Galactic Radio Explorer: an all-sky monitor for bright radio bursts

The advent of wide-field, broad band radio surveys combined with our ability to search data at high time resolution has led to a number of novel discoveries. The fast radio burst (FRB) phenomenon in particular has radically changed the radio astronomy landscape (Cordes & Chatterjee, 2019; Petroff et al., 2019). In response, many FRB surveys have been built or proposed.

The detection of a Galactic FRB from SGR 1935+2154 by both STARE2 and the Canadian Hydrogen Intensity Mapping Experiment (CHIME/FRB) was the most significant step to date in connecting extragalactic FRBs to a known phenomenon (The CHIME/FRB Collaboration et al., 2020; Bochenek et al., 2020b). The value of having such objects nearby is difficult to overstate, and detecting more ultra-bright bursts is essential to understanding the connection between magnetars and FRBs.

We propose the Galactic Radio Explorer (GReX, rhymes with “T-Rex”) as a complement to deeper, high-spatial resolution surveys. This is similar to how X-ray all-sky monitors and more sensitive instruments work symbiotically; GReX will detect rare ultra-bright Galactic bursts that cannot be discovered without nearly continuous all-sky monitoring. We are therefore building on the STARE2 design (Bochenek et al., 2020a), but with greater sensitivity, a five-times larger bandwidth, and clusters of antennas dispersed around the world. The GReX design is meant to maximize the detection rate of $\sim$ MJy bursts per unit cost.

FRBs have been detected over the frequency range 0.1 GHz to 8 GHz, though not simultaneously (Gajjar et al., 2018; Pastor-Marazuela et al., 2020; Pleunis et al., 2020). The angular distribution is approximately isotropic and the daily all-sky rate is $\sim$ $10^{3}$ with fluence above a few Jy ms (Petroff et al., 2019). The typical observed pulse widths are a few milliseconds, set by the $\sim$ ms back-ends of most blind surveys; some FRBs are known to be tens to hundreds of microseconds in duration. A subset of 22 FRBs is known to repeat¹¹1https://www.chime-frb.ca/repeaters(Fonseca et al., 2020; CHIME/FRB Collaboration et al., 2019), and two of those repeaters appear to do so periodically on weeks to months timescales (Chime/Frb Collaboration et al., 2020; Rajwade et al., 2020). It is still unclear if repeating FRBs and those that have not been seen to repeat form two physically distinct classes, though there is evidence that their pulse morphology (Pleunis & Chime/Frb Collaboration, 2021) and widths have different statistical distributions (Fonseca et al., 2020; Connor et al., 2020). Whether all FRBs are repeaters but with a large variation in time between bursts, or if there is a population of genuine once-off FRBs, remains one of the main outstanding questions in the field.

The events leading up to and during April 28, 2020 resulted in a dramatic link between magnetars and FRBs. On 27 April 2020, the Swift Burst Alert Telescope reported multiple bursts from the soft $\gamma$-ray repeater (SGR) 1935+2154, signaling that the magnetar had entered a phase of heightened activity. The next day the CHIME/FRB collaboration reported a dispersed burst with $\mathcal{F}$ of few kJy ms (0.4–0.8 GHz) in a side-lobe announced via ATel (Scholz & Chime/Frb Collaboration, 2020), but this fluence value was later revised by a factor of $10^{2}$ (The CHIME/FRB Collaboration et al., 2020). The daily inspection of recorded STARE2 triggers was then expedited, and ST 200428A was found at approximately at the same time and dispersion measure (DM) as the CHIME/FRB event. However, ST 200428A had a fluence that was a one thousand times greater than that reported by CHIME/FRB (Figure 2). Because this event was found in the complex sidelobes of CHIME, its fluence was simpler to measure in the STARE2 data. On 30 April 2020, the Five hundred metre Aperture Spherical Telescope (FAST) reported a weak (0.06 Jy ms) radio pulse and localized to SGR 1935+2154, with a DM consistent with the CHIME/FRB and STARE2 events.

