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...@@ -12,120 +12,3 @@ The Green Bank Telescope is the obvious replacement platform for the ALPACA PAF ...@@ -12,120 +12,3 @@ The Green Bank Telescope is the obvious replacement platform for the ALPACA PAF
With the loss of AO, which will increase pressure on the GBT, many ongoing and planned observational campaigns would greatly benefit from a very widefield, multibeam, L-band survey instrument on the GBT. We describe below several examples of expressed interest from the broader scientific community in what could be accomplished using the ALPACA receiver on the GBT. None of these studies are funded directly under the ALPACA construction project, but the scientific community will find it of great utility as a facility instrument available on the GBT. With the loss of AO, which will increase pressure on the GBT, many ongoing and planned observational campaigns would greatly benefit from a very widefield, multibeam, L-band survey instrument on the GBT. We describe below several examples of expressed interest from the broader scientific community in what could be accomplished using the ALPACA receiver on the GBT. None of these studies are funded directly under the ALPACA construction project, but the scientific community will find it of great utility as a facility instrument available on the GBT.
### New Pulsars as Probes of Fundamental Physics and Astronomy
Pulsars are unrivaled laboratories for studying a wide range of phenomena in fundamental physics and astronomy. Even though we have been searching for (and finding) pulsars for over 60 years, we have only scratched the surface of the Galactic pulsar population (Lorimer et al. 2006; Levin et al. 2013). Pulsar surveys are still sensitivity limited, and new surveys invariably uncover unique, scientifically valuable gems. An incomplete list of the science enabled by new pulsars includes:
* ***Building a better gravitational wave detector:*** Pulsars are being used to make the next great advance in gravitational wave (GW) astronomy – exploring the low-frequency GW universe . Nanohertz-frequency GWs are produced by supermassive black hole binaries (SMBHBs; Jenet et al. 2005) during the early stages of inspiral (and possibly more exotic sources such as cosmic strings; Siemens et al. 2007), and the influence of those GWs on the Earth causes a slight deviation in the arrival time of pulses from high-precision millisecond pulsars (MSPs). The quadrupolar nature of GWs gives rise to a unique angular correlation pattern between pairs of pulsars which can be detected using a “pulsar timing array” (PTA) of dozens of MSPs distributed across the sky (Hellings & Downs 1983). The North American Nanohertz Observatory for Gravitational Waves (NANOGrav; McLaughlin 2013) leads the world in this technique by observing over 70 MSPs with the GBT, the Canadian $`\rm HI`$ Intensity Mapping Experiment, the Very Large Array, and (until recently) Arecibo. The most recent NANOGrav data release showed evidence for a common process between pulsars that is “not inconsistent” with a stochastic GW background, although the quadrupolar signature has not yet been detected (Arzoumanian et al. 2020). If this is a true GW signature, confirmation should be forthcoming in the next couple of years (Pol et al 2021).
Detection is only the first step. Over the next decade NANOGrav has the potential to characterize the GW background (caused by an ensemble of SMBHBs) and detect individual binaries, enabling a wealth of multi-messenger astrophysics (Kelley et al 2019). To do so, more pulsars need to be added to the array – simulations suggest that up to 200 MSPs may be suitable. The sensitivity of a PTA to the stochastic GW background increases linearly with the number of pulsars (Siemens et al. 2013). Large-area pulsar surveys and targeted, deep surveys of regions of the sky where few MSPs are known are crucial for these efforts, and ALPACA on the GBT will be a perfect instrument for both strategies.
