Identification of a Local Sample of Gamma-Ray Bursts Consistent with a Magnetar Giant Flare Origin
E. Burns, D. Svinkin, K. Hurley, Z. Wadiasingh, M. Negro, G. Younes, R. Hamburg, A. Ridnaia, D. Cook, S. B. Cenko, R. Aloisi, G. Ashton, M. Baring, M. S. Briggs, N. Christensen, D. Frederiks, A. Goldstein, C. M. Hui, D. L. Kaplan, M. M. Kasliwal, D. Kocevski, O. J. Roberts, V. Savchenko, A. Tohuvavohu, P. Veres, C. A. Wilson-Hodge
DDraft version January 25, 2021
Typeset using L A TEX twocolumn style in AASTeX61
IDENTIFICATION OF A LOCAL SAMPLE OF GAMMA-RAY BURSTS CONSISTENT WITH A MAGNETARGIANT FLARE ORIGIN
E. Burns, D. Svinkin, K. Hurley, Z. Wadiasingh,
4, 5
M. Negro, G. Younes,
7, 8
R. Hamburg, A. Ridnaia, D. Cook, S. B. Cenko,
4, 11
R. Aloisi,
12, 13
G. Ashton, M. Baring, M. S. Briggs, N. Christensen, D. Frederiks, A. Goldstein, C. M. Hui, D. L. Kaplan, M. M. Kasliwal, D. Kocevski, O. J. Roberts, V. Savchenko, A. Tohuvavohu, P. Veres, and C. A. Wilson-Hodge Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA Ioffe Physical-Technical Institute, Politekhnicheskaya 26, St. Petersburg, 194021, Russia Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720-7450, USA NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA Universities Space Research Association Columbia, Maryland 21046, USA University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA Department of Physics, The George Washington University, Washington, DC 20052, USA Astronomy, Physics and Statistics Institute of Sciences (APSIS), The George Washington University, Washington, DC 20052, USA Department of Space Science, University of Alabama in Huntsville, Huntsville, AL 35899, USA IPAC/Caltech , 1200 E California Blvd, Pasadena, CA 91125, USA Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA. University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA Department of Astronomy, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706, USA OzGrav: The ARC Centre of Excellence for Gravitational Wave Discovery, Clayton VIC 3800, Australia Department of Physics and Astronomy, Rice University, MS-108, P.O. Box 1892, Houston, TX 77251, USA Artemis, Universit´e Cˆote d’Azur, Observatoire de la Cˆote d’Azur, CNRS, Nice 06300, France Science and Technology Institute, Universities Space Research Association, Huntsville, AL 35805, USA Astrophysics Office, ST12, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, 1290, Versoix, Switzerland Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4 Canada
ABSTRACTCosmological Gamma-Ray Bursts (GRBs) are known to arise from distinct progenitor channels: short GRBs mostlyfrom neutron star mergers and long GRBs from a rare type of core-collapse supernova (CCSN) called collapsars.Highly magnetized neutron stars called magnetars also generate energetic, short-duration gamma-ray transients calledMagnetar Giant Flares (MGFs). Three have been observed from the Milky Way and its satellite galaxies and theyhave long been suspected to contribute a third class of extragalactic GRBs. We report the unambiguous identificationof a distinct population of 4 local ( < > × erg of R MGF = 3 . +4 . − . × Gpc − yr − place MGFs as thedominant gamma-ray transient detected from extragalactic sources. As previously suggested, these rates imply thatsome magnetars produce multiple MGFs, providing a source of repeating GRBs. The rates and host galaxies favorcommon CCSN as key progenitors of magnetars. Keywords: gamma rays: general, methods: observation a r X i v : . [ a s t r o - ph . H E ] J a n INTRODUCTIONThe history of GRBs and magnetars are intertwined.Short bursts of gamma-rays were recorded by the Velasatellites beginning in 1967 (Klebesadel et al. 1973),and were given the phenomenological name GRBs.GRB 790305B was localized by the InterPlanetary Net-work (IPN) to the Large Magellanic Cloud (Mazetset al. 1979; Evans et al. 1980). It was unique in beingthe brightest event seen at Earth, the prompt emissionhad a long-lasting, exponentially-decaying, periodic tail(Barat et al. 1979) and additional, weaker bursts werelocalized to the same source (Mazets et al. 1979). Im-mediately there were papers investigating if the mainevent shared a common origin with other GRBs (Mazetset al. 1982; Cline et al. 1980). It is now known to be thefirst signal identified from a magnetar.Key results on the nature of GRBs in the subsequentdecades were often proven by population-level statisticalanalysis before direct “smoking-gun” proof. Perhaps thegreatest debate was whether these events had a galac-tic or an extragalactic origin, with the latter initiallydisfavored as it would require