Identification of a Local Sample of Gamma-Ray Bursts Consistent with a Magnetar Giant Flare Origin

The histories of gamma-ray bursts (GRBs) and magnetars are intertwined. Short bursts of gamma rays were recorded by the Vela satellites beginning in 1967 (Klebesadel et al. 1973) and were given the phenomenological name GRBs. The InterPlanetary Network (IPN) localized GRB 790305B to the Large Magellanic Cloud (Mazets et al. 1979; Evans et al. 1980). It was unique in being the brightest event seen at Earth, the prompt emission had a long-lasting, exponentially decaying periodic tail (Barat et al. 1979), and additional weaker bursts were localized to the same source (Mazets et al. 1979). Immediately, there were papers investigating whether the main event shared a common origin with other GRBs (Cline et al. 1980; Mazets et al. 1982). It is now known to be the first signal identified from a magnetar.

Key results on the nature of GRBs in the subsequent decades were often proven by population-level statistical analysis before direct "smoking-gun" proof. Perhaps the greatest debate was whether these events had a galactic or an extragalactic origin, with the latter initially disfavored, as it would require intrinsic energetics beyond anything previously known. Proof came first indirectly via statistical studies of the spatial distribution of GRBs (Meegan et al. 1992) and then directly from redshift measurements (Metzger et al. 1997).

Studies of the prompt GRB emission provided strong evidence in favor of two populations (Kouveliotou et al. 1993), with short and long GRBs traditionally separated at 2 s, as measured by the T₉₀ parameter. Long GRBs were tied to broad-line type Ic core-collapse supernovae (CCSNe) called collapsars (Galama et al. 1998). The Neil Gehrels Swift Observatory (Swift) mission enabled successful detections of afterglow from a sample of short GRBs. Circumstantial evidence pointed toward a neutron star merger origin (Eichler et al. 1989; Fong et al. 2015), with direct confirmation that some GRBs arise from binary neutron star mergers coming with GW170817 and GRB 170817A (Abbott et al. 2017).

Yet another debate on the behavior of GRBs is whether or not the sources repeated. This is best explained using modern parlance. Soft gamma-ray repeaters (SGRs) are galactic magnetars named phenomenologically for the weak, recurrent short bursts that first identified them before their physical origin was known. SGR flares are classified as distinct from GRBs and have recently been tied to radio emission similar to the cosmological fast radio bursts (Bochenek et al. 2020). The flare on 1979 March 5 and the subsequent similar events GRB 980827 (Hurley et al. 1999; Mazets et al. 1999b) and GRB 041227 (Palmer et al. 2005; Frederiks et al. 2007a) from magnetars in the Milky Way are referred to as magnetar giant flares (MGFs). The designation for the prompt emission of MGFs often carries the GRB designation, which we use here. GRBs are not thought to repeat as collapsars and neutron star mergers are cataclysmic events. While several galactic magnetars have been observed to produce multiple SGR flares, none have been observed to produce multiple giant flares (though this is not surprising). The historic debate on potential repeating GRBs was likely confounded by magnetar transients before the separation of SGR flares from GRBs.

We here refer to GRBs 790305B, 980827, and 041227 as the known MGF sample. The detection of three from the Milky Way and its satellite galaxies implies a high intrinsic rate on a per-galaxy or volumetric basis. These events should be detectable to extragalactic distances by GRB monitors such as Konus-Wind (Aptekar et al. 1995), Swift-BAT (Barthelmy et al. 2005), and Fermi-GBM (Meegan et al. 2009). However, at these distances, only the immediate bright spike would be detectable, and the event should resemble a short GRB (Hurley et al. 2005). There are two events discussed in previous literature as extragalactic MGF candidates, GRB 051103 (Ofek et al. 2006; Frederiks et al. 2007b; Hurley et al. 2010) and GRB 070201 (Mazets et al. 2008; Ofek et al. 2008), whose chance alignment coincidence was measured to be ∼1% (Svinkin et al. 2015).

