ASKAP J1755-2527: Galactic Long-Period Radio Transient

• ASKAP J1755-2527 is a Galactic long-period radio transient characterized by coherent bursts across 160 MHz to 3.1 GHz, high polarization, and a period of approximately 1.16 hours.
• Precise timing using ASKAP, MWA, MeerKAT, and ATCA data has yielded detailed dispersion and scattering parameters that align with expectations from Galactic electron-density models.
• Its physical interpretation remains unsettled, with debates over neutron star and white dwarf scenarios influenced by its intermittent activity and complex polarimetric behavior.

Searching arXiv for the source papers and closely related long-period transient literature. ASKAP J175534.9−252749.1, usually abbreviated J1755−2527, is a Galactic long-period radio transient (LPT) discovered in the ASKAP Variables and Slow Transients survey as a single highly polarised coherent burst and later established as a repeating source with a period of about $1.16$ hours. Observations with ASKAP, MWA, MeerKAT, and ATCA show pulses from $\approx 160$ MHz to $3.1$ GHz, strong low-frequency scattering consistent with Galactic electron-density models, a large and stable rotation measure of about $962\,\mathrm{rad\,m^{-2}}$, and intrinsic intermittency on month-long timescales. Its physical interpretation remains unsettled: the discovery study emphasized neutron-star-like polarimetric phenomenology, whereas the later timing study argued that a white-dwarf-based binary scenario is favoured over a neutron star (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

1. Discovery and observational establishment

J1755−2527 was first identified in the VAST Galactic plane program with ASKAP, using a RACS-like setup with center frequency $887.5$ MHz, bandwidth $288$ MHz, $1$ MHz channelization for imaging, $10$ s integrations, and a field of view of about $30\,\mathrm{deg^2}$ per pointing. The transient search was image-domain rather than a beamformed single-pulse search. In the early VAST subset, the source was the only object that both appeared in a single detected epoch and showed significant circular polarization in that epoch (Dobie et al., 2024).

The discovery observation was ASKAP scheduling block SB47253 on 2023-01-21. In a $10$ s image the source had peak flux density $\approx 160$0 mJy and circular polarization fraction of about $\approx 160$1, but dynamic spectra recovered a much brighter intrinsic event. The main burst was a single coherent episode with onset about $\approx 160$2 minutes after observation start, time above half-maximum of about $\approx 160$3 s, significant emission above $\approx 160$4 for about $\approx 160$5 s, and peak flux density of order $\approx 160$6 mJy near $\approx 160$7 GHz. A weaker linearly polarized leading excess may have extended the total emitting episode to about $\approx 160$8 s, or to about $\approx 160$9 s if symmetric (Dobie et al., 2024).

The later study converted that initial single-burst detection into a securely periodic source through new detections across four interferometers. It reported ASKAP detections at $3.1$0 MHz, MWA detections at $3.1$1 MHz and $3.1$2–$3.1$3 MHz, MeerKAT detections at $3.1$4 MHz and in UHF band centered at about $3.1$5 MHz, and ATCA detections in a $3.1$6 GHz setup with band edge extending to $3.1$7 GHz. These detections span 2023–2024, with most pulses obtained in 2024, and securely place J1755−2527 within the emerging LPT population (McSweeney et al., 19 Jul 2025).

2. Period determination, dispersion, and scattering

Using barycentred times of arrival from all telescopes, the later study fit a timing model including period and dispersion measure. Because the low-frequency pulses are strongly scattered, each total-intensity pulse profile was modeled as an exponentially modified Gaussian, and the time of arrival was taken as the parameter $3.1$8 of the underlying Gaussian before scattering. The authors fixed the $3.1$9 GHz scattering timescale at $962\,\mathrm{rad\,m^{-2}}$0 and obtained a stable ephemeris over about $962\,\mathrm{rad\,m^{-2}}$1 years, with timing residuals scattered at the level of a few seconds (McSweeney et al., 19 Jul 2025).