intrinsic energetics be-yond anything previously known. Proof came first indi-rectly via statistical studies on the spatial distributionof GRBs (Meegan et al. 1992) and then directly fromredshift measurements (Metzger et al. 1997).Studies of the prompt GRB emission provided strongevidence in favor of two populations (Kouveliotou et al.1993), with short and long GRBs traditionally sepa-rated at 2 s as measured by the T parameter. LongGRBs were tied to broad-line type Ic core-collapse su-pernovae called collapsars (Galama et al. 1998). The Neil Gehrels Swift Observatory (Swift) mission enabledsuccessful detections of afterglow from a sample of shortGRBs. Circumstantial evidence pointed towards a neu-tron star merger origin (Eichler et al. 1989; Fong et al.2015) with direct confirmation that some GRBs arisefrom binary neutron star mergers came with GW170817and GRB 170817A(Abbott et al. 2017).Yet another debate on the behavior of GRBs iswhether or not the sources repeated. This is bestexplained using modern parlance. Soft Gamma-rayRepeaters (SGRs) are galactic magnetars named phe-nomenologically for the weak, recurrent short burststhat first identified them before their physical originwas known. SGR flares are classified as distinct fromGRBs, and have recently been tied to radio emissionsimilar to the cosmological Fast Radio Bursts (Boch-enek et al. 2020). The flare on March 5, 1979 and thesubsequent similar events GRB 980827 (Mazets et al.1999b; Hurley et al. 1999a) and GRB 041227 (Palmeret 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 ofMGFs often carries the GRB designation, which we usehere. GRBs are now not thought to repeat as collapsarsand neutron star mergers are cataclysmic events. Whileseveral galactic magnetars have been observed to pro-duce multiple SGR flares, none have been observed toproduce multiple giant flares (though this is not surpris-ing). The historic debate on potential repeating GRBswas likely confounded by magnetar transients before theseparation of SGR flares from GRBs.We here refer to GRBs 790305B, 980827, and 041227as the known MGF sample. The detection of threefrom the Milky Way and its satellite galaxies impliesa high intrinsic rate on a per-galaxy or volumetric basis.These events should be detectable to extragalactic dis-tances by GRB monitors such as Konus-
Wind (Aptekaret al. 1995),
Swift -BAT (Barthelmy et al. 2005), and
Fermi -GBM (Meegan et al. 2009). However, at thesedistances only the immediate bright spike would be de-tectable and the event should resemble a short GRB(Hurley et al. 2005). There are two events discussed inprevious literature as extragalatic MGF candidates, be-ing 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 coincidencewas measured to be ∼
1% (Svinkin et al. 2015).There have been population-level searches for addi-tional events, which identified no additional candidates(Popov & Stern 2006; Ofek 2007; Svinkin et al. 2015).However, these studies allow us to constrain the frac-tion of detected short GRBs that have an MGF origin:Ofek (2007) show that the rate of galactic events requiresthis to be > <
8% (Tikhomirova et al. 2010; Svinkin et al. 2015;Mandhai et al. 2018). These studies and tehir conclu-sions generally assumed that the brightest MGFs couldbe detectable to tens of Mpc.Recently, GRB 200415A was identified as the thirdand likeliest extragalactic MGF (Svinkin et al. 2021).In this work, we perform a new population-level searchutilizing the largest GRB sample, new galaxy catalogsthat are both more complete and provide additional in-formation, and develop a new formalism to determine ifwe can prove extragalactic MGFs contribute to the ob-served GRB population. Section 2.4 details the searchformalism which identifies four nearby events, identify-ing an additional extragalactic candidate. The progeni-tors of our identified sample are investigated in Section 3,the implications of which are discussed in Section 4. Weconclude with discussions in Section 5. xtragalactic MGFs as SGRBs LOCAL GRBSThe “smoking-gun” evidence of an MGF is the longperiodic tails which are modulated by the rotation pe-riod of the neutron star (Hurley et al. 1999b) and alsoshow quasi-periodic oscillations related to the modes ofthe neutron star itself (Barat et al. 1983; Strohmayer& Watts 2005; Israel et al. 2005; Watts & Strohmayer2006). However, these signatures are not unambiguouslyidentifiable at extragalactic distances with existing in-struments. As such, we follow prior population-levelsearches 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. Wecombine existing GRB and galaxy catalogs to build themost complete set of information from existing litera-ture. For each individual burst we quantify our be-lief that it is an MGF from a known galaxy throughcomparison of two PDFs, which are discussed below.These PDFs are generated in HEALPix (Gorski et al.2005). The resolution of HEALPix maps is defined bythe NSIDE parameter, where the number of total pixelsis equal to the square of the NSIDE times twelve. Themaps were generated with NSIDE=8192, correspondingto a pixel width of ∼ The GRB Sample
