On August 15, 1977, the Ohio State University "Big Ear" radio telescope recorded a powerful, narrowband radio signal at 1420.455 MHz near the 21-centimeter neutral hydrogen line. Characterized by the alphanumeric printout "6EQUJ5", the signal lasted the full 72-second beam transit, matched the telescope antenna pattern, and displayed a signal-to-noise ratio exceeding 30. Despite hundreds of follow-up observations, the signal was never detected again. In 2024, archival analysis by the Arecibo WOW project identified similar narrowband emissions associated with rare magnetar-stimulated interstellar hydrogen masers.
| Parameter / Probe | Observational Value | Survey / Instrument Anchor | State |
|---|---|---|---|
| Observed Frequency | 1420.4556 ± 0.005 MHz | Big Ear Radio Observatory (Channel 2) | 0.05 MHz from H I Line (1420.4058 MHz) |
| Spectral Bandwidth | < 10 kHz (Narrowband) | Big Ear 50-channel Receiver | Unprecedented for Natural Emitters |
| Peak Signal-to-Noise Ratio | SNR ≈ 30 (Flux ~ 30 Jy) | Equaled Level "U" (30x RMS noise) | Strongest Signal in Big Ear History |
| Sky Localization (J2000) | RA: 19h 28m, Dec: -26° 57' or -27° 03' | Dual Feed Horn Ambiguity | Constrained in Sagittarius |
On August 15, 1977, the Ohio State University Radio Observatory—affectionately known as the "Big Ear"—was conducting a continuous SETI search. The Big Ear was a meridian transit telescope, using Earth's rotation to sweep across the sky. Because of its fixed antenna geometry, a stationary celestial source would take exactly 72 seconds to sweep through the antenna's 8-minute-wide primary beam, rising in intensity for 36 seconds and falling symmetrically for 36 seconds.
Reviewing computer printouts several days later, astronomer Jerry Ehman spotted an extraordinary sequence in Channel 2:
6 - E - Q - U - J - 5.
In the Big Ear's recording system, digits 0–9 represented signal intensities relative to noise, while letters represented multiples of ten (A = 10, B = 11... U = 30). The signal peaked at "U", representing a signal-to-noise ratio of 30, corresponding to an equivalent flux density of approximately 30 Janskys. Surprised by the perfection of the light curve, Ehman circled the characters in red ink and scribbled the margin note: "Wow!".
Crucially, the signal possessed three defining physical properties:
1. Extreme Narrowband Character: The signal was confined to a single 10-kHz-wide channel (1420.455 MHz), with no detected emission in adjacent channels. Natural astrophysical continuum emitters (pulsars, synchrotron, thermal bremsstrahlung) radiate across gigahertz bandwidths.
2. The "Water Hole" Protected Frequency: The frequency lay within the 1400–1427 MHz internationally protected radio astronomy band, centered right beside the 1420.4058 MHz hyperfine spin-flip transition of neutral atomic hydrogen (H I)—the exact interstellar communication channel proposed by Philip Morrison and Giuseppe Cocconi in their seminal 1959 paper.
3. Perfect Antenna Pattern Fit: The bell-shaped intensity progression over 72 seconds precisely matched the transit of a point source at infinite distance, ruling out birds, aircraft, or nearby ground reflections.
Despite extensive efforts by the Big Ear team, the Very Large Array (VLA), and the Robert C. Byrd Green Bank Telescope, the signal was never detected again. Because the Big Ear utilized two feed horns and the recording system could not distinguish which horn received the signal, two possible coordinates in Sagittarius remain.
In 2024, the Arecibo WOW Project, led by Abel Méndez at the Planetary Habitability Laboratory, analyzed historical archival data from the 305-meter Arecibo Observatory. They discovered transient, narrowband emissions near the 21 cm hydrogen line associated with cold, diffuse interstellar clouds. They proposed that the Wow! Signal may have been the first recorded observation of a natural hydrogen astrophysical maser: an intense flare from a background magnetar or soft gamma repeater temporarily pumped the energy levels of atomic hydrogen in a foreground cloud, producing stimulated coherent narrowband emission at 1420.4 MHz. Whether the Wow! Signal was an unprecedented natural astrophysical laser or an intentional technological transmission remains one of astronomy's most compelling mysteries.