Shortly thereafter, a constellation of space-borne instruments reported a one-second-long X-ray (1–250 keV) burst from the direction of SGR 1935+2154 that occurred at precisely the same time as the CHIME/FRB bursts and ST 200428A. This partnership between radio facilities (The CHIME/FRB Collaboration et al., 2020; Bochenek et al., 2020b; Zhang et al., 2020) is summarized graphically in Figure 2. Following the mega Janksy burst of 28 April, SGR 1935+2154 emitted intermittently bursts with fluence of 100 Jy ms (Kirsten et al., 2020) and has now finally become an intermittent pulsar at the X-ray period of 3.25 s and fluence of 40 mJy ms (Zhu et al., 2020). Clearly, magnetars are capable of emitting mega Jansky bursts but not all magnetar X-ray flares are accompanied by intense radio bursts. We need more observations of this phenomenon to understand the currently murky X-ray-radio connection.

In this paper we first discuss the science that can be done with a global network of antennas searching the sky continuously at tens of microseconds. We then describe the novel hardware and digital back-end that have been developed for DSA-110 and DSA-2000, which will be modified for GReX. This includes an ultra-wide band feed, extremely low-noise amplifiers (LNAs), and a sub-band single pulse search strategy. In Sect. 4, we model the Galactic pulsar and magnetar distribution in order the forecast FRB and giant radio pulse science that will emerge from GReX.

X-ray astronomy has shallow but very wide-field “all-sky monitors” or ASMs (e.g., Rossi X-ray Timing Explorer; MAXI) and also highly sensitive instruments (e.g., Rossi Proportional Counting Array; Chandra X-ray Observatory). The ASMs have historically played a major role in identifying rare but bright events that are missed by narrower field instruments. In the same way, as demonstrated by STARE2, radio astronomy would benefit from having a powerful radio ASM. GReX will fill this role.

Now let us examine the science drivers for GReX in some detail. Connor (2019) has analyzed the performance of FRB studies and argues that there likely exist many temporally narrow bursts missed due to instrumental smearing. Giant radio pulses (GRP) from pulsars have typical widths of microseconds but a few at the nanoseconds timescale have been detected (Soglasnov et al., 2004). The computational complexity is enormous for major FRB surveys like those of ASKAP and CHIME/FRB, which search many radio beams and seek both to localize FRBs and detect them at a high rate. As a result, these searches limit their blind detection temporal resolution for detection to $\sim$ 1 ms. In contrast, each unit of GReX receives only one signal stream with modest daily upload of candidates to Caltech. Thus, each GReX unit can undertake sub-millisecond pulses and in due course routinely explore the radio sky at greater resolution. Next, FRBs appear to have strong frequency structure. The usual approach of realizing higher sensitivity as $\sqrt{B}$ where $B$ is the bandwidth is no longer optimal. However, by undertaking sub-band searches, the ultra-wideband of GReX acts as a very broad frequency field of view.

The dynamic spectra of extragalactic FRBs show several distinct features. They are often band-limited, with downward drifting subpulses (Hessels et al., 2019), known as the “sad trombone” effect. While CHIME/FRB’s detection of FRB 200428 had significant time and frequency structure, it is not currently known if the mega bursts emitted by Galactic magnetars have similar dynamic spectra to extragalactic FRBs. The ultra-wideband receivers of GReX allow us to “catch” the narrowband bursts that STARE2 might have missed, but also to study their dynamic spectra over a 3:1 band—five times larger than the band of STARE2—and with almost 100 times better temporal resolution.

If Galactic mega bursts such as ST 200428A are found to be the same physical phenomenon as extragalactic FRBs, we will be able to answer major open questions in the FRB field. For example, is the coherent radio emission produced in the neutron star’s magnetosphere, or is it produced in a relativistic shock well outside of the light cylinder? What is the origin of periodic activity in repeating FRBs? If bright bursts from Galactic magnetars are found to be meaningfully distinct from other FRBs, then that is proof that multiple mechanisms can produce $\sim$ 10³⁰ erg Hz^-1 radio pulses; this would be evidence for the multiple-class interpretation of the FRB population.