* ***Understanding Strong Gravity:*** Pulsars are often found in highly relativistic binary systems with white dwarfs or other neutron stars, making them excellent test masses that probe space-time in the strong-field regime. For example, the double neutron star system (discovered in a large-area survey with the Parkes telescope; Lyne et al. 2004) provides the most precise overall tests of general relativity (GR; Kramer et al. 2006). The white-dwarf pulsar binary J0348+0432 (discovered in a large-area survey with the GBT; Lynch et al. 2013) uniquely constrains tensor-vector-scalar theories of gravity due to system’s large mass ratio (Antoniadis et al. 2013). The MSP triple system J0337+1715 (also discovered in a large area survey with the GBT; Ransom et al. 2014) places the best strong-field limits on the equivalence principle (Archibald et al. 2018). In all cases the data match the predictions of GR with astounding accuracy, but we know GR must break down at some point because it is incompatible with quantum mechanics. The long-sought after discovery of a pulsar in close proximity to a BH would provide the best tests yet (e.g. Liu et al. 2014; Seymour et al. 2018). ALPACA on the GBT will be a superb instrument for finding pulsars in the Galactic plane, which is the likeliest location of a pulsar-stellar mass BH binary.
* ***Determining the Neutron Star Equation of State*** We do not understand how matter behaves at the extreme densities found within neutron stars. However, every theoretical model for the equation of state (EoS) of neutron stars predicts a maximum mass above which the star will collapse. Finding massive neutron stars can thus directly rule out some EoSs. Perhaps the best example is PSR J1614-2230 (Demorest et al. 2010). The current record holder for the most massive neutron star is PSR J0740+6620 (Cromartie et al. 2020), which was only recently discovered in a wide-area GBT survey (Lynch et al. 2018). These results are extremely important for nuclear physics, and ALPACA on the GBT has the potential to find dozens of MSPs, increasing the probability of finding yet more massive neutron stars.
* ***Learning About the Life-Cycle of Pulsars:*** Pulsars are born in supernovae and typically emit for ~$`10^7`$ years. Pulsars in binary systems may then be recycled via mass transfer, forming a partially recycled pulsar with a rotational period of 10s of ms, or a fully recycled MSP. However, there is tremendous variety in the paths along which pulsars may evolve. For example, evidence from low-eccentricity double neutron star systems suggests that some pulsars are born in electron capture supernovae, instead of the typical iron core collapse variety (e.g. Ferdman et al. 2013). Only recently have we found MSPs that are in the act of transitioning between accreting, low-mass X-ray binaries and radio-loud MSPs (e.g. J1023+0038, discovered in a large-area pulsar survey with the GBT; Archibald et al. 2009). ALPACA on the GBT could uncover other interesting systems that reveal more detail about the rich life-cycles of pulsars.
* ***Finding a Connection Between Magnetars and FRBs:*** The origin of fast radio bursts (FRBs) is one of the greatest outstanding mysteries in modern astronomy. The discovery of FRB-like bursts from a Galactic magnetar – neutron stars with extremely strong magnetic fields that are typically distinguished by their high-energy emission – suggests a link between the two (Bochenek et al. 2020; CHIME/FRB Collaboration 2020), but with only six radio-emitting magnetars known[^footnote1], it is difficult to draw definite conclusions. A Galactic plane survey using ALPACA on the GBT will be well suited for finding radio-loud magnetars (which are young objects found at low Galactic latitudes). Indeed, one such object was found in a large-area survey using Parkes while in an X-ray quiescent state (Levin et al. 2010).
* ***Understanding How Pulsars Shine:*** Despite promising theoretical advances (e.g. Philippov et al. 2020), we still do not have a comprehensive understanding of pulsar emission mechanisms. A successful theory will need to explain not only typical emission featuress, but also the radio emission of magnetars, nulling, mode changing (e.g. Wang et al. 2007), and rotating radio transients (RRATs; McLaughlin et al. 2006; discovered in large-area pulsar surveys), and correlated changes between pulse activity and rotation (e.g. Kramer et al. 2006; Lyne et al. 2010). Many of these phenomena are more common in non-recycled long-period pulsars than in recycled MSPs, and such objects are concentrated at low Galactic latitudes. An ALPACA survey on the GBT has the potential to discover hundreds of long-period pulsars.