We utilize data from
CGRO -BATSE (Fishman et al.1989), Konus-
Wind (Aptekar et al. 1995),
Swift -BAT(Barthelmy et al. 2005),
Fermi -GBM (Meegan et al.2009), and additional information from the IPN . Trig-gers from the same events were matched utilizing tem-poral information for all events and spatial information(Ashton et al. 2018) when available. The total samplecontains more than 11,000 GRBs observed, with > T < T used is the shortest reported by any triggering in-strument. Second, we require the bolometric fluence(1 keV-10 MeV) determined from a broadband instru-ment (Konus, BATSE, or GBM), converting from theinstrument-specific ranges as necessary. Intercalibra-tion uncertainties are within 25%. For the trigger times,duration, and spectral properties we utilized the latestcatalog information (Paciesas et al. 1999; Svinkin et al.2016; Lien et al. 2016; von Kienlin et al. 2020), updatedonline catalogs , GCN circulars, and performed dedi-cated analysis when necessary. ssl.berkeley.edu/ipn3/index.html Lastly, we require well-localized GRBs, constructedfrom all available information. For BATSE localizationwe utilize the latest catalogs (Goldstein et al. 2013) andapply the largest systematic error (Briggs et al. 1999).
Swift -BAT positions are taken from the updated
Swift -BAT Catalog and Swift -XRT localizations are utilizedwhen available . Fermi -GBM localizations are quasi-circular and were generated using the latest methods(Goldstein et al. 2020) for all bursts.KONUS localizations are an ecliptic band which aresummarized in the IPN catalogs. The IPN compiles lo-calization information for GRBs, including the timingannuli derived from the relative arrival times of gamma-rays at distant spacecraft. Information used here is fromthe IPN localizations of Konus short GRBs through 2020(Pal’Shin et al. 2013) and the IPN list kept up to dateonline . Additional IPN localizations were compiled formore than 100 additional short GRBs for this work,which were added to the online table. The locationinformation, including systematic error, from the au-tonomous localizations, timing annuli, and Earth occul-tation selections are converted to the HEALpix formatusing the GBM Data Tools . These independent PDFsare combined into a final PDF referred to as P GRB .The localization threshold is set to a 90% confidencearea < .
125 deg when including systematic error. Thisvalue is chosen as it is 1/10,000 the area of the sky, iscomparable to the sum of the angular size of galaxies (asdefined in the following section) within 200 Mpc, and isbetween previously used thresholds (Svinkin et al. 2015).With the bolometric fluence measure requirement andthe removal of bursts with known redshift (Lien et al.2016) beyond the distance where the event may be adetected MGF, we are left with a sample of 250 shortGRBs. We do not apply more stringent cuts on spectralor temporal information at this stage as the relevantparameters are not uniformly reported in GRB catalogs.2.2. The Galaxy Sample
For the galaxies considered in this work we require theposition (RA, Dec, Distance), angular extent (if non-negligible at our spatial resolution; represented here asellipses), and the current Star Formation Rate (SFR).The z=0 Multiwavelength Galaxy Synthesis (z0MGS)Catalog (Leroy et al. 2019) combines the ultraviolet ob- https://swift.gsfc.nasa.gov/results/batgrbcat/index.html https://swift.gsfc.nasa.gov/archive/grb_table/ https://fermi.gsfc.nasa.gov/ssc/data/analysis/gbm/gbm_data_tools/gdt-docs/ servations from GALEX (Morrissey et al. 2007) with theinfrared observations of WISE (Wright et al. 2010) touniformly measure gas and dust for galaxies within ap-proximately 50 Mpc. As a result, for galaxies containedin this catalog these measures of the distance and SFRare our default values. The angular size of galaxies isrepresented as an ellipse when data allows or as a cir-cle when the axial ratio is not known. Angular extentis taken from the input catalogs, but is generally theHolmberg isophote, i.e. where the B band brightness is26.5 mag arcsecond .The Census of the Local Universe (CLU) Catalog(Cook et al. 2019) aims to provide the most completecatalog of galaxies out to 200 Mpc. We use the CLUmeasures of distance and SFR when they are not pro-vided by z0MGS, and we use the CLU measures forangular size (which are not provided by the z0MGS).When missing, we add position angle information fromHyperLEDA (Paturel et al. 2003). The SFR measuresof these two catalogs correct for internal extinction us-ing WISE4/FUV luminosities. To ensure completenesswithin <