The resulting timing relation is

$962\,\mathrm{rad\,m^{-2}}$2

with

$962\,\mathrm{rad\,m^{-2}}$3

and frequency-dependent scattering described by

$962\,\mathrm{rad\,m^{-2}}$4

This correction is especially important at MWA frequencies, where scattering delays can reach tens of seconds and bias times of arrival by a substantial fraction of the pulse width (McSweeney et al., 19 Jul 2025).

Parameter	Value	Notes
Period $962\,\mathrm{rad\,m^{-2}}$5	$962\,\mathrm{rad\,m^{-2}}$6	$962\,\mathrm{rad\,m^{-2}}$7
PEPOCH	$962\,\mathrm{rad\,m^{-2}}$8 MJD	Reference epoch
DM	$962\,\mathrm{rad\,m^{-2}}$9	Joint timing fit
$887.5$0	$887.5$1	Fixed in timing analysis
$887.5$2	$887.5$3	Consistent with no measurable spin-down or spin-up

The DM estimate in the discovery paper was $887.5$4, inferred from ASKAP dynamic spectra with $887.5$5 s sampling. The later paper refined this to $887.5$6, while emphasizing that low-frequency DM estimates can be badly biased if scattering is ignored. In the Appendix, the authors showed that for $887.5$7 and $887.5$8 around $887.5$9, the inferred DM can be biased by several hundred $288$0. Empirically, they measured an in-band DM at $288$1 MHz of $288$2, about $288$3 above the higher-frequency value, and therefore recommended deriving DM primarily from MeerKAT and ASKAP data where scattering is much smaller (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

The measured scattering is consistent with Galactic electron-density models rather than requiring local exotic scattering. For DM near $288$4–$288$5, NE2001 predicts $288$6 and YMW16 predicts $288$7, both close to the adopted $288$8 s value (McSweeney et al., 19 Jul 2025).

3. Burst morphology and polarimetric phenomenology

Across all observations, the pulses are single and relatively smooth. Their intensity profiles are approximately Gaussian, with asymmetric exponential tails at low frequencies due to scattering. The intrinsic Gaussian width parameter $288$9 is typically $1$0–$1$1 s; observed widths at about $1$2–$1$3 MHz are tens of seconds; and at higher frequencies the widths are about $1$4–$1$5 s. Peak flux densities span from about $1$6–$1$7 Jy in ASKAP $1$8 MHz data, to about $1$9–$10$0 Jy in MWA $10$1–$10$2 MHz data, with one $10$3 MHz pulse reaching about $10$4 Jy, and about $10$5 mJy for detected ATCA $10$6 GHz pulses. The corresponding fluences range from about $10$7 Jy s at GHz frequencies to about $10$8 Jy s at $10$9 MHz. Over months of MWA observations, pulse-to-pulse morphological variation is very small, suggesting a stable emission beam and scattering environment (McSweeney et al., 19 Jul 2025).

The discovery burst was broadband across the ASKAP band and had a steep spectral index

$30\,\mathrm{deg^2}$0

Its brightness temperature was estimated as $30\,\mathrm{deg^2}$1, exceeding the incoherent synchrotron limit and confirming coherent emission. Using $30\,\mathrm{deg^2}$2 kpc and $30\,\mathrm{deg^2}$3 Jy at peak, the discovery paper gave an isotropic-equivalent spectral luminosity $30\,\mathrm{deg^2}$4, placing the event squarely in the ULP regime rather than among stellar coherent bursts (Dobie et al., 2024).

Polarisation is one of the defining observational features of J1755−2527, but it is also one of its most ambiguous diagnostics. The discovery paper reported a highly polarised ASKAP burst with linear fraction $30\,\mathrm{deg^2}$5 at the main peak, $30\,\mathrm{deg^2}$6, and a smooth S-shaped position-angle swing. The normalized $30\,\mathrm{deg^2}$7 trajectory followed a smooth arc on the Poincaré sphere, and the authors successfully modeled it with the partial coherence model of Oswald et al. (2023b), involving two orthogonal intrinsically elliptical modes with a coherence fraction $30\,\mathrm{deg^2}$8 and a phase offset varying linearly with time (Dobie et al., 2024).