GRPs have been detected from at least ten sources, mostly either young pulsars or MSPs (Kuzmin, 2007; Mahajan et al., 2018; Kuiack et al., 2020). Staelin & Reifenstein (1968) discovered the Crab pulsar via its GRPs only days before an independent group at Arecibo found its normal periodic emission in a custom integer FFT developed for pulsar searching. This history was recounted by Lovelace & Tyler (2012). All GRPs were detected after the pulsar or its supernova remnant were discovered, sometimes in targeted searches of pulsars with large light-cylinder magnetic field strengths (Knight et al., 2005). Pulsars are typically not discovered blindly via their giant pulses. The Crab is $10^{3}$ years old, and we expect $\mathcal{O}(10)$ such young neutron stars in the Milky Way, given the Galactic core-collapse rate (Rozwadowska et al., 2021) as well as recent High-Altitude Water Cherenkov (HAWC) observations (Albert et al., 2020). Their normal radio emission may be beamed away from us, but giant pulses are not from near the polar cap and may have different beaming properties (Philippov et al., 2020). The central compact objects of supernova remnants, which do not produce normal radio emission, may generate sudden bursts of SGPs powered by multipolar magnetic fields (De Luca, 2017). While they show no sign of strong pulsar wind nebulae and appear to have weak dipolar magnetic fields, their strong multipolar magnetic fields offers a potential energy source for bright bursts (Viganò & Pons, 2012). We emphasize that this scenario is speculative as such bursts have not been observed. The central compact objects of supernova remnants (De Luca, 2017), which do not produce normal radio emission, may generate sudden bursts of SGPs powered by multipolar magnetic fields. While they do not exhibit strong pulsar wind nebulae, It is likely that some SGPs from unknown pulsars may be detected by the unprecedented blind search of GReX. A pulse of $30\rm\,MJy$ in flux corresponds to $\nu L_{\nu}=1.4\times 10^{38}\rm\,erg\,s^{-1}$ at a distance of 2 kpc, so the (non-)detection of such pulses will test whether giant pulse isotropic-equivalent luminosity can (temporarily) exceed the spin-down luminosity of the pulsar.

We remark on two questions at the end of this section. STARE2 could see approximately 25$\%$ of the northern sky, meaning at most Earth rotational phases it would not have seen the FRB-like event from SGR 1935+2154. This explains the importance of covering the entire sky for rare but bright events. A world-wide GReX network (as described above) can be built for under a million dollars. Brilliant bursts such as FRB 200428 can be detected via the side-lobes of powerful facilities; indeed, that is how CHIME/FRB discovered FRB 200428. In the northern hemisphere, CHIME/FRB and GReX-I will cover a large portion of the visible sky from 400 MHz to 2 GHz, with 100 MHz of overlap at 750 MHz. This will allow for future symbioses similar to the joint discovery of FRB 200428-like events.

Low frequency facilities such as MWA and LWA naturally enjoy large field-of-view. However, computational costs and interstellar scattering increase as one attempts blind searches at lower frequencies. In the following section we carry out detailed modelling of the magnetars whose bursts we hope to detect.