10 Mpc we supplement these two catalogs withthe Local Volume Galaxy (LVG) Catalog (Karachent-sev & Kaisina 2013). The three catalogs are matchedby name, with help from the NASA/IPAC Extragalac-tic Database (NED) , and position information.We consider galaxies between 0.5 Mpc (excluding theMilky Way and its satellite galaxies) and 200 Mpc (be-yond where MGFs can be detected), which leaves morethan 100,000 galaxies. The SFR is a key parameter inour method and our inferences also rely on scaling theproperties of our host galaxy. The Milky Way SFR usedhere is 1.65 ± (cid:12) /yr (Licquia & Newman 2015).We specify the SFR for NGC 3256, which was identifiedin Popov & Stern (2006) as being a likely source of de-tectable extragalactic MGFs. We searched the literaturefor values of the active SFR in this galaxy and take thevalue of ∼ (cid:12) /yr from Lehmer et al. (2015) which isinferred using UV information and is among the middlereported values.2.3. MGF Spatial Distribution
We seek an all-sky PDF, P MGF , representing theprobability that a given position is to produce a MGFwith a particular fluence at Earth. Note that this is de-termined by the fluence of each burst considered, but isconstructed independently of the location of the burstitself, P GRB . The comparison of the two PDFs gener-ated for each burst quantifies the likelihood that a givenshort GRB has an MGF origin, which is performed in https://ned.ipac.caltech.edu/ the next section. This section details the burst-specificconstruction of P MGF .If a given burst has an MGF origin it should arise froma cataloged galaxy and its intrinsic energetics should fallinto the expected range. To construct this we computea weight for each galaxy representing how likely it is tohave produced the observed fluence for the burst underconsideration. This weight has two-components: a lin-ear weighting with SFR and a more complex weightingthat compares the inferred intrinsic energetics (deter-mined by the burst fluence and potential host galaxydistance) against an assumed PDF.Magnetars are expected to be able to produce MGFsonly for a short period of time (approximately 10 kyr Be-niamini et al. 2019), tying the predicted rate of MGFsto the rate of their formation. The rate of CCSN can beinferred from the SFR since the lifetimes of stars thatundergo core-collapse is much shorter than the timescaleprobed by the SFR tracers (Botticella et al. 2012). Un-der the assumption that the dominant formation channelfor magnetars is CCSN (which is explored in Section 4)we can infer the rate of MGFs from a galaxy from itsSFR. Thus, each galaxy is linearly weighted with SFR.We use the far ultraviolet measure of SFR (Lee et al.2010) when available as it should track massive starslikely to undergo core-collapse, otherwise we use the H α measure (Kennicutt Jr 1998) scaled by the average dif-ference from galaxies with both measures to account forthe lack of dust correction in the LVG catalog.Next we can determine the total isotropic-equivalentenergetics of a potential burst-galaxy pair as E iso =4 πd S where S is the burst fluence and d the distanceto the potential host. This value can be compared toan assumed intrinsic energetics PDF to determine howlikely the event is to be an MGF. For example, a partic-ularly high fluence short GRB spatially aligned with adistant galaxy would require an intrinsic energetics farbeyond what has been observed in the galactic MGFs,excluding an MGF origin. We note that some studiesutilize the peak luminosity L Max iso but we work with an E iso distribution as there is stronger theoretical guid-ance on the maximum total energy that can be released(related to the magnetic fields of the magnetar) than onthe timescale that it is released.We now construct an informed intrinsic energeticsfunction, assuming a power-law distribution with an as-sumed minimum and maximum value, which is similarto the behavior of lower energy magnetar flares (Chenget al. 1996). Our method bypasses the need for an as-sumed detection threshold, which is difficult to quantifywhen considering many instruments over 30 years. The xtragalactic MGFs as SGRBs E iso [erg]10 P D F Figure 1.