The later paper measured significant polarisation in three pulses: the original 2023 ASKAP $30\,\mathrm{deg^2}$9 MHz pulse, a 2024 ASKAP pulse, and a 2024 MeerKAT UHF pulse. It found a highly stable rotation measure: $10$0 for the 2023 ASKAP pulse, $10$1 for the 2024 ASKAP pulse, and $10$2 in magnitude for the 2024 MeerKAT pulse after correcting a sign-convention difference in the pipeline. Linear-polarisation spectral indices were $10$3, $10$4, and $10$5, respectively. At MWA frequencies, no polarised emission was detected even with $10$6 kHz channels; given RM near $10$7, substantial bandwidth depolarisation is expected at $10$8 MHz (McSweeney et al., 19 Jul 2025).

A central interpretive issue is that the later pulses do not preserve the discovery paper’s simple pulsar-like polarimetric picture. The classical rotating vector model,

$10$9

describes the 2023 ASKAP pulse reasonably well, but not the newer events. The 2024 ASKAP pulse has an essentially flat PA across the pulse, and the 2024 MeerKAT pulse is curved but not in a conventional RVM-like way. The later paper explicitly notes that the PA behaviour differs significantly from that expected in the classical RVM in two of the three polarised pulses. This suggests that either the active emission region changes between epochs, or propagation effects such as magnetospheric birefringence, mode switching, or Faraday conversion significantly modify the observed polarisation, or a simple single-dipole pulsar geometry does not apply (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

4. Intermittency, duty cycle, and observational coverage

Before the period was known, the discovery paper emphasized non-repeatability. Across ASKAP/VAST, MeerKAT, MWA GPM, and Parkes/Murriyang UWL, only a single burst was detected in more than $\approx 160$00 hours of radio coverage, implying an overall GHz-band duty cycle of about $\approx 160$01. Under an assumed active window of $\approx 160$02 days around the observed burst, all periods up to $\approx 160$03 minutes were ruled out and most periods up to $\approx 160$04 hours were also ruled out; under a more conservative scenario using only the detection observation and an immediately preceding overlapping field, periods $\approx 160$05 minutes and periods between about $\approx 160$06 and $\approx 160$07 minutes were excluded (Dobie et al., 2024).

Once the $\approx 160$08-hour ephemeris was established, historical non-detections could be reassessed more sharply. Re-reduction of ASKAP 2023 follow-up revealed one low-S/N pulse at the expected time of arrival on 2023-04-06, but other ASKAP data with predicted pulse arrivals still showed no detection. In the MWA GPM 2022 run, bi-weekly scans from 2022-06-02 to 2022-09-08 at $\approx 160$09–$\approx 160$10 MHz yielded no pulses, even though the 2024-measured pulse fluxes imply that dozens should have been detected if the source had been emitting at similar levels. In early 2024, three MWA snapshots containing predicted arrival times again showed no detection, whereas from 2024-06-20 onward the source was consistently detected in every MWA pointing that included a predicted time of arrival. ATCA then detected two faint pulses on 2024-10-04, but on 2024-10-12 saw no pulses even though four were predicted within a $\approx 160$11-hour track (McSweeney et al., 19 Jul 2025).

The later paper therefore concluded that J1755−2527 is intrinsically intermittent, with active periods lasting on the order of months and separated by months to years of quiescence. The ATCA non-detection on 2024-10-12 could still be explained by ordinary pulse-to-pulse amplitude variability because the 2024-10-04 detections were only $\approx 160$12–$\approx 160$13, but the 2022 MWA non-detections cannot reasonably be attributed to sensitivity alone. The authors compared this behaviour to GLEAM-X J162759.5−523504.3 and GCRT J1745−3009, both of which show on/off behaviour on month-to-year timescales (McSweeney et al., 19 Jul 2025).