* ***Taking a Galactic Pulsar Census:*** Simulations suggest that the Galaxy is home to about 100,000 pulsars (Lorimer et al. 2006). However, these simulations are based on the results of large pulsar surveys, and the sensitivity and biases of these surveys must be well understood. To-date, most models rely heavily on the Parkes Multi-beam Pulsar Survey, the most successful survey ever. ALPACA on the GBT will enable a complementary survey in the northern hemisphere, which will provide a more accurate picture of the Milky Way pulsar population as well as providing key objects for gravitational wave detection and tests of theories of gravity mentioned above.
* ***Mapping the Magneto-ionic ISM:*** Pulsar signals are affected by dispersion, scattering, and Faraday rotation as they traverse the ionized interstellar medium (ISM). As such, pulsars serve as critical inputs to models of the free electron content of the Galaxy (e.g. Cordes & Lazio 2001; Yang et al. 2017) and its magnetic field (Wahl et al. 2021; Han et al. 2018). We have come to appreciate that the ISM is rich in filamentary structures that can be probed using pulsars (Brisken et al. 2010; Pen & Levin 2014). A better understanding of these structures is important for characterizing nuisance parameters in PTAs (Lam et al. 2016). ALPACA on the GBT could add hundreds of new sight-lines through the ISM, especially in complex regions near the Galactic center, which is not visible with the full sensitivity of FAST (and would not have been visible with ALPACA on Arecibo).
### An Example Pulsar Survey Using ALPACA on the GBT
The case for large pulsar surveys is strong, but how successful could ALPACA on the GBT be, especially in light of past and future surveys with next-generation instruments such as FAST?
To answer this question, former GBO summer student Lulu Agazie (currently a graduate student at the University of Wisconsin – Milwaukee) simulated a variety of potential GBT pulsar surveys, considering different receivers and regions of the sky, and taking into account the likely yield from other, sensitive telescopes. This project determined that an L-Band survey of the Galactic plane visible to the GBT, but skipping the parts of the plane visible to FAST, would be the most fruitful (see Figure the figure below). We estimated that a total survey duration of ~5000 hours would be “reasonable” and could detect approximately 500 new pulsars, about 40-50 of which would be MSPs. This would be one of the most successful pulsar surveys ever, but would likely require more than a decade to complete.
![Science_case_fig_1](uploads/709d0dbaa175af6cd379813ac6d69831/Science_case_fig_1.png)
*Figure: Optimal pulsar surveys in different parts of the sky. Colors indicate different past and future surveys: CRAFTS with FAST (green), the PALFA survey with Arecibo (yellow), the HTRU survey with Parkes and Effelsberg (dark purple), an ultra-wideband survey with the GBT (gold), and an L-Band survey with the GBT (pink). Light blue indicates regions with few pulsars. The GBT surveys use a dwell-time of 180 seconds, and different dwell times change the optimal survey region slightly by making the survey more sensitive. The colored lines are of constant declination: -45 degrees (the limit of the GBT, blue), -12 degrees (the lower limit of FAST, orange), and 60 degrees (the upper limit of FAST, green). We use the three regions where a GBT L-Band survey is best (in pink) for the rest of our analysis.*
At the time, the only GBT receivers that could be used for such a survey were the single pixel L-Band receiver and the 7-beam FLAG PAF. Surprisingly, FLAG only offered a marginal benefit over the L-Band receiver for a fixed amount of telescope time because its wider field of view (FOV) was mostly offset by a narrower bandwidth and higher band-averaged Tsys.
We have repeated this analysis for ALPACA, and, it is a game-changer. As can be seen in the Table below, a survey that would take over 6,000 hours with the L-Band receiver or FLAG could be completed in less than 800 hours (assuming an observing efficiency of 75%), while surveying a larger region of the Galaxy and finding more pulsars. For context, the soon-to-be-complete Green Bank North Celestial Cap Survey (which has been ongoing for more than a decade) has averaged about 500 hours per year and discovered about 200 pulsars. We can therefore expect ALPACA to accomplish more in less than two years than what a single-pixel L-Band survey would accomplish in over a decade!