The initial assumed MGF energetics distribution,with E iso , min and E iso , max set to the x-axis boundaries. ThePDF form is (1 − α ) E − α iso / ( E − α Max − E − α Min ). As described inthe text, α = 1 . ± . E iso values from the known MGFs used to constrain the slope areshown as black vertical lines. assumed and inferred values are reported below, withthe initially determined distribution shown in Figure 1.The slope of a power-law can be determined via max-imum likelihood, independent of an assumed maximumvalue, as α = 1 + n (cid:34) n (cid:88) i =1 ln (cid:18) E iso , i E iso , min (cid:19)(cid:35) − , σ α = α − √ n + O ( n − )(1)where the sum is over the observed E iso and E iso , min is the lowest considered value Newman (2005); Bauke(2007). We set E iso , min as 1 . × erg which is afactor of a few below the lowest value measured in aknown MGF as shown in Table 1 but above the bright-est SGR flare that lacked the periodic tail emission(Mazets et al. 1999a). Iterating over the E iso values ofthe known MGFs (GRBs 790305B, 090827, and 041227)gives α = 1 . ± . O ( n − ) error contribution. In order to min-imize the required computation we assume the centroid( α = 1 .
3) in what follows; the effect of this assumptionon our results is discussed in the closing paragraph ofthis section.There must be a physical maximum energy for anMGF, which should be related to the total magneticenergy. This is supported by the lack of detectionsof more energetic events otherwise consistent with anMGF origin. The highest E iso observed for a knownMGF is 2 . × erg which comes from the magne-tar with the highest reported magnetic field at thesurface of 2 . × G (Olausen & Kaspi 2014). Wenote this reported value is approximately 3 times largerthan the dipolar spin-down inferred magnetic field value of 7 × G (Younes et al. 2017), but we have con-firmed this does not affect our results. To determine an E iso , max for our search we assume a dipole field, wherethe available energy scales as B , and a nominal maxi-mum magnetic field strength of ∼ × G. This gives E iso , max = 2 . × erg × (1 . × G / . × G) =5 . × erg.This allows us to determine the burst-specific two-component weight for each of the > E iso PDF for the inferredenergetics considering the burst fluence and galaxy dis-tance. The sum of the galaxy weights is normalized tounity. Then, P MGF is built by placing the calculatedweights at the position of the host galaxy. If the angulardiameter of the galaxy is larger than the effective reso-lution of our discrete sky representation ( ∼ arcminute )then its weight is uniformly distributed over its angularextent. 2.4. The Search
For each of the 250 short GRBs in our sample wegenerate P GRB from the observations of the GRB and P MGF from theoretically motivated expectations. Wequantify the likelihood that a given GRB has an MGForigin using Ω = 4 π (cid:80) i P GRBi P MGFi /A i where P GRBi and P MGFi indicate the probability for each PDF in the i th sky region, which has area A i (Ashton et al. 2018).Significance is determined by the empirical FalseAlarm method (e.g Messick et al. 2017) with Ω as ourranking statistic. Our backgrounds are generated bysimulating different galaxy distributions. Each iterationis generated by uniform rotation of the 2D (RA, Dec)positions of the galaxies in our sample, which main-tains the distance and SFR distributions as well as localstructure. Population-level confidence intervals createdthrough comparison of each rotation against our fullGRB sample with results are shown in Figure 2. At3 and 4 events the short GRB sample has an excesssurpassing 5 σ discovery significance, with individualsignificance values of the four bursts between 1.2 × − and 4 . × − as given in Table 1.Three of the four are discussed in the literatureas extragalatic MGF candidates. The Konus- Wind lightcurves are shown in Figure 3. GRB 070201 has theleast robust association to a nearby galaxy; however,the localization is comparatively large ( ∼
10x the otherevents) and M31 has the largest angular size of anygalaxy in our sample, together lowering Ω even for realassociations. We confirm this by checking GRB 790305Bwith the Large Magellanic Cloud (Evans et al. 1980;
Figure 2.
The discovery of a local but extragalactic popula-tion of GRBs. Ω is a statistic that ranks how believable theevent is to be an extragalactic MGF, with values for the truepopulation is shown in orange. The background confidenceintervals at 1, 3, and 5 σ are shown in blue. The four mostsignificant events together surpass 5 σ discovery significance. Cline et al. 1982), which has even larger angular extentthan M31, giving Ω = 500.We perform a number of sanity checks to ensure ourassumptions do not significantly affect our results. Thesearch we run assuming our centroid α = 1 . α = 0 . , .