There have been population-level searches for additional events, which identified no additional candidates (Popov & Stern 2006; Ofek 2007; Svinkin et al. 2015). However, these studies allow us to constrain the fraction of detected short GRBs that have an MGF origin; Ofek (2007) showed that the rate of galactic events requires this to be >1%, while the lack of additional candidates found in several searches constrains the upper bound to be <8% (Tikhomirova et al. 2010; Svinkin et al. 2015; Mandhai et al. 2018). These studies and their conclusions generally assumed that the brightest MGFs could be detectable to tens of megaparsecs.

Recently, GRB 200415A was identified as the third and likeliest extragalactic MGF (Svinkin et al. 2021). In this work, we perform a new population-level search utilizing the largest GRB sample and new galaxy catalogs that are both more complete and provide additional information, and we develop a new formalism to determine if we can prove that extragalactic MGFs contribute to the observed GRB population. Section 2.4 details the search formalism that identifies four nearby events, identifying an additional extragalactic candidate. The progenitors of our identified sample are investigated in Section 3, the implications of which are discussed in Section 4. We conclude with discussions in Section 5.

The "smoking-gun" evidence of an MGF is the long periodic tails, which are modulated by the rotation period of the neutron star (Hurley et al. 1999) and also show quasi-periodic oscillations related to the modes of the neutron star itself (Barat et al. 1983; Israel et al. 2005; Strohmayer & Watts 2005; Watts & Strohmayer 2006). However, these signatures are not unambiguously identifiable at extragalactic distances with existing instruments. As such, we follow prior population-level searches and focus on spatial information; if a well-localized short GRB is an MGF, it should occur within ∼50 Mpc and be consistent with a cataloged galaxy. We combine existing GRB and galaxy catalogs to build the most complete set of information from existing literature. For each individual burst, we quantify our belief that it is an MGF from a known galaxy through comparison of two probability distribution functions (PDFs), which are discussed below. These PDFs are generated in HEALPix (Gorski et al. 2005). The resolution of the HEALPix maps is defined by the NSIDE parameter, where the number of total pixels is equal to the square of the NSIDE times 12. The maps were generated with NSIDE = 8192, corresponding to a pixel width of ∼05.

2.1. The GRB Sample

2.2. The Galaxy Sample

2.3. MGF Spatial Distribution

We seek an all-sky PDF, P^MGF, representing the probability that a given position will produce a MGF with a particular fluence at Earth. Note that this is determined by the fluence of each burst considered but is constructed independently of the location of the burst itself, P^GRB. The comparison of the two PDFs generated for each burst quantifies the likelihood that a given short GRB has an MGF origin, which is performed in the next section. This section details the burst-specific construction of P^MGF.

If a given burst has an MGF origin it should arise from a cataloged galaxy and its intrinsic energetics should fall into the expected range. To construct this, we compute a weight for each galaxy representing how likely it is to have produced the observed fluence for the burst under consideration. This weight has two components: a linear weighting with SFR and a more complex weighting that compares the inferred intrinsic energetics (determined by the burst fluence and potential host galaxy distance) against an assumed PDF.

Magnetars are expected to be able to produce MGFs only for a short period of time (approximately 10 kyr; Beniamini et al. 2019), tying the predicted rate of MGFs to the rate of their formation. The rate of CCSNe can be inferred from the SFR, since the lifetimes of stars that undergo core collapse are much shorter than the timescale probed by the SFR tracers (Botticella et al. 2012). Under the assumption that the dominant formation channel for magnetars is CCSNe (which is explored in Section 4), we can infer the rate of MGFs from a galaxy from its SFR. Thus, each galaxy is linearly weighted with SFR. We use the far-UV measure of SFR (Lee et al. 2010) when available, as it should track massive stars likely to undergo core collapse; otherwise, we use the Hα measure (Kennicutt 1998) scaled by the average difference from galaxies with both measures to account for the lack of dust correction in the LVG catalog.