| **GBT Pulsar Survey Yield and Duration** | ALPACA | Single Pixel L-Band |
| --------: | :-------: | :-------: |
| Dwell Time (s) | 600 | 180 |
| New PSRs | 519 | 467 |
| New MSPs | 45 | 43 |
| Total Survey Area (sq deg) | 2397 | 1712 |
| Total Survey Duration[^footnote2] (hr) | 770 | 6460 |
It is important to note that these results do not account for the deleterious effects of radio frequency interference (RFI), which decreases sensitivity and hence survey yield, especially for long-period pulsars (Lazarus et al. 2016). However, ALPACA can be tuned to a region of the spectrum relatively free of RFI, and we have added a survey degradation factor to account for the loss of some bandwidth near the edges of this range. Thus, while the estimates in the Table are probably somewhat optimistic, we do not expect the fundamental result to change – *ALPACA on the GBT would be revolutionary for pulsar science*.
The benefits of ALPACA on the GBT are so significant that it opens up entirely new possibilities. With 85% of the celestial sphere visible from the GBT, it may be feasible to survey a much larger region at higher Galactic latitudes, or to conduct more sensitive searches in especially promising regions of the Galaxy, such as the Galactic center (which was not accessible with Arecibo and will not be accessible with the full sensitivity of FAST), or parts of the sky with a dearth of MSPs. These approaches would be especially important for improving sensitivity to GWs and for finding young pulsars.
### Fast Radio Bursts: Understanding Extreme Source Physics and Probing the Magnetoionic Universe
FRBs originate primarily from extragalactic sources and occur at a rate of about 5000 per day over the whole sky. Widefield telescopes such as ASKAP and CHIME have dramatically increased the detection rate in recent years to of order one per day, but still a small fraction of the total rate. As with pulsars, some FRBs have greater value than others for showing unique phenomenology that can lead to a better understanding of their extreme emission physics ($`10^{10}`$ times brighter than Galactic pulsars), their dynamics (e.g. spin and precession), and cosmological population statistics. Finding these rare, key objects necessarily requires a large detection sample to choose from. At the same time, large FRB samples will ultimately allow tomographic mapping of the electron density and magnetic field of the intergalactic medium and galaxy clusters along with probing the interstellar media of host galaxies as well as our own.
ALPACA on the GBT will yield FRB detections as a by-product of pulsar surveys.
Although the detection rate will be smaller than the rate from the very wide-field telescopes, ALPACA allows some unique coverage of the discovery parameter space. FRBs discovered at L-band complement those found by CHIME in the lower 0.4-0.8 GHz range. Some FRBs that show repeated bursts are detected at L-band much more frequently than at low frequencies; a notable example is the first repeater FRB 121102 discovered at Arecibo, which has been detected only a few times with CHIME but more than 2000 bursts have been found at L-band, suggesting that other FRBs are likely missed by CHIME (and vice versa for L-band surveys). The FAST telescope, with much larger collecting area than the GBT, has provided most of the detections from FRB 121102, but its correspondingly smaller field of view (even with its 19-beam feed), implies a smaller survey speed, which seems to be more important for new FRB discoveries. The spectral properties of FRBs are much different than those of pulsars and it is clear that wide frequency coverage is needed to get a clear picture of the emission process, which may be modified by frequency-dependent propagation effects in intervening media. The GBT with ALPACA will provide significant coverage of L-band in FRB surveys that have a larger survey speed than FAST. While the instantaneous field of view is similar to that of the VLA at L-band, VLA surveys are limited to only a coarse time resolution that causes many bursts to be missed. GBT backend processors will not have this limitation.