2) identi-fies the same four bursts as significant outliers and doesnot identify other candidates. Running the search atgreater NSIDE affects our Ω values by < E iso , min , so long as we donot exclude known events, as events of this strength arenot detected far into the universe. There are a few eventswith Ω > E iso , max marginally identifies GRB 100216A (Ω = 10) which in-deed has a potential host galaxy within 200 Mpc (Perleyet al. 2010), which is inconsistent with expectations forMGFs. PROGENITOR INVESTIGATIONSTo determine the origin of these four bursts we firstdetermine if the known GRB progenitors are compati-ble. Collapsars power long GRBs with durations (cid:38) ∼
15% brightest bursts detected by Konusbetween 1994 and 2020) rejects the null hypothesis thatthey are drawn from the same population at > E iso values are orders of magnitudefainter than cosmological GRBs, where only the unusualGRB 170817A (Abbott et al. 2017) is comparable. Thisparameter depends on the distance to the source, whichis not directly observable from prompt emission. Forsome cosmological GRBs direct distance (redshift) de-termination is made from follow-up observations. How-ever, for most short GRBs the distance is determined byfirst robustly associating the short GRB to an aligned ornearly aligned host galaxy, and then determining the dis-tance to the host (Fong et al. 2015). We adapt this lastapproach for MGFs to enable the use of larger promptemission localizations and expected host galaxy proper- xtragalactic MGFs as SGRBs GRB 200415A GRB 070222
GRB 051103
GRB 070201
Time [s] N o r m a li z e d C o un t s Figure 3.
The lightcurves of the candidate extragalactic MGFs in order of significance from Extended Data Table 1. Theseare from Konus-
Wind and plotted with 2 ms resolution (Frederiks et al. 2007b; Mazets et al. 2008; Svinkin et al. 2021), withGRB 070222 reported here for the first time. While GRBs 200415A and 051103 are strikingly similar (Svinkin et al. 2021) andGRB 070201 is broadly consistent with a single emission episode, GRB 070222 has two temporally and spectrally distinct pulses(see Appendix B), suggesting varied behavior. ties. For each GRB and potential host galaxy we cal-culate Ω
Host = 4 π (cid:80) i P GRBi P Hosti /A i with P Host theweighted spatial distribution of that galaxy. Each GRBhas only a single likely host, providing robust associa-tion. GRB 051103 has been discussed in the literature asbelonging to the M81 Group of galaxies (Frederiks et al.2007b), which is dominating by the interacting galaxiesM81 and M82. Our galaxy catalog selection and methodassigns the burst to M82.The inferred E iso values for each extragalatic MGFcandidate is given in Table 1. For the population com-parison we add the E iso distribution of GBM shortGRBs (Abbott et al. 2017) to the sample of Konus burstswith measured redshift (Tsvetkova et al. 2017). To-gether these give 23 short GRBs with E iso determinedby a broadband instrument, which is the largest suchsample to date. The extragalactic MGFs are clearlyinconsistent with the broader population, rejecting thenull hypothesis at > Known ExtragalacticMGF Event 790305B 980827 041227 200415A 070222 051103 070201OriginFalse Alarm Rate 0 0 0 4 . × − . × − . × − . × − BNS Excl. [Mpc] 6.7 5.2 3.5Galaxy PropertiesCatalog Name LMC MW MW NGC253 M83 M82 M31Distance [Mpc] 0.054 0.0125 0.0087 3.5 4.5 3.7 0.78SFR [ M (cid:12) /yr ] 0.56 1.65 1.65 4.9 4.2 7.1 0.4GRB PropertiesDuration [s] < < < ∼ ∼ ∼ L Max iso [10 erg/s] 0.65 2.3 35 140 40 180 12 E iso [10 erg] 0.7 0.43 23 13 6.2 53 1.6Index -0.7 0.0 -1.0 -0.2 -0.6 E peak [keV] 500 1200 850 1080 1290 2150 280 Table 1.