Next, we can determine the total isotropic-equivalent energetics of a potential burst–galaxy pair as E_iso = 4π d² S, where S is the burst fluence and d is the distance to the potential host. This value can be compared to an assumed intrinsic energetics PDF to determine how likely the event is to be an MGF. For example, a particularly high fluence short GRB spatially aligned with a distant galaxy would require an intrinsic energetics far beyond what has been observed in the galactic MGFs, excluding an MGF origin. We note that some studies utilize the peak luminosity ${L}_{\mathrm{iso}}^{\mathrm{Max}}$, but we work with an E_iso distribution, as there is stronger theoretical guidance on the maximum total energy that can be released (related to the magnetic fields of the magnetar) than on the timescale on which it is released.

We now construct an informed intrinsic energetics function, assuming a power-law distribution with an assumed minimum and maximum value, which is similar to the behavior of lower-energy magnetar flares (Cheng et al. 1996). Our method bypasses the need for an assumed detection threshold, which is difficult to quantify when considering many instruments over 30 yr. The assumed and inferred values are reported below, with the initially determined distribution shown in Figure 1.

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The slope of a power law can be determined via maximum likelihood, independent of an assumed maximum value, as

$$\begin{eqnarray}&&\alpha =1+n{\left[\displaystyle \sum _{i=1}^{n}\mathrm{ln}\left(\displaystyle \frac{{E}_{\mathrm{iso},{\rm{i}}}}{{E}_{\mathrm{iso},\min }}\right)\right]}^{-1},{\sigma }_{\alpha }=\displaystyle \frac{\alpha -1}{\sqrt{n}}+O({n}^{-1}),\end{eqnarray} \tag{ 1 }$$

where the sum is over the observed E_iso and ${E}_{\mathrm{iso},\min }$ is the lowest considered value (Newman 2005; Bauke 2007). We set ${E}_{\mathrm{iso},\min }$ as 1.0 × 10⁴⁴ erg, which is a factor of a few below the lowest value measured in a known MGF, as shown in Table 1, but above the brightest SGR flare that lacked the periodic tail emission (Mazets et al. 1999a). Iterating over the E_iso values of the known MGFs (GRBs 790305B, 090827, and 041227) gives α = 1.3 ± 0.9 at 90% confidence, where we have included the O(n⁻¹) error contribution. In order to minimize the required computation, we assume the centroid (α = 1.3) in what follows; the effect of this assumption on our results is discussed in the closing paragraph of this section.

There must be a physical maximum energy for an MGF, which should be related to the total magnetic energy. This is supported by the lack of detection of more energetic events otherwise consistent with an MGF origin. The highest E_iso observed for a known MGF is 2.3 × 10⁴⁶ erg, which comes from the magnetar with the highest reported magnetic field at the surface of 2.0 × 10¹⁵ G (Olausen & Kaspi 2014). We note that this reported value is approximately three times larger than the dipolar spin-down inferred magnetic field value of 7 × 10¹⁴ G (Younes et al. 2017), but we have confirmed that this does not affect our results. To determine an ${E}_{\mathrm{iso},\max }$ for our search, we assume a dipole field, where the available energy scales as B², and a nominal maximum magnetic field strength of ∼1.0 × 10¹⁶ G. This gives ${E}_{\mathrm{iso},\max }=2.3\times {10}^{46}\,\mathrm{erg}\times {\left(1.0\times {10}^{16}{\rm{G}}/2.0\times {10}^{15}{\rm{G}}\right)}^{2}$ $=\ 5.75\ \times \ {10}^{47}$ erg.

This allows us to determine the burst-specific two-component weight for each of the >100,000 galaxies in our sample, which are weighted linearly by SFR multiplied by the value of the E_iso PDF for the inferred energetics considering the burst fluence and galaxy distance. The sum of the galaxy weights is normalized to unity. Then, P^MGF is built by placing the calculated weights at the position of the host galaxy. If the angular diameter of the galaxy is larger than the effective resolution of our discrete sky representation (∼arcmin²), then its weight is uniformly distributed over its angular extent.