ALPACA on the GBT will also allow follow-up observations to search for repeated bursts from FRBs, particularly those found with other telescopes that have poor sky localizations and lower sensitivity. ALPACA on the GBT will also provide the ability to search efficiently for FRBs in the directions of nearby galaxies (e.g. M31) and galaxy clusters (e.g. Virgo), that are extended at scales much larger than an individual beam of a single-pixel GBT receiver. The recent discovery of a Galactic FRB, the mega-Jansky burst from the soft gamma repeater SGR 1935+2154, underscores interest in sampling a large number of galaxies efficiently to capture such relatively rare events. The Galactic SGR/FRB demonstrates that the range of burst luminosities covers many orders of magnitude and that large samples of bursts from galaxies at similar distances will help disentangle the luminosity function from inverse square-law effects. It is also possible that FRB sources fall into multiple classes of objects (e.g. neutron stars vs. black holes, cosmic strings, etc.).
### Extra-galactic Surveys:
* ***Wide area $`\rm HI`$ surveys of the local Universe:*** A primary science goal for ALPACA on Arecibo was to use the wide multibeam field of view for efficient surveys of the low-mass end of the $`\rm HI`$ mass function. Existing large area $`\rm HI`$ surveys such as HIPASS (Zwaan et al. 2005), EBHIS (Kerp et al. 2011), and ALFALFA (Haynes et al. 2018) have provided valuable insights into the $`\rm HI`$ properties of galaxies and their variation with environments as well as revealing new classes of objects (e.g. Adams et al. 2013). These surveys are also vital for mapping out the large-scale structures in the local universe using peculiar velocities and the Tully-Fisher relation (e.g. Cosmicflows-4, Dupuy et al. 2021). Such large-area surveys have only been possible with multibeam receivers, compared to the current GBT $`\rm HI`$ receiver, ALPACA on the GBT would be capable of surveying 20 times faster. As such, it will be capable of improving on these surveys with better sensitivity than HIPASS, faster survey speeds than EBHIS, and more sky coverage than ALFALFA. Further, it will have more sky coverage and better sensitivity to extended $`\rm HI`$ features than the WSRT APERTIF surveys and better sky coverage than FAST.
* ***The low mass end of the $`\rm HI`$ mass function:*** A robust census of the lowest mass galaxies can provide fundamental constraints on the physics of dark matter, cosmic reionization and the interplay between gas, stars and star formation within their host dark matter halos. Gas-rich galaxies detectable in the $`\rm HI`$ 21 cm line play a critical role in the determination of the low mass end of the galaxy mass function and offer not only the gas mass but its dynamics. In $`\Lambda {\rm CDM}`$ scenarios, a turndown in the $`\rm HI`$ mass function (HIMF) is expected at masses below $`\rm M_{HI}`$ ~ $`10^8`$ $`{\rm M}_\odot`$ as galaxies are no longer able to retain the baryonic fuel needed to form stars. The turnover $`\rm HI`$ mass directly reflects the primordial power spectrum, the processes of galaxy formation and the interplay between baryons and dark matter (e.g. Bullock & Boylan-Kolchin 2017). The ALFALFA extragalactic $`\rm HI`$ survey (Giovanelli et al 2005; Haynes et al 2018) was specifically designed to sample the low mass ($`\rm M_{HI} < 10^8`$ $`{\rm M}_\odot`$) end of the HIMF and did so (e.g. Jones et al 2018). However, its results at the low-mass end remain limited by the statistics of the sample size, uncertainty in the galaxy distances and potential variations with local galaxy environment. Further work is needed particularly to establish the reality of the turnover of the HIMF and to explore the imprint of large-scale structure on baryon retention at the lowest masses.
Many next-generation $`\rm HI`$ line surveys are currently underway, including the planned CRAFTS (Zhang et al 2021) survey with the FAST telescope and the various surveys being undertaking with the SKA pathfinder arrays. As has been shown by Jones et al (2015), the singular niche of single-dish $`\rm HI`$ line surveys lies in the local universe where beam confusion is not a serious issue and where survey volume requires wide area coverage. A carefully planned moderately-deep survey with ALPACA on the GBT, which would have much larger sky coverage than the existing Arecibo/AGES (Auld et al. 2006) survey or the APERTIF medium-deep survey, would uniquely probe the low mass end of the $`\rm HI`$ mass function in a range of environments, extending from regimes of extreme isolation to filaments, groups and clusters over distances to 50 Mpc. The discovery of faint, low mass galaxies will also help populate the low mass end of the baryonic Tully-Fisher relation, in the regime where current cosmological models predicts a steepening of the slope due to the changing baryon retention fraction. It is likely that such a spectroscopic survey could be combined with one looking for transient/burst continuum sources.