A summary of the MGF sample. Significance for extragalactic events is from this text. BNS Excl. refers to theneutron star merger exclusion distances from LIGO. LMC refers to the Large Magellanic Cloud and MW refers to the MilkyWay. Individual significance is determined by comparison of the individual Ω against the full background sample. Distancesfor 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). GRB parameters include E peak as the energy of peak output, Index isthe low-energy power-law from the spectral fit, and the rest are discussed in the text. GRB measures for the galactic events arefrom the literature; GRB measures for extragalactic events are all measured from Konus- wind data. millisecond variation of the prompt emission (Svinkinet al. 2021; Roberts et al. 2021). Newly identified isGRB 070222 which is in-class with key properties ofMGFs. However, it has two distinct but overlappingpulses, which is not known to occur from galactic events.This requires either a broader morphology of MGFs, adistinct and unknown origin, or a 1 in 100,000 chancealignment (Table 1). However, given the range of (quasi-)periodic oscillations seen from magnetar emission sucha morphology is not necessarily surprising.To summarize the observational case for an MGF ori-gin: these events localize to the nearby universe and inparticular to star-forming regions or star-forming galax-ies. The prompt emission is inconsistent with a collap-sar origin and gravitational wave observations excludea compact merger involving neutron stars and/or blackholes. The event rates, quantified below, are in excessof the majority of energetic astrophysical transients butare consistent with predictions from the known MGFs.The properties of the prompt emission are distinct fromthe larger short GRB population but again consistentwith the properties from the known MGFs. There isadditional evidence for individual events in partner anal-yses. We conclude that we have confirmed a sample of extragalatic MGFs that match prior predictions on de-tection rates and properties from both theoretical andobservational studies.A remaining question is: why have we not identifiedMGFs to greater distances? Previously, MGFs werethought to be detectable to tens of Mpc. The spec-tra of the initial pulse of GRBs 200415A, 051103, andGRB 070222 are particularly spectrally hard with shal-low spectral index and high peak energies, which is con-sistent with GRB 041227 (Frederiks et al. 2007a). As-suming a cut-off power-law spectrum for bright MGFswith a low-energy spectral index ≈ . ≈ ∼ ∼ xtragalactic MGFs as SGRBs Time to Peak [s] E v e n t s p e r B i n Galactic MGF Rise TimesNearby GRBsBright Short GRBs E iso E v e n t s p e r b i n Galactic MGFsShort GRBsNearby GRBs
Figure 4.
Key parameter comparison of the extragalacticMGF candidates against the wider short GRB populationand the known MGFs.
Top shows the Time to Peak Konusdistributions and bottom the E iso distributions. The onlycomparable E iso value for a burst from a neutron star mergeris the off-axis GRB 170817A.4. INFERENCESWe now proceed to make population-level inferencesutilizing the three known MGFs and treating all four ofour events as extragalatic MGFs.4.1.
Intrinsic Energetics Distribution
The power-law distribution of the energetics of normalSGR flares gave hints to the physical process that pro-duces them (Cheng et al. 1996). Thus, it is interestingto measure the slope of the E iso distribution for MGFs.We assign our search volume and detection threshold byempirical means, selecting 2 . × − erg cm − for theIPN and a maximal detection distance of ∼ E iso PDF as our search method; however, we can- not utilize the maximum likelihood estimate becauseit requires the assumption that the observed sample iscomplete, which is not true for MGFs at extragalacticdistances. Instead we simulate a large number of ex-tragalactic MGFs by drawing E iso from PDFs over arange of α values, assigning them to specific host galax-ies weighted by their SFR, and setting the event dis-tance as the host galaxy distance. Events that wouldbe detected are those where the sampled E iso and dis-tance produce a flux greater than our detection thresh-old. E iso , min = 3 . × erg is determined by sam-pling the Kolmogorov-Smirnov test statistic value overa range of viable options (Bauke 2007). Then, we cal-culate an Anderson-Darling k-sample value for a rangeof potentially viable α values. We take the 5% rejec-tion values as the bounds on a 90% confidence interval,and determine the mean assuming a symmetric Gaus-sian distribution, giving α = 1 . ± .