2.4. The Search

For each of the 250 short GRBs in our sample, we generate P^GRB from the observations of the GRB and P^MGF from theoretically motivated expectations. We quantify the likelihood that a given GRB has an MGF origin using ${\rm{\Omega }}=4\pi {\sum }_{i}{P}_{i}^{\mathrm{GRB}}{P}_{i}^{\mathrm{MGF}}/{A}_{i}$, where P^GRB _i and P^MGF _i indicate the probability for each PDF in the ith sky region, which has area A_i (Ashton et al. 2018).

Significance is determined by the empirical false-alarm method (e.g., Messick et al. 2017) with Ω as our ranking statistic. Our backgrounds are generated by simulating different galaxy distributions. Each iteration is generated by uniform rotation of the 2D (R.A., decl.) positions of the galaxies in our sample, which maintains the distance and SFR distributions, as well as local structure. Population-level confidence intervals created through comparison of each rotation against our full GRB sample with results are shown in Figure 2. At three and four events, the short GRB sample has an excess surpassing 5σ discovery significance, with individual significance values of the four bursts between 1.2 × 10⁻⁴ and 4.9 × 10⁻⁶, as given in Table 1.

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Table 1. A Summary of the MGF Sample

	Known			Extragalactic
MGF Event	790305B	980827	041227	200415A	070222	051103	070201
Origin
False-alarm rate	0	0	0	4.9 × 10−6	7.8 × 10−6	1.5 × 10−5	1.2 × 10−4
BNS excl. (Mpc)					6.7	5.2	3.5
Galaxy Properties
Catalog name	LMC	MW	MW	NGC 253	M83	M82	M31
Distance (Mpc)	0.054	0.0125	0.0087	3.5	4.5	3.7	0.78
SFR (M⊙ yr−1)	0.56	1.65	1.65	4.9	4.2	7.1	0.4
GRB Properties
Duration (s)	<0.25	<1.0	<0.2	0.100	0.038	0.138	0.010
Rise time (ms)	∼2	∼4	∼1	2	4	2	24
${L}_{{\rm{i}}{\rm{s}}{\rm{o}}}^{{\rm{m}}{\rm{a}}{\rm{x}}}$ (1046 erg s−1)	0.65	2.3	35	140	40	180	12
Eiso (1045 erg)	0.7	0.43	23	13	6.2	53	1.6
Index			−0.7	0.0	−1.0	−0.2	−0.6
Epeak (keV)	500	1200	850	1080	1290	2150	280

Note. The significance for extragalactic events is from this text. Here BNS excl. refers to the neutron star merger exclusion distances from LIGO, LMC refers to the Large Magellanic Cloud, and MW refers to the Milky Way. Individual significance is determined by comparison of the individual Ω against the full background sample. Distances for the known magnetars come from Olausen & Kaspi (2014); extragalactic distances are taken from the host galaxy values (which have minor variations with our catalog values). The GRB parameters include E_peak as the energy of peak output and index as the low-energy power law from the spectral fit, and the rest are discussed in the text. The GRB measures for the galactic events are from the literature; GRB measures for extragalactic events are all measured from Konus-Wind data.

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Three of the four are discussed in the literature as extragalatic MGF candidates. The Konus-Wind lightcurves are shown in Figure 3. The GRB 070201 has the least robust association with a nearby galaxy; however, the localization is comparatively large (∼10× the other events), and M31 has the largest angular size of any galaxy in our sample, together lowering Ω even for real associations. We confirm this by checking GRB 790305B with the Large Magellanic Cloud (Evans et al. 1980; Cline et al. 1982), which has an even larger angular extent than M31, giving Ω = 500.

