGRS TGS845Z440 nearer the body–tail junction are both described as plausible hosts, but the paper states that precise association requires high-resolution multi-frequency radio imaging, polarimetry, and optical spectroscopy (Smeaton et al., 13 Aug 2025).

This suggests that Stingray 2 belongs to a small class of environmentally distorted synchrotron sources whose taxonomic assignment cannot be made from morphology alone. A plausible implication is that its importance lies less in establishing a new source class than in probing how diffuse radio plasma evolves under strong external interaction.

6. Physical scales, comparison with Stingray 1, and observational limits

Using the measured spectral index to scale to 1 GHz and the total area from the defined regions, the diffuse-source paper gives $\approx 10$6 and $\approx 10$7 mJy, implying $\approx 10$8 (Smeaton et al., 13 Aug 2025). The source is described as extremely faint, consistent with a distant, aging synchrotron source and/or surface-brightness dimming at non-zero redshift.

If associated with the $\approx 10$9 cluster, the total length would be $\nu = 88$0 Mpc; if associated with the $\nu = 88$1 galaxy 2dFGRS TGS845Z440, the total length would be $\nu = 88$2 Mpc (Smeaton et al., 13 Aug 2025). Such scales are described as favoring an evolved, environment-shaped radio galaxy and disfavoring typical cluster halo or relic interpretations.

The paper adopts the $\nu = 88$3-corrected monochromatic luminosity

$\nu = 88$4

with $\nu = 88$5 and diffuse EMU 944 MHz flux $\nu = 88$6 mJy (Smeaton et al., 13 Aug 2025). If $\nu = 88$7 and $\nu = 88$8 Mpc, then $\nu = 88$9, giving $15''$0 (Smeaton et al., 13 Aug 2025). If $15''$1, with $15''$2–$15''$3 Gpc, then $15''$4 would scale to $15''$5–$15''$6 (Smeaton et al., 13 Aug 2025). The paper notes that both estimates are consistent with low-power, evolved radio galaxies.

In direct comparison with Stingray 1, Stingray 2 is fainter and has a much steeper radio spectrum: $15''$7 versus $15''$8 (Smeaton et al., 13 Aug 2025). Stingray 2 also has a lower 1 GHz surface brightness, $15''$9, than Stingray 1, $= -56{:}42{:}06.0$00, and lacks a clear central host galaxy (Smeaton et al., 13 Aug 2025). Both share the head–tail morphology, but the evidence favors a more evolved, possibly remnant synchrotron population in Stingray 2.

The same paper is explicit about the present limitations. The steep spectral indices rely on subtracting scaled point-source contributions at MWA bands under the assumption $= -56{:}42{:}06.0$01; if embedded sources are steeper, the diffuse spectrum could be somewhat flatter, although an independent GLEAM fit confirms a steep tail (Smeaton et al., 13 Aug 2025). The lack of polarization and RM measurements precludes constraints on ordered fields and depolarization. No high-resolution, multi-frequency mapping above $= -56{:}42{:}06.0$02 GHz is available, because WALLABY does not cover the field, preventing spectral-curvature tests and spatially resolved aging diagnostics. The H I absorption analysis uses the 16.2 arcmin Parkes beam, so higher-resolution H I could refine the line-of-sight structure (Smeaton et al., 13 Aug 2025).

7. Distinction from the Relay 2 nanosecond burst

The second paper introduces a separate phenomenon that shares the "Stingray 2" nickname within ASKAP’s internal transient taxonomy (James et al., 13 Jun 2025). That event was detected on 2024-09-12 05:13:09.035 UTC by ASKAP/CRACO over 695.5–1031.5 MHz, localized through near-field time-delay analysis to the long-decommissioned Relay 2 satellite, and shown to have best-fit dispersion measure $= -56{:}42{:}06.0$03, corresponding to 69.7 TECU (James et al., 13 Jun 2025). After coherent dedispersion, the burst was found to be less than 30 ns in width, with average flux density at least 300 kJy and true peak likely at least 3 MJy (James et al., 13 Jun 2025). Its favored physical interpretation is an electrostatic discharge, with plasma discharge following a micrometeoroid impact treated as an alternative (James et al., 13 Jun 2025).

The near-field curvature analysis yielded a best-fit source distance $= -56{:}42{:}06.0$04 km with rms timing residuals 0.075 ns and uncertainty $= -56{:}42{:}06.0$05 km (James et al., 13 Jun 2025). Skyfield propagation of space-track.org TLEs produced a single viable match, NORAD ID 737, at line-of-sight range 4322 km and offset 3.2 arcmin from the imaged burst position (James et al., 13 Jun 2025). The paper states that this event helped cement criteria for classification of such near-field satellite-origin nanosecond bursts: only inner baselines image coherently under far-field assumptions, measurable ns-scale curvature delays are present, dedispersed nanosecond structure is observed, DM and TECU are consistent with a single ionospheric pass, and a satellite ephemeris match exists (James et al., 13 Jun 2025).

These properties are categorically different from those of the diffuse synchrotron source in the EMU paper (Smeaton et al., 13 Aug 2025). The former is a nanosecond-duration, near-field transient associated with an artificial satellite; the latter is a resolved, low surface-brightness continuum structure behind the Magellanic Stream. The shared nickname is therefore a nomenclatural overlap rather than an indication of common physical origin.

Taken together, the literature identifies Stingray 2 primarily as a diffuse, extragalactic, steep-spectrum synchrotron source behind the Magellanic Stream, with morphology and spectral properties most consistent with a head–tail radio galaxy undergoing strong environmental interaction (Smeaton et al., 13 Aug 2025). At the same time, the same nickname has also been used within ASKAP for a near-field satellite burst from Relay 2 (James et al., 13 Jun 2025). For the diffuse source, confirmation of the preferred interpretation hinges on identification of the host galaxy, deeper multi-frequency radio imaging, polarimetry, and improved optical and X-ray characterization (Smeaton et al., 13 Aug 2025).