* ***Mapping gas flow onto galaxies:*** Relocating ALPACA to the GBT will transform our ability to understand the flow of gas from the cosmic web onto individual galaxies. We know from statistical studies of UV absorption lines that the circumgalactic medium (CGM) of low redshift galaxies contains about half of the total gas mass of galaxy halos (Werk et al. 2014). These studies, however, tell us little about how the gas is distributed within individual galaxy halos or how it flows onto galaxy disks. For this, we need to make maps of $`\rm HI`$ emission.
Deep pointed observations with the GBT of atomic neutral hydrogen (HI) have shown the presence of $`\rm HI`$ in the CGM of nearby galaxies (e.g. Das et al. 2020), but detailed maps of the CGM show that this gas can be quite clumpy (Wolfe et al. 2013, 2016) resulting in a low covering fraction of $`\rm HI`$ (Howk et al. 2017). Together, these imply that to understand the flow of gas in the CGM onto galaxy disks requires excellent spatial resolution and sensitivity combined with large areal coverage out to the virial radius of individual galaxies.
A number of deep $`\rm HI`$ surveys of individual galaxies are currently underway with MeerKAT, such as MHONGOOSE (de Blok et al. 2020). However, interferometers have far less sensitivity to extended emission than single-dish telescopes. As such, the GBT with ALPACA is the ideal combination for studying the CGM of nearby galaxies.
As an example, Wolfe et al. (2016) mapped 12 square degrees of the halo of M31, spending 400 hours to reach a detection limit of $`{\rm N_{HI}} = 4 \times 10^{17} {\rm cm}^{-2}`$, orders of magnitude deeper than most $`\rm HI`$ emission surveys. To map the entire virial area of M31 would require a map of 1477 square degrees. To reach the same sensitivity as Wolfe et al. (2016), would require almost 50,000 hours (over 5 years) with the existing L-band receiver on the GBT! With ALPACA, the survey speed of the GBT is almost 20 times faster and such a survey becomes feasible in under 3000 hours and would revolutionize our understanding of the CGM of nearby galaxies. Such surveys could also be done commensally with transient surveys further increasing their scientific return.
### Galactic Studies:
The GBT has revolutionized studies of Galactic HI, OH, and other species because it provides high sensitivity to low surface-brightness emission over most of the celestial sphere. These capabilities remain unequalled to this day and have resulted in major advances in our understanding of many aspects of the Milky Way. Detection of low surface-brightness emission requires long integrations, thus limiting the sky coverage that can be achieved in the available time. ALPACA on the GBT will be every bit as transformational for these studies as the GBT has been, and will allow advances in many areas, a few of which are listed here.
* ***Galactic nuclear outflows of $`\rm HI`$ and OH:*** Most large galaxies like the Milky Way have nuclear outflows driven by star formation, an AGN, or both (Veilleux et al 2020). These winds can shape the subsequent evolution of a galaxy by redistributing gas and even, in some cases, quenching future star formation. The Milky Way's hot nuclear wind has entrained a population of neutral clouds, detectable in the 21cm line of $`\rm HI`$ (McClure-Griffiths et al 2013; Di Teodoro et al. 2018; Lockman et al. 2020). These clouds are unique tracers of the wind’s kinematics, energetics, and mass flow. They offer us the opportunity to study a nearby example of a universal phenomenon, revealing details not accessible in extra-galactic systems. The combination of GBT studies of faint $`\rm HI`$ emission with UV spectroscopy from HST reveals fundamental properties of the entrained gas such as abundances and ionization state (e.g., Ashley et al. 2020).
