The Breakthrough Listen Search For Intelligent Life Near the Galactic Center I

Brief

Gajjar et al. (2021) report preliminary results from the Breakthrough Listen program's deep survey of the Galactic Center (GC), using 7.0 hours on Parkes (1–4 GHz) and 11.2 hours on GBT (3.9–8 GHz) out of a planned 600-hour campaign spanning 0.7–93 GHz. The narrowband drifting-signal search constrains ETI transmitters to EIRP ≥4×10¹⁸ W across 60 million line-of-sight stars at 1–4 GHz and ≥5×10¹⁷ W across ~500,000 stars at 3.9–8 GHz; for the first time, artificially dispersed broadband transients are also bounded at EIRP ≥1×10¹⁴ W/Hz with repetition period ≤4.3 hours. The astrophysical yield includes several hundred bursts from the GC magnetar SGR J1745-2900, with no new radio-loud magnetar found above a peak luminosity limit of ≥10³¹ erg s⁻¹ at a burst rate ≥0.23 burst-hr⁻¹. The authors argue the GC is the optimal Schelling point for technosignature searches because it maximizes both the number of potential targets and the energy efficiency of any hypothetical beacon.

Metadata

Category

Search

Venue

The Astronomical Journal

Type

Peer-reviewed

Year

2021

Authors

Vishal Gajjar, Karen I. Perez, Andrew P. V. Siemion, Griffin Foster, Bryan Brzycki, Shami Chatterjee, Yuhong Chen, James M. Cordes, Steve Croft, Daniel Czech, David DeBoer, Julia DeMarines, Jamie Drew, Michael Gowanlock, Howard Isaacson, Brian C. Lacki, Matt Lebofsky, David H. E. MacMahon, Ian S. Morrison, Cherry Ng, Imke de Pater, Danny C. Price, Sofia Z. Sheikh, Akshay Suresh, Claire Webb, S. Pete Worden

DOI

10.3847/1538-3881/abfd36

arXiv

2104.14148

Access

Open access

Length

5.6 M

Programs

Breakthrough Listen

Instruments

Robert C. Byrd Green Bank Telescope (GBT), Parkes Observatory, Ultra Wideband Low (UWL) receiver, BL Digital Backend (BLDB), NuSTAR, Swift

Data sources

Breakthrough Listen GBT C-band observations (3.9–8 GHz, 11.2 h), Parkes UWL observations (1–4 GHz, 7.0 h), SGR J1745-2900 burst catalog

Tags

SETI, technosignature, narrowband beacon, Galactic Center, magnetar, Fast Radio Bursts, astrobiology, habitability

Key points

• EIRP upper limit of ≥4×10¹⁸ W placed on narrowband ETI transmitters at 1–4 GHz, covering 60 million Galactic Center stars in 7.0 hours of Parkes observations.p.1
• A tighter constraint of ≥5×10¹⁷ W applies to ~500,000 stars at 3.9–8 GHz from 11.2 hours of GBT C-band observations.p.1
• First-ever constraint on artificially dispersed broadband transients: EIRP ≥1×10¹⁴ W/Hz at 3.9–8 GHz with repetition period ≤4.3 hours.p.1
• Several hundred bursts detected from SGR J1745-2900; no new GC magnetar found above 10³¹ erg s⁻¹ at a rate ≥0.23 burst-hr⁻¹, constraining the radio-loud magnetar population.p.1
• The full survey plan covers 0.7–93 GHz in 600 hours across Parkes (352 h, UWL receiver) and GBT (280 h, C through W band), constituting the largest fractional bandwidth SETI search toward any single target.p.3
• Galactic habitability models (Gowanlock et al. 2011; Morrison & Gowanlock 2015) extended to R<2.5 kpc show the inner disk hosts the greatest fraction of stars with habitable planets, motivating GC as the primary SETI target.p.4
• SGR J1745-2900's rotation measure of −66960 ± 50 rad m⁻² is the highest of any known Galactic source, and its 1 GHz scattering time of 1.3 s is far below model predictions.p.5
• Schelling point analysis and energy-budget calculations (Benford et al. 2010a,b) favor the GC as the location a transmitting civilization would choose to maximize targets per watt, since required transmitter power scales as D².p.4

Verbatim

• “A line-of-sight towards the Galactic Center (GC) offers the largest number of potentially habitable systems of any direction in the sky.”

  p.1

• “For the first time, we were able to constrain the existence of artificially dispersed transient signals across 3.9–8 GHz with EIRP ≥ 1 × 10 14 W/Hz with a repetition period ≤ 4.3 hours.”

  p.1

• “We detected several hundred transient bursts from SGR J1745 − 2900, but did not detect any new transient burst with the peak luminosity limit across our observed band of ≥ 10 31 erg s − 1 and burst-rate of ≥ 0.23 burst-hr − 1 .”

  p.1

• “This constitutes the largest fractional bandwidth search for radio signals towards any source to date for SETI.”

  p.3

• “Comisso & Asenjo (2021) suggested that it is possible to extract energy from the spinning black hole at the GC.”

  p.4

• “the rotation measure (RM), − 66960 ± 50 rad m − 2 , is the highest RM value for any known Galactic source.”

  p.5

Most interesting

• The paper proposes that an ETI civilization could power a beacon for billions of years by extracting rotational energy from Sgr A*, the 4×10⁶ solar-mass black hole at the Galactic Center.
• Negative or artificially imposed dispersion measure, the opposite of what the interstellar medium produces, may be the only unambiguous indicator that a broadband pulse is non-astrophysical in origin.
• SGR J1745-2900 has been detected at frequencies up to 225 GHz, the highest frequency radio detection ever recorded for a magnetar, and its spectrum is remarkably flat compared to ordinary pulsars.
• The planned 600-hour survey spans a factor of ~130 in frequency (0.7 to 93 GHz), a range no prior SETI survey toward any single target has attempted.
• Within 0.8 kpc of the GC, stellar flybys are energetic enough to strip Oort clouds, yet the authors note this may paradoxically improve long-term habitability by depleting cometary reservoirs that would otherwise bombard inner planets.
• Breakthrough Listen's budget for this decade-long program is stated as US$100 million, making it the largest privately funded SETI effort in history.