The basic unit of GReX has a field-of-view of $\sim$ 1.5 steradian and a frequency range of 0.7–2 GHz. We will search data down to $\sim$ 10 $\mu$s. RFI rejection and crude localization (via timing) requires three units separated by at least one hundred kilometers. We call such a triplet as “cluster”. To cover the entire sky would require eight clusters (four in the North separated by 80 degrees in longitude and four in the South). Thus, a full-up GReX network would have 24 unit systems. Reducing the unit cost is important and is an engineering requirements in the GReX pilot phase. During GReX Phase I we will build a cluster with a 3:1 radio band and, thanks to novel LNAs, sensitivity at least twice that of STARE2. Combined, the increase in sensitivity relative to STARE2 could be as much as a factor of five. We are able to achieve such improvements by piggy-backing on the advances in electronics and antenna design provided by DSA-110 and DSA-2000. Leveraging this work, we will have the lowest system noise temperature (approximately 20 K) yet achieved without cryogenic cooling in the proposed frequency range. While this is our starting point, there is enough flexibility to cater the feed and back-end design to GReX’s science goals, for example trading field of view for forward gain or shifting the frequency range on a per-site basis to avoid RFI. Similarly, the RFI excision for each cluster of GReX antennas will likely need to be tailored to the local RFI environment, even if the algorithms are common across locations.

We have chosen the GReX configuration over, for example, a focal-plane phased-array without a reflector²²2https://old.astron.nl/r-d-laboratory/ska/embrace/embrace for the following reasons: The added complexity and cost related to developing such a system would be non-trivial, as would the increased cost of digitization, channelization, and beamforming compute hardware that follow from having more feeds. The current feed design provides a frequency-independent beamwidth, and a single Stokes I beam requires only two digital backends per antenna, one for each polarization. Therefore, if our goal is to monitor the whole sky continuously with high time and frequency resolution, we find that the most effective way is to deploy simple single radiometer systems around the world. Another suggestion for an all-sky Galactic FRB survey relied on Citizens-Science and cellular communication devices to search for $\sim$ 10 GJy bursts (Maoz & Loeb, 2017). However, the detection of ST 200428A just above the STARE2 threshold established that the Galactic FRB brightness distribution cannot be very flat and Giga bursts are likely exceedingly rare.

With a world-wide GReX system that includes both more sky coverage and more exposure to the Galactic plane, we anticipate more than an order of magnitude increase in detection rate over STARE2. To achieve this, we aim to create an assembly kit that can be shipped at cost to interested parties around the world.

By sending GReX clusters to southern latitudes we will gain exposure to the bulk of the Galactic plane, where magnetars reside. We also aim to increase our coverage in right ascension by deploying clusters at a range of longitudes. Using molecular gas and star formation as a proxy for the magnetar distribution in the Milky Way, we model the fraction of Galactic magnetars to which GReX will be exposed at a given time. Combining this with the increased sensitivity of each unit, we can forecast the expected Galactic FRB rate and speculate on the GRPs we can find from pulsars.

We take the Planck HFI 857 GHz map and assume we have clusters in the Southwestern United States, India, Western Europe and Australia. Based on Figure 4, we take the GReX antenna’s FWHM to be 80 deg. We compute to total visible material in the Planck 857 GHz map at a given time to estimate the fraction of Galactic magnetars that are observable at each cluster. The top panel of Figure 9 shows this fraction as a function of hour angle at four locations. Clearly, the heightened exposure to the inner Galactic plane gives the southern stations an advantage. The bottom panel show the same fraction, but as a function of coordinated universal time (UTC), including the instantaneous total exposure of all stations combined. For more than half of the time, this configuration is exposed to more than 30$\%$ of the magnetars in our Galaxy. Eventually the network will see the whole sky all of the time.

We have proposed GReX, a radio all-sky monitor that will detect the brightest bursts in the Galactic sky on sub-millisecond timescales. GReX Phase II will be an international network of ultra-wideband, high performance antennas that will continuously search for Galactic FRBs and super giant pulses from radio pulsars. Each hardware component will be low-cost and replicable, such that clusters of at least three GReX antennas can be shipped as an assembly kit around the world to span a wide range of latitude and longitude. The software and firmware back-end will also be standardized such that each system will require little intervention once running. We expect to find multiple new FRB-like events from Galactic magnetars each year. As the first wide-field, blind single-pulse survey at the microsecond level, we should to find new super giant pulses from Galactic pulsars, as well as previously-unknown phenomena.

We thank Dale Gary and Dan Werthimer for helpful discussions, as well as an anonymous referee for valuable advice on the manuscript.