[Paper Review] The JCMT Transient Survey: Single Epoch Transients and Variability of Faint Sources

Doug Johnstone, Bhavana Lalchand+14 more|arXiv (Cornell University)|Aug 16, 2022

Stellar, planetary, and galactic studies4 citations

TL;DR

This study analyzes 5.5 years of JCMT 850µm observations across eight Gould Belt star-forming regions to detect single-epoch transients and long-term variability in faint submillimeter sources. The only robust transient detected was JW 566, a unique T Tauri binary flare eight times above the 4.5σ threshold (55 mJy/beam), while Source 2864 in NGC 2023 is identified as a highly variable blazar, suggesting variability across the EM spectrum aids source classification in crowded fields.

ABSTRACT

Short-duration flares at millimeter wavelengths provide unique insights into the strongest magnetic reconnection events in stellar coronae, and combine with longer-term variability to introduce complications to next-generation cosmology surveys. We analyze 5.5 years of JCMT Transient Survey 850 micron submillimeter monitoring observations toward eight Gould Belt star-forming regions to search for evidence of transient events or long-duration variability from faint sources. The eight regions (30 arcmin diameter fields), including ~1200 infrared-selected YSOs, have been observed on average 47 times with integrations of approximately half an hour, or one day total spread over 5.5 years. Within this large data set, only two robust faint source detections are recovered: JW 566 in OMC 2/3 and MGM12 2864 in NGC 2023. JW 566, a Class II TTauri binary system previously identified as an extraordinary submillimeter flare, remains unique, the only clear single-epoch transient detection in this sample with a flare eight times bright than our ~4.5 sigma detection threshold of 55 mJy/beam. The lack of additional recovered flares intermediate between JW 566 and our detection limit is puzzling, if smaller events are more common than larger events. In contrast, the other submillimeter variable identified in our analysis, Source 2864, is highly variable on all observed timescales. Although Source 2864 is occasionally classified as a YSO, the source is most likely a blazar. The degree of variability across the electromagnetic spectrum may be used to aid source classification.

Motivation & Objective

• Identify single-epoch transients and long-term variability in faint submillimeter sources across eight Gould Belt star-forming regions.
• Assess the prevalence of millimeter flares from young stellar objects (YSOs), particularly those associated with magnetic reconnection and coronal activity.
• Distinguish between YSOs and extragalactic contaminants (e.g., blazars) using multi-wavelength variability patterns.
• Establish detection thresholds for radio and X-ray luminosities of flares, comparing them to known super-flare populations.
• Use variability metrics to improve source classification in mid-IR color-color diagrams where YSOs and active galactic nuclei overlap.

Proposed method

• Conducted 47 average epochs of 30-minute integrations per field across eight 30′ diameter regions using JCMT's SCUBA-2 instrument at 850µm.
• Generated normalized standard deviation maps from all epochs to identify sources with significant flux variability.
• Applied false alarm probability (FAP) analysis to assess statistical significance of detected variability, with FAP < 0.01% for JW 566 and <3% for Source 2864.
• Compared flux variability across timescales to distinguish short-duration flares from long-term variability.
• Used radio-to-X-ray luminosity scaling relations (L_X / L_R ~ 10^15 Hz) to infer X-ray luminosities from detected radio flares.
• Cross-identified variable sources with known YSO catalogs and mid-IR color-color diagrams to assess source type and contamination.

Experimental results

Research questions

• RQ1 What is the rate of single-epoch submillimeter transients in faint YSO populations across the Gould Belt regions?
• RQ2 How do the luminosities of detected flares compare to known super-flare populations in X-ray and radio bands?
• RQ3 Can variability across the electromagnetic spectrum distinguish between YSOs and extragalactic contaminants like blazars?
• RQ4 What is the upper limit on radio and X-ray luminosities for undetected flares in the JCMT Transient Survey data?
• RQ5 Does the lack of additional transients above the 4.5σ threshold suggest a rarity of small-to-moderate flares or a limitation in sensitivity?

Key findings

• Only one robust single-epoch transient, JW 566 in OMC 2/3, was detected, with a flare amplitude eight times above the 4.5σ detection threshold of 55 mJy/beam.
• JW 566 remains the only confirmed submillimeter transient in the survey, with a peak flux of ~440 mJy/beam, indicating a rare and energetic event.
• No additional single-epoch brightening events were detected above 5.5σ (65 mJy/beam) or 4.5σ (55 mJy/beam) at YSO locations, implying a low incidence of moderate flares.
• Source 2864 in NGC 2023 is a highly variable source across all observed timescales and is most likely a background blazar, the first extragalactic variable detected in the survey.
• The non-detection of flares above 55 mJy/beam implies a radio luminosity threshold of L_R ~ 10^19 erg/s/Hz, corresponding to X-ray luminosity L_X ~ 10^34 erg/s, near the upper end of super-flare populations.
• Among 1200 monitored YSOs, only one burst was detected over ~40 epochs, with a 70-day equivalent observation of the Ophiuchus core yielding an upper limit of L_R ~ 10^18 erg/s/Hz for flares.