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We perform a number of sanity checks to ensure our assumptions do not significantly affect our results. The search we run assumes the centroid α = 1.3 value; however, we have confirmed that running the search at the 90% confidence interval bounds (α = 0.5, 2.2) identifies the same four bursts as significant outliers and does not identify other candidates. Running the search at greater NSIDE affects our Ω values by <10%. Rerunning the search where the linear SFR weighting is altered to the stellar mass results in identification of the same galaxies but with generally lower Ω values. Running with a specific SFR returns similar results. Together, these suggest a progenitor that tracks SFR. Our results are insensitive to the assumed ${E}_{\mathrm{iso},\min }$, so long as we do not exclude known events, as events of this strength are not detected far into the universe. There are a few events with Ω > 1 that are either excluded as events of interest for our MGF search or insignificant given our sample. Lastly, significantly raising the assumed ${E}_{\mathrm{iso},\max }$ marginally identifies GRB 100216A (Ω = 10), which indeed has a potential host galaxy within 200 Mpc (Perley et al. 2010), which is inconsistent with expectations for MGFs.

To determine the origin of these four bursts, we first determine if the known GRB progenitors are compatible. Collapsars power long GRBs with durations ≳2 s and are followed immediately by afterglow and then broad-line type Ic supernovae. This origin is excluded, as all four events have durations of 0.1 s or less. Additionally, no subsequent supernovae were reported in any case (Li et al. 2011b; though see Gehrels et al. 2006; Grupe et al. 2007). A neutron star merger origin is excluded by LIGO nondetections in gravitational waves for three of the four events (Abbott et al. 2008; Abadie et al. 2012; Aasi et al. 2014), but observations are insufficiently sensitive to inform on the origin of GRB 200415A. One may consider whether off-axis GRBs could explain these events. The best-known such event is GRB 170817A, where the duration was longer and spectrum softer than the bulk of the short GRB population, which is inconsistent with the prompt emission from these four local events. Further, the rates of these local events (discussed in the following section) are orders of magnitude higher than cosmological GRBs (Siegel et al. 2019), even considering events that are oriented away from Earth.

To determine the progenitors of these events, we follow the historical procedure, where we begin by population comparison of prompt emission parameters. The only additional potential progenitors for extragalatic GRBs commonly discussed in the literature are MGFs, where, contrary to the works that identified the two confirmed progenitors, we have the advantage of observations of galactic events, which are summarized in Table 1. The parameters relevant for only the main peak of the flare that appear distinct from cosmological GRBs are the rise time and intrinsic energetics. Figure 4 contains the population comparison of these parameters.

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First, MGFs have rise times of order a few milliseconds, far shorter than most cosmological short GRBs (Hakkila et al. 2018). Rise times are not reported in most GRB catalogs. As a proxy for the rise time, we define the time to peak as the time from the start of the emission to the beginning of the peak 2 ms counts interval. An Anderson–Darling k-sample test against 75 bright Konus short GRBs (∼15% brightest bursts detected by Konus between 1994 and 2020) rejects the null hypothesis that they are drawn from the same population at >99.9% confidence.

Second, MGF E_iso values are orders of magnitude fainter than cosmological GRBs, where only the unusual GRB 170817A (Abbott et al. 2017) is comparable. This parameter depends on the distance to the source, which is not directly observable from prompt emission. For some cosmological GRBs, the direct distance (redshift) determination is made from follow-up observations. However, for most short GRBs, the distance is determined by first robustly associating the short GRB with an aligned or nearly aligned host galaxy and then determining the distance to the host (Fong et al. 2015). We adapt this last approach for MGFs to enable the use of larger prompt emission localizations and expected host galaxy properties. For each GRB and potential host galaxy, we calculate ${{\rm{\Omega }}}_{{\rm{h}}{\rm{o}}{\rm{s}}{\rm{t}}}=4\pi {\sum }_{i}{P}_{i}^{{\rm{G}}{\rm{R}}{\rm{B}}}{P}_{i}^{{\rm{h}}{\rm{o}}{\rm{s}}{\rm{t}}}/{A}_{i}$, with P^host the weighted spatial distribution of that galaxy. Each GRB has only a single likely host, providing a robust association. In the literature, GRB 051103 has been discussed as belonging to the M81 Group of galaxies (Frederiks et al. 2007b), which is dominated by the interacting galaxies M81 and M82. Our galaxy catalog selection and method assign the burst to M82.