The $`\rm HI`$ cloud population in the Milky Way's nuclear wind extends over at least 400 square degrees on the sky. It has taken many hundreds of hours on the GBT to obtain our current, quite limited understanding, and $`\rm HI`$ clouds are detected down to the sensitivity limit of the data. There is much more to be discovered. ALPACA on the GBT would allow us to refine properties of the entrained nuclear gas -- e.g., kinematics, cloud evolution, energetics -- that will never be possible with the current single-pixel L-band system and available telescope time. Some of the entrained clouds contain molecular gas (Di Teodoro et al 2020), so it is certain that clouds will be detectable in OH as well. With ALPACA we can undertake a systematic study to map the column density, excitation, and kinematics of the molecular component of the outflow.
* ***The Milky Way’s Gaseous Halo:*** The second publication from the GBT reported the discovery of a population of $`\rm HI`$ clouds in the interface between the Galaxy’s disk and its gaseous halo (Lockman, 2002). Since then GBT has been a critical instrument in advancing our knowledge of the circumgalactic medium of the Milky Way: the volume outside the Galaxy’s disk that contains material expelled from the disk as well as gas infalling to fuel future star formation (Putman et al. 2012). GBT maps of faint $`\rm HI`$ have traced the structure and kinematics of the Magellanic Stream, have shown the complex interaction between infalling gas and the Milky Way disk, and have measured the magnetic field far from the Galactic disk (Nidiver et al. 2010; Barger et al. 2020; Betti et al. 2019). ALPACA on the GBT would revolutionize these studies, all of which require sensitive measurements over large areas of the sky, measurements, in some cases, an order of magnitude more sensitive than existing surveys.
* ***Molecular gas studies:*** Combining the large field-of-view with the large bandwidth of ALPACA will also transform studies of molecular gas, via OH emission, and ionized gas, via radio recombination lines (RRLs). Previous work with the GBT has shown that OH emission is a unique tracer of molecular gas that is not detectable in CO or other traditional tracers (Engelke et al. 2019, Busch et al. 2019). Dark molecular gas now appears to be ubiquitous in the outer Galaxy (Busch et al. 2021) but its OH lines are only a few mK in intensity, requiring long integrations. The beam of the GBT is well-matched to the size of these diffuse, dark molecular clouds making ALPACA ideally suited for these studies and allowing for deep, wide-area surveys of this gas in a systematic manner for the first time.
While there have been many studies of RRLs tracing ionized gas in star-forming regions in the Milky Way and other galaxies (Haffner et al. 2009, Alves et al. 2015), ALPACA, with its wide bandwidth and high spectral resolution, will allow for a deeper, wide-area spectroscopic survey of the diffuse ionized gas traced by multiple RRLs. This will allow astronomers to measure the physical conditions of the gas surrounding star-forming regions in the Milky Way and will complement the past surveys with Parkes and interferometers. Such surveys would require a prohibitive amount of time with the existing L-band receiver at GBT.
### SETI:
* ***Breakthrough Listen technosignature searches:*** Breakthrough Listen (Worden et al. 2017), launched in 2015, is the world’s largest search for technosignatures – indicators of technology that may have been developed by extraterrestrial intelligence. Listen has deployed a receiver / detector backend (MacMahon et al. 2018) to the GBO which has been in operation since the beginning of 2016, and represents the most capable digital spectrometer (with available frequency resolution as fine as hertz scales, over billions of channels) currently in operation anywhere in the world. Listen’s purchase of 20% of the available time on GBT has enabled the team, based at the University of California, Berkeley SETI Research Center, to undertake the most comprehensive, sensitive, and intensive search for extraterrestrial intelligence in history. To date, Listen has primarily used the single pixel receivers at L, S, C, and X-band, to perform technosignature searches of thousands of individual stars (e.g., Price et al. 2020) in addition to surveys of the Galactic center and some of the Galactic plane (Gajjar et al. 2021). Listen’s flexible digital backend is also capable of ingesting data from multi-beam receivers and has been commissioned for observations with KFPA and higher frequency receivers.
