A search for optical bursts from RRAT J1819-1458: II. Simultaneous ULTRACAM-Lovell Telescope observations

The Rotating RAdio Transient (RRAT) J1819-1458 exhibits ~3 ms bursts in the radio every ~3 min, implying that it is visible for only ~1 per day. Assuming that the optical light behaves in a similar manner, long exposures of the field would be relatively insensitive due to the accumulation of sky photons. A much better way of detecting optical emission from J1819-1458 would then be to observe with a high-speed optical camera simultaneously with radio observations, and co-add only those optical frames coincident with the dispersion-corrected radio bursts. We present the results of such a search, using simultaneous ULTRACAM and Lovell Telescope observations. We find no evidence for optical bursts in J1819-1458 at magnitudes brighter than i’=19.3 (5-sigma limit). This is nearly 3 magnitudes fainter than the previous burst limit, which had no simultaneous radio observations.

💡 Research Summary

The paper presents a coordinated high‑speed optical and radio campaign aimed at detecting optical bursts from the rotating radio transient (RRAT) J1819‑1458. This source is known for its extremely intermittent radio emission: ∼3 ms bursts occurring roughly every three minutes, amounting to about one second of radio activity per day. Because such brief, sporadic events are easily swamped by sky background in conventional long‑exposure imaging, the authors adopted a strategy of simultaneous observations with the fast optical camera ULTRACAM and the 76‑m Lovell radio telescope at Jodrell Bank. By recording the exact arrival times of the radio bursts, correcting for interstellar dispersion, and aligning them with the timestamps of the ULTRACAM frames (which have ∼50 µs relative and ∼1 ms absolute timing accuracy), they were able to co‑add only those optical frames that coincided with the dispersion‑corrected radio bursts.

Two observing runs were carried out. The first, on 2008 August 6, used ULTRACAM on the 4.2‑m William Herschel Telescope (WHT) in drift mode with 51.1 ms exposures and 1.4 ms dead time, yielding 112 588 frames over 1.5 h. The second, on 2010 June 14, employed ULTRACAM on the 3.5‑m New Technology Telescope (NTT) with 86.5 ms exposures and 3.5 ms dead time, producing 68 274 frames. Simultaneous Lovell observations recorded the radio bursts with an analogue filterbank in 2008 (64 × 1 MHz channels, 100 µs sampling) and a digital filterbank in 2010 (1024 × 0.5 MHz channels, 1 ms sampling). After de‑dispersion (the DM of 196 pc cm⁻³ corresponds to a 415 ms delay at 1.4 GHz) and barycentric correction, 24 bursts in 2008 and 25 bursts in 2010 were matched to optical frames.

The authors performed three complementary analyses. First, they summed the optical frames that matched the radio bursts and inspected the resulting images for any point source at the known X‑ray position of J1819‑1458; none was seen. Second, they also summed the frames immediately before and after each burst (n‑1, n+1, and n±1) to allow for possible optical lag or broader pulse width; again no source appeared. Third, they extracted a light curve from a small aperture centred on the RRAT position for every frame and searched for >5σ excursions coincident with the radio burst times. The light curves showed no such excursions; the distribution of points was consistent with pure Gaussian noise.

From these non‑detections the authors derive a 5σ upper limit of i′ = 19.3 mag for any optical burst associated with the radio events. This limit is roughly three magnitudes deeper than the previous ULTRACAM+WHT burst limit of i′ = 16.6 mag obtained without simultaneous radio data. The result implies that either optical bursts are intrinsically fainter than this limit, or that the emission mechanism does not produce detectable optical flashes simultaneous with the radio bursts. The paper discusses the comparison with the Crab pulsar, where the optical pulse is about five times wider and leads the radio pulse by ∼200 µs; the lack of a similar signature in J1819‑1458 suggests a different magnetospheric configuration or emission geometry. The authors also note that the radio bursts are distributed over three distinct rotational phases, and the bursts observed during the optical runs are statistically typical of the source’s overall activity, indicating that the source was not unusually active or quiet during the campaign.

In conclusion, the simultaneous high‑speed optical–radio observations set the most stringent optical burst limit for J1819‑1458 to date, constraining models that predict bright, short‑duration optical counterparts to RRAT radio bursts. The study demonstrates the power of coordinated multi‑wavelength, high‑time‑resolution observations for probing the emission physics of intermittent neutron stars and suggests that future observations with larger telescopes (e.g., the ELT) and even faster detectors could push the limits another 2 mag deeper, potentially revealing faint optical activity if it exists.