The inferred E_iso values for each extragalatic MGF candidate are given in Table 1. For the population comparison, we add the E_iso distribution of GBM short GRBs (Abbott et al. 2017) to the sample of Konus bursts with measured redshift (Tsvetkova et al. 2017). Together, these give 23 short GRBs with E_iso determined by a broadband instrument, which is the largest such sample to date. The extragalactic MGFs are clearly inconsistent with the broader population, rejecting the null hypothesis at >99.9% confidence.

Host galaxy studies of GRBs have been key in determining prior progenitor channels (e.g., Fong et al. 2015). As discussed in the design of our method, MGFs are expected to arise in star-forming galaxies or regions. Within our maximal detection distance for these bright events, the galaxies with the highest SFR are M82, M83, NGC 253, and NGC 4945 (Mattila et al. 2012). GRB 051103 is associated with M82 by our method or consistent with star-forming knots on the outskirts of M81 (Ofek et al. 2006), GRB 070222 with M83, and GRB 200415A with the star-forming core of NGC 253 (Svinkin et al. 2021). GRB 790305B is associated with the star-forming Large Magellanic Cloud. This is consistent with a massive-star progenitor, as expected for an MGF origin.

Individually, GRBs 200415A and 051103 are the most robust identifications of extragalactic MGFs based on our significance assessment and the results of partner analyses including lightcurve morphology and submillisecond variation of the prompt emission (Roberts et al. 2021; Svinkin et al. 2021). Newly identified is GRB 070222, which is in-class with key properties of MGFs. However, it has two distinct but overlapping pulses, which is not known to occur from galactic events. This requires either a broader morphology of MGFs, a distinct and unknown origin, or a 1 in 100,000 chance alignment (Table 1). However, given the range of (quasi)periodic oscillations seen from magnetar emission, such a morphology is not necessarily surprising.

To summarize the observational case for an MGF origin: these events localize to the nearby universe, particularly to star-forming regions or galaxies. The prompt emission is inconsistent with a collapsar origin, and gravitational wave observations exclude a compact merger involving neutron stars and/or black holes. The event rates, quantified below, are in excess of the majority of energetic astrophysical transients but consistent with predictions from the known MGFs. The properties of the prompt emission are distinct from the larger short GRB population but again consistent with the properties from the known MGFs. There is additional evidence for individual events in partner analyses. We conclude that we have confirmed a sample of extragalactic MGFs that match prior predictions on detection rates and properties from both theoretical and observational studies.

A remaining question is, why have we not identified MGFs to greater distances? Previously, MGFs were thought to be detectable to tens of megaparsecs. The spectra of the initial pulse of GRBs 200415A, 051103, and GRB 070222 are particularly spectrally hard, with a shallow spectral index and high peak energies, which is consistent with GRB 041227 (Frederiks et al. 2007a). Assuming a cutoff power-law spectrum for bright MGFs with a low-energy spectral index of ≈0.0 and peak energies of ≈1.5 MeV produces only 15%–20% of the photons in the nominal triggering energy range of 50–300 keV, as compared to a typical short GRB (assuming an index of 0.4 and peak energy of 0.6 MeV; Goldstein et al. 2017). The GRB monitors are triggered by photon counts, which suggests that the harder spectrum reduces the detectable distance by a factor of ∼5 and therefore the volume by a factor of more than 100. Instrument-specific comments are given in Appendix A. Further, there is a local overdensity within ∼5 Mpc of CCSNe (Mattila et al. 2012), which provides an additional explanation of detections within this range and the lack of detections beyond it.