The 40 beams provided by ALPACA on the GBT would enable a dramatic increase in survey speed across the covered frequencies, enabling Listen to place much tighter constraints on the prevalence of technosignatures, at a given luminosity limit, without using additional telescope time. In particular, extended objects such as the Galactic center and plane, could be surveyed much more efficiently than with the existing single-pixel L-band receiver, enabling a northern-hemisphere counterpart to completement the current Breakthrough Listen Galactic plane survey at Parkes. Targets such as external galaxies, as well as globular clusters and other extended objects (Lacki et al. 2020) would also be well-suited to observations with ALPACA. Even for individual star observations, the presence of background stars in the GBT beam can be used to infer more stringent technosignature limits (Wlodarczyk-Sroka et al. 2020) and the increased field of view provided by ALPACA would enable surveys of much larger volumes in a given amount of observing time.
ALPACA’s multiple beams also provide a key capability for SETI that is lacking at GBT in the current single-pixel receiver suite at X-band and below. One of the key challenges for technosignature searches is RFI rejection, requiring alternating pointings on and off the target position. By providing simultaneous on and off measurements, ALPACA will enable much better rejection of time-varying RFI – particularly important as the RFI environment at L-band becomes more challenging with the increasing number of satellites transmitting in this range.
The Breakthrough Listen backend is also made available by the Listen team for up to 50 hours of shared risk observing each semester. To date, guest users have primarily undertaken studies of pulsars and FRBs, but the flexibility of the DSP hardware (allowing voltage capture, as well as total power spectrograms with easily-configurable frequency and time resolution), combined with the field of view of ALPACA, make this an attractive combination for all kinds of surveys.
* ***ALPACA interface with the Breakthrough Listen backend:*** The Breakthrough Listen instrument currently consists of a 64-node GPU-equipped cluster (MacMahon et al. 2018) that interfaces with the VEGAS backend. ALPACA will bypass VEGAS entirely, enabling additional flexibility in ingesting data from the receiver. The Listen instrument is capable of ingesting beamformed voltages directly from ALPACA if this mode is available. Alternatively Listen can ingest F-Engine outputs directly, as coarsely-channelized array element data streams, running beamforming on the Listen hardware. This mode would allow independent beamformer weights to be implemented, for example for commensal SETI searches alongside other users.
The ALPACA switch has 23 spare 100 GbE ports. Data can be transmitted via QSFP to the Listen backend as unicast ethernet packets during Listen’s 20% primary user time, or potentially as multicast for additional commensal searches. Commensal capabilities would provide a win-win in terms of science return from, for example, simultaneous technosignature and pulsar searches of the Galactic center region.
ALPACA provides a 305.2 MHz real-time processing bandwidth, within an overall frequency coverage of 1300 – 1720 MHz. User selection of subsets of coarse channels will be supported, enabling technosignature surveys to avoid regions of the spectrum that are essentially unusable due to persistent RFI, and thereby covering most of the band delivered from ALPACA.
### Acknowledgements:
James Cordes (Cornell University), Martha Haynes (Cornell University),
F. Jay Lockman (GBO), Ryan Lynch (GBO), D.J. Pisano (West Virginia University/University of Cape Town), Amit Vishwas (Cornell University), and Steve Croft (UC Berkeley / Breakthrough Listen) contributed to the Science Justification.
### [References](uploads/b33ac588ec93b0ebc9016804c3abdbdf/Science_case_v5_Sept_11_2021_references.pdf)
[Downloadable pdf version of this section](uploads/99c4dc7440834ef39091976853414472/Science_case_v5_BDJ_Sept_11_2021.pdf)
[//]: <> (Delete this file: uploads/580714ada2b228dd2b0f5cd7552917c6/Science_case_v5_BDJ_Sept_11_2021.pdf)
[Section 3.1 >>](3-front-end-design/3.1)
#### Footnotes
[^footnote1]: See http://www.physics.mcgill.ca/~pulsar/magnetar/main.html
[^footnote2]: We assume an observing efficiency of 75%.
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