Two broad mechanisms were proposed for the intermittency. In a geometric or orbital scenario, radio pulses are visible only over certain orbital phases, or over a longer apparent timescale produced by precession or a spin-orbit resonance. In a plasma or wind-driven scenario, the emission requires a sufficient supply of plasma from a companion’s wind or environment, so activity windows can be aperiodic or quasi-periodic and last months or longer. The paper suggested that month-scale intermittency might arise from beat phenomena when the spin-orbital or beat-orbital ratio is close to a small integer, or from variable companion winds, but stated that long-term monitoring is required to distinguish these possibilities (McSweeney et al., 19 Jul 2025).

5. Distance, Galactic setting, and counterpart constraints

The discovery paper used the YMW16 Galactic electron-density model to infer a distance $\approx 160$14 kpc from $\approx 160$15, with a $\approx 160$16 upper-limit distance $\approx 160$17 kpc for $\approx 160$18. The later paper adopted the distance estimate from Dobie et al. and noted that NE2001 and YMW16 both give $\approx 160$19 kpc for this line of sight. The observed scattering times, about $\approx 160$20 ms at $\approx 160$21 GHz, are likewise consistent with a path length of about $\approx 160$22–$\approx 160$23 kpc through the inner Galactic plane (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

The radio position was refined over time. The discovery paper gave $\approx 160$24 and $\approx 160$25 after beam-by-beam astrometric correction. The later paper reported a precise MeerKAT position at MJD $\approx 160$26: $\approx 160$27, $\approx 160$28. The source lies essentially in the Galactic plane. One paper quotes Galactic latitude $\approx 160$29, while the later paper quotes $\approx 160$30; in either case, the source is extremely close to the plane and on an inner-Galaxy line of sight (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

Multi-wavelength counterpart searches have so far been negative. The discovery paper reported no X-ray counterpart in three pre-burst Swift/XRT observations and no source in a post-burst XMM-Newton/PN observation, with absorbed $\approx 160$31–$\approx 160$32 keV flux limits of $\approx 160$33 and $\approx 160$34, respectively, for an absorbed power law with $\approx 160$35 and Galactic $\approx 160$36. Swift/UVOT also gave a non-detection with $\approx 160$37 in UVM2. These limits were described as not especially constraining given the distance, extinction, and delayed X-ray coverage (Dobie et al., 2024).

Optical and infrared limits are likewise dominated by severe extinction. The discovery paper reported no coincident source in DECaPS, VVV, Pan-STARRS1, or ZTF, with co-added limits of $\approx 160$38, $\approx 160$39, $\approx 160$40, $\approx 160$41, $\approx 160$42 in DECaPS and $\approx 160$43, $\approx 160$44, $\approx 160$45 in VVV. It estimated extinction toward extragalactic sources of $\approx 160$46 mag and $\approx 160$47 mag. The later paper summarized the counterpart situation as no optical/IR counterpart down to about $\approx 160$48 AB mag in archival PanSTARRS and UKIDSS data, and noted that the absence of an optical counterpart currently prevents direct spectroscopic confirmation of a companion or white dwarf (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

The discovery study specifically assessed and rejected a low-mass stellar origin. It argued that the combination of very high linear polarisation, broadband steep spectrum, and $\approx 160$49 is incompatible with known flare stars, ultra-cool dwarfs, or other stellar coherent emitters, whose radio luminosities are orders of magnitude lower. Using the $\approx 160$50-band limit and ultra-cool dwarf catalogues, it concluded that any such counterpart would be far too radio-faint to explain the ASKAP burst (Dobie et al., 2024).

6. Physical interpretation and relation to the long-period transient population

J1755−2527 sits at an intersection of two interpretive frameworks that evolved between the discovery and follow-up studies. The discovery paper stressed that its polarisation phenomenology resembles classical pulsars and magnetars: high fractional linear polarisation, significant circular polarisation, a smooth PA swing, and a Poincaré-sphere arc consistent with partial coherent addition of orthogonal modes in a birefringent magnetosphere. It therefore treated a neutron-star family interpretation as attractive, while explicitly leaving open white dwarf and other possibilities (Dobie et al., 2024).