We now proceed to make population-level inferences utilizing the three known MGFs and treating all four of our events as extragalatic MGFs.

4.1. Intrinsic Energetics Distribution

4.2. Rates

4.3. Magnetar Formation Channel

We summarize our conclusions as follows.

• 1.  
  We have shown that four short GRBs that occurred within ∼5 Mpc are the closest events by an order of magnitude in distance. Our analysis was the first to identify GRB 070222 as a local event.
• 2.  
  They are inconsistent with a collapsar or neutron star merger origin.
• 3.  
  Their prompt emission is inconsistent with the properties of cosmological GRBs but consistent with the observations of the known MGFs.
• 4.  
  They originate from star-forming regions or galaxies, including those with metallicity that prevents collapsars from occurring.
• 5.  
  All together, this matches expectations for an MGF origin, which appears to produce 4 out of 250 events. This would be ∼2% of detected short GRBs (consistent with the 1%–8% range from the literature; Ofek 2007; Svinkin et al. 2015) or ∼0.3% of all detected GRBs.
• 6.  
  Modeling the intrinsic energetics distribution of MGFs as a power law constrains the index to be 1.7 ± 0.4.
• 7.  
  The volumetric rates are ${R}_{\mathrm{MGF}}={3.8}_{-3.1}^{+4.0}\times {10}^{5}$ Gpc⁻³ yr⁻¹.
• 8.  
  The rates and host galaxies of these events favor CCSNe as the dominant formation channel for magnetars, requiring at least 0.5% of CCSNe to produce magnetars.
• 9.  
  We estimate the rate of MGFs per magnetar to be ≲0.02 yr⁻¹.
• 10.  
  Our results suggest that some magnetars produce multiple MGFs; this would be the first known source of repeating GRBs.
• 11.  
  GRB 070222 suggests that MGFs can have multiple pulses.
• 12.  
  The MGFs may not be detectable to tens of megaparsecs with existing instruments due to their spectral hardness.

Our analysis suggests that additional extragalactic MGFs may be identified with improved analysis, but "smoking-gun" confirmation likely requires future instruments. The inferred rates are sufficiently high that they may contribute to the stochastic background of gravitational waves. This, along with the recent observations of a fast radio burst to lower-energy gamma-ray flares from magnetars (Bochenek et al. 2020; Li et al. 2020; Marcote et al. 2020; Ridnaia et al. 2020), suggests that the coming years will bring new insights into the physics and emission of magnetars.

N.C. is supported by NSF grant PHY-1806990. The Fermi-GBM Collaboration acknowledges the support of NASA in the United States under grant NNM11AA01A and DRL in Germany. The CLU galaxy list made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology, and was supported by the Global Relay of Observatories Watching Transients Happen (GROWTH) project funded by the National Science Foundation under PIRE grant No. 1545949.

We present rough estimates for the maximal detection distance of bright MGFs with representative active instruments. Konus-Wind can detect bright MGFs to ∼13–16 Mpc, based on GRBs 051103 and 200415A (Svinkin et al. 2021). This can be taken as the approximate detection distance of the IPN (Svinkin et al. 2015). The following investigations assume a hard spectrum based on the time-integrated values for the most energetic bursts, with a low-energy spectral index of ≈0.0 and peak energies of ≈1.5 MeV. This has only 15% (20%) of the number of photons over the 15–150 keV (50–300 keV) energy range, reducing the detection distance by ∼5× and thus the volume by >100.

