Discovery of a new Soft Gamma Repeater: SGR J0418+5729

On 2009 June 5, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray Space Telescope triggered on two short, and relatively dim bursts with spectral properties similar to Soft Gamma Repeater (SGR) bursts. Independent localizations of the bursts by triangulation with the Konus-RF and with the Swift satellite, confirmed their origin from the same, previously unknown, source. The subsequent discovery of X-ray pulsations with the Rossi X-ray Timing Explorer (RXTE), confirmed the magnetar nature of the new source, SGR J0418+5729. We describe here the Fermi/GBM observations, the discovery and the localization of this new SGR, and our infrared and Chandra X-ray observations. We also present a detailed temporal and spectral study of the two GBM bursts. SGR J0418+5729 is the second source discovered in the same region of the sky in the last year, the other one being SGR J0501+4516. Both sources lie in the direction of the galactic anti-center and presumably at the nearby distance of ~2 kpc (assuming they reside in the Perseus arm of our galaxy). The near-threshold GBM detection of bursts from SGR J0418+5729 suggests that there may be more such dim SGRs throughout our galaxy, possibly exceeding the population of bright SGRs. Finally, using sample statistics, we conclude that the implications of the new SGR discovery on the number of observable active magnetars in our galaxy at any given time is <10, in agreement with our earlier estimates.

💡 Research Summary

The paper reports the discovery of a new soft gamma repeater, SGR J0418+5729, based on observations made on 2009 June 5 by the Gamma‑ray Burst Monitor (GBM) aboard the Fermi Gamma‑ray Space Telescope. GBM triggered on two short, relatively faint bursts at 20:40:48.883 UT and 21:01:35.059 UT. Initial GBM localizations, although subject to a systematic uncertainty of 2–3°, placed both events within a common region distinct from the previously known SGR J0501+4516. Independent triangulation using Konus‑RF (on the CORONAS‑PHOTON spacecraft) and Swift/BAT provided annuli that intersected precisely at the new source position, ruling out the known SGR as the origin.

Swift/XRT observations on 2009 July 8 identified a new X‑ray source with a 3.6″ error radius at RA = 04h 18m 33.70s, Dec = +57° 32′ 23.7″. To refine the position, a 23.8 ks Chandra/HRC imaging observation was performed on 2009 July 12, yielding an absolute position of RA = 04h 18m 33.867s, Dec = +57° 32′ 22.91″ with a 0.35″ (95 % confidence) uncertainty. Near‑infrared imaging with Palomar/WIRC in the Kₛ band reached a 5σ detection limit of 19.6 mag but did not reveal a convincing counterpart within the Chandra error circle; two nearby sources (Kₛ ≈ 17.7 mag and 18.8 mag) lie just outside the refined Swift error region, and a very faint source (Kₛ ≈ 21.6 mag) is at the detection limit and cannot be confirmed.

Temporal analysis of the GBM bursts using Time‑Tagged Event (TTE) data shows extremely short durations: the first burst has T₉₀ = 40 ± 7 ms (T₅₀ = 10 ± 4 ms) and the second T₉₀ = 80 ± 6 ms (T₅₀ = 34 ± 4 ms). Spectral fitting with the RMFIT package indicates that an optically‑thin thermal bremsstrahlung (OTTB) model provides the best description for both events, with characteristic temperatures of ~30 keV and ~35 keV, respectively. These spectral and temporal properties are consistent with typical SGR bursts and differ from simple power‑law or blackbody models.

A systematic search of GBM continuous data and the Interplanetary Network (IPN) archive revealed only one additional, untriggered, very weak burst occurring ~5 minutes before the first GBM trigger; no further activity was detected in the weeks surrounding the event. This suggests that SGR J0418+5729 was active only briefly during the observed interval.

The discovery of SGR J0418+5729, together with the earlier detection of SGR J0501+4516 in the same sky region, underscores the capability of wide‑field instruments like GBM to uncover faint magnetar activity near the detection threshold. The authors argue that many similar low‑luminosity SGRs may remain undiscovered, potentially outnumbering the known bright population. Using simple population statistics, they estimate that the number of observable active magnetars in the Milky Way at any given time is less than ten, a figure consistent with previous estimates. This work therefore refines our understanding of magnetar demographics and highlights the importance of continuous, all‑sky monitoring for revealing the hidden population of soft gamma repeaters.