The later paper, informed by the secure $\approx 160$51-hour periodicity and by accumulating identifications in the wider LPT population, instead argued that a white-dwarf-based scenario is favoured over a neutron star. Several LPTs now have optical identifications as white-dwarf plus main-sequence systems, including ILT J1101+5521 and GLEAM-X J0704−37, whose spectroscopically confirmed orbital periods equal their radio periods; CHIME/ILT J1634+44 appears to be a binary hosting a white dwarf; and AR Sco, J1912−4410, and SDSS J2306+24 are confirmed white dwarf pulsars. In this context the authors conjectured that J1755−2527 may host a white dwarf in a binary orbit, perhaps a strongly magnetised white dwarf interacting with a companion through a magnetic propeller, magnetospheric interaction, coherent curvature or cyclotron maser emission, or reconnection events modulated by orbital motion (McSweeney et al., 19 Jul 2025).

The measured period,

$\approx 160$52

is central to that argument. The later paper noted that this is slightly shorter than the canonical orbital period minimum of cataclysmic variables, especially for strongly magnetic systems or polars, where the observed minimum is about $\approx 160$53 hours. If the radio periodicity is orbital, J1755−2527 would therefore fall below the known minimum and raise questions about whether it is detached, pre-bounce, post-bounce, or subject to a magnetic modification of the period minimum. Because no optical spectroscopy is available, the $\approx 160$54-hour period could still represent the orbital period, the white-dwarf spin period, or a beat period (McSweeney et al., 19 Jul 2025).

Within the published LPT sample, J1755−2527 has the fifth longest period. Its period is comparable to GCRT J1745−3009 at $\approx 160$55 hours, somewhat shorter than ILT J1101+5521 at $\approx 160$56 hours and GLEAM-X J0704−37 at $\approx 160$57 hours, and much shorter than ASKAP J1839−0756 at $\approx 160$58 hours. Its duty cycle is low, its pulses are narrow compared with the period, its DM is relatively high, and its month-scale activity windows resemble those of GLEAM-X J1627−5235 and GCRT J1745−3009. The later paper emphasized, however, that it remains unclear whether all LPTs constitute a single physical class or a mixture of white-dwarf systems, neutron stars, and possibly other objects (McSweeney et al., 19 Jul 2025).

For neutron-star interpretations, the main difficulty is the extreme period. The later paper noted that if $\approx 160$59 hours were a neutron-star spin period, the source would be a hyper-long-period object well beyond the range of ordinary radio pulsars, and the absence of a stable RVM-like PA swing in the newer data weakens a straightforward pulsar analogy. It also did not detect a significant $\approx 160$60, so standard spin-down inferences are unavailable. The white-dwarf interpretation is therefore easier to reconcile with the period, pulse width, broad duty-cycle phenomenology, and binary-modulated intermittency, although the discovery paper’s pulsar-like polarimetric analysis still argues that magnetospheric propagation physics analogous to that of pulsars may be relevant (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).

Future work identified in the two studies converges on the same priorities. Long-term radio monitoring with wide-field facilities is needed to determine whether the month-scale intermittency is periodic or aperiodic and to search for secular $\approx 160$61. Higher-S/N multi-frequency polarimetry is needed to test whether any stable RVM component exists and whether the PA variability is frequency-dependent. Deep optical and infrared observations may reveal a heavily extinguished counterpart and establish whether the radio period is orbital through radial-velocity spectroscopy. Deeper X-ray observations could help discriminate between neutron-star and white-dwarf scenarios. A plausible implication is that J1755−2527 will remain a benchmark object for deciding whether the LPT population is primarily a white-dwarf phenomenon, a neutron-star phenomenon, or a heterogeneous mixture of both (Dobie et al., 2024, McSweeney et al., 19 Jul 2025).