The GBM GRB trigger algorithms cover 50–300 keV, where the short GRB sensitivity is usually quoted over the 64 ms timescale. With the assumed spectral and energetics values, the GBM would have only triggered these onboard algorithms out to ∼15–20 Mpc. At greater distances, only the peak flux interval would be visible, which would be spectrally harder and reduce this distance. The GBM localizations alone are insufficient to associate events with any specific burst. Ground-based searches for GRBs and terrestrial gamma-ray flashes should be able to recover additional events but may require confirmation on other GRB instruments

The INTEGRAL SPI-ACS and IBIS are especially sensitive to hard and short bursts, and additional extragalactic MGFs have likely triggered the SPI-ACS real-time pipeline in the past. However, SPI-ACS and IBIS lack the capacity to discriminate extragalactic MGFs from high cosmic-ray effects that appear similar to real events in these instruments. The real-time IBAS pipeline has not been tuned to favor short and hard events. We estimate that SPI-ACS would record sufficient signal from extragalactic MGFs for association with another instrument up to 25–35 Mpc but would only independently report much brighter events out to 15–20 Mpc. The sensitivity of IBIS is close to or better than that of SPI-ACS in about 10% of the sky, and in the majority of directions, IBIS would only yield detectable signal for extragalactic MGF flares out to at most 10 Mpc. However, PICsIT may often be more suitable for triangulation, owing to better time resolution, and can provide some spectral characterization.

Swift/BAT has >500 different rate trigger criteria running in real time on board, continuously sampling and testing trigger timescales from 4 ms up to 64 s, each of which is evaluated for 36 different combinations of energy ranges and focal plane regions. While the BAT detector is sensitive to photons with energies up to 500 keV, the transparency of the lead tiles in the mask above 200 keV limits its imaging energy range (necessary for a successful autonomous trigger) to 15–150 keV. This narrow and low energy range limits the BAT's sensitivity to hard events, such as MGFs, despite its high effective area. Due to the number and complexity of the onboard triggering algorithms, the varying computer load on the BAT CPU, the evolving state of the BAT detector array, and the changing operational choices for trigger vetoes/thresholds, modeling the likelihood of an onboard autonomous trigger is quite difficult. In addition, due to the BAT's high effective area, continuous time-tagged event data cannot be downlinked, making it difficult to assess the relative completeness of the triggering algorithms versus ground searches, though this is partly ameliorated by GUANO (Tohuvavohu et al. 2020). Under the assumed energetics and spectral values, we estimate that as of 2020 (averaging half of the original detector array online), Swift/BAT should reliably trigger on MGFs out to ∼25 Mpc in the highest coded region of its field of view. Ground analyses in the downlinked BAT event data can extend this, but the availability of these data will often depend on an external trigger (e.g., GUANO). We note that operational changes to the BAT onboard triggering thresholds with the goal of increasing sensitivity to extragalactic MGFs and local low-luminosity GRBs have been previously attempted. In 2012, the threshold for a successful trigger from an image was lowered from the usual value of 6.5–5.7, with the condition that triggers in this range had to be localized to within 12' projected offset from a local cataloged galaxy stored in the BAT onboard catalog. No local GRB-like source was ever identified in this program.

As GRB 070222 has not been reported elsewhere, we describe its basic analysis here. The event was detected by Konus-Wind, HEND on Mars Odyssey, and both SPI-ACS and PICsIT on INTEGRAL. Combination of the two best annuli produces a localization with a 90% containment region of 0.004 deg². This location and its consistency with M83 are shown in Figure 5.

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This burst is distinct from the separate candidates as having two separate pulses. Time-resolved analysis of this burst is summarized in Table 2, while time-integrated analysis is reported in the Second Konus GRB Catalog (Svinkin et al. 2016).

Table 2. The Time-resolved Analysis of the Two Pulses of GRB 070222

Tstart	Tstop	Index	Epeak	Flux
(s)	(s)		(keV)	(1 × 10−6 erg s−1 cm −2)
−0.006	0.012	${0.14}_{-0.24}^{+0.28}$	${733}_{-99}^{+138}$	${153.4}_{-16.5}^{+21,2}$
0.026	0.038	$-{0.27}_{-0.36}^{+0.48}$	${193}_{-14}^{+25}$	${24.5}_{-3.0}^{+3.0}$

Note. Errors are quoted at 68% confidence. The main pulse is spectrally hard, similar to the time-integrated fits of GRB 200415A and GRB 051103.

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