4. We note thatthis is consistent with the reported slope values of 5 / Rates
Utilizing the same sample and selection above we canconstrain the intrinsic volumetric rate of MGFs. Thedominant sources of uncertainty are the Poisson uncer-tainty and the imprecisely known sample completeness.The latter is limited by the uncertainty on the power-law index of the intrinsic energetics function, where fora steep index the majority of events will be missed(with most events below 1 . × erg missed in oursample volume) and for a shallow index most eventsare recovered. The α distribution is taken as a Gaus-sian. The SFR within 5 Mpc is 35.5 M (cid:12) /yr which isscaled to a volumetric rate by considering the totalSFR within 50 Mpc, which is ∼ (cid:12) /yr from ourgalaxy sample. We infer a volumetric rate of R MGF =3 . +4 . − . × Gpc − yr − .4.3. Magnetar Formation Channel
Magnetars may be generated in a variety of eventsincluding common CCSN, low-mass mergers (Price &Rosswog 2006), a rare evolution of white dwarfs (Dessartet al. 2007), or a rare sub-type of CCSN such as collap-sars or superluminous supernovae (Nicholl et al. 2017).Each of these is consistent with the observed associationof magnetars to supernova remnants (Beniamini et al.2019). Low-mass merger events have long inspiral timesand should track total stellar mass rather than the cur-rent SFR, which is disfavored given our model preferencefor SFR over stellar mass and the discovery of the firstMGF from the LMC. A CCSN origin would arise from0regions with high rates of star formation. This is con-sistent with our observations and bolstered by both thelack of detections beyond 5 Mpc due to the local SFRoverdensity and the detection of GRB 790305B from thelow-mass, star-forming Large Magellanic Cloud. Thehost galaxies of our extragalactic sample and the MilkyWay itself have larger mass and higher metallicity thanis typically seen in hosts of collapsars or superluminoussupernovae (Taggart & Perley 2019). Therefore, thetypes of host galaxies favor common CCSN as the dom-inant formation channel of magnetars.Additional support for this conclusion is providedfrom the event rates. We can relate our inferredMGF rates to progenitor formation rates as R MGF = R Event f M τ Active r MGF/M (Tendulkar et al. 2016) where R Event is the rate of events that may form magnetars, f M is the fraction that successfully form magnetars, τ Active the timescale that magnetars can produce MGFs,and r MGF/M the rate of MGFs per magnetar. We take τ Active ≈ yr limited by the decay of the magneticfield (Beniamini et al. 2019). Given the incompletenessof our known magnetar sample and lack of understand-ing which magnetars can produce MGFs, we use onlythe 3 known to be capable to estimate an upper boundof r MGF/M < .
02 yr − per magnetar. We note this issignificantly weaker than those reported in the literaturethat consider all known SGRs, being ∼ × − yr SGR(e.g. Ofek 2007; Svinkin et al. 2015).Of the discussed formation channels only CCSN areexpected to track star-forming regions and have a com-parable rate, being 7 × Gpc − yr − in the local uni-verse (Li et al. 2011a). A fiducial value on f M is 0.4with a 2 σ confidence interval of 0.12-1.0 (Beniaminiet al. 2019); other estimates range between 0.01 and0.1 (e.g. Woods & Thompson 2004; Gull´on et al. 2015).We require either that some magnetars produce multi-ple MGFs or that both f M ≈ R MGF is near our 95% lower bound. Alternatively, us-ing the CCSN rate and the 95% lower limit on R MGF we can place observational constraints using our resultsof f M > . CONCLUSIONSTo summarize our conclusions: • We have shown that 4 short GRBs occurred within ∼ • They are inconsistent with a collapsar or neutronstar merger origin. • Their prompt emission is inconsistent with theproperties of cosmological GRBs, but is consistentwith the observations of the known MGFs. • They originate from star-forming regions or star-forming galaxies, including those with metallicitythat prevents collapsars from occurring. • Altogether this matches expectations for an MGForigin, which appear to produce 4 out of 250events. This would be ∼
2% of detected shortGRBs (consistent with the 1-8% range from theliterature Ofek 2007; Svinkin et al. 2015) or ∼ • Modeling the intrinsic energetics distribution ofMGFs as a power-law constrains the index to be1 . ± . • The volumetric rates are R MGF = 3 . +4 . − . × Gpc − yr − . • The rates and host galaxies of these events favorCCSN as the dominant formation channel for mag-netars, requiring at least 0.5% of CCSN to producemagnetars. • We estimate the rate of MGFs per magnetar to be (cid:46) .
02 yr − . • Our results suggest that some magnetars producemultiple MGFs: this would be the first knownsource of repeating GRBs. • GRB 070222 suggests MGFs can have multiplepulses. • MGFs may not be detectable to tens of Mpc withexisting instruments due to their spectral hard-ness.Our analysis suggests additional extragalactic MGFsmay be identified with improved analysis but “smoking-gun” confirmation likely requires future instruments.The inferred rates are sufficiently high that they maycontribute to the stochastic background of gravitationalwaves. This, and the recent observations of a fast radioburst to lower-energy gamma-ray flares from magnetars(Bochenek et al. 2020; Marcote et al. 2020; Ridnaia et al.2020; Li et al. 2020), suggest the coming years will bringnew insights into the physics and emission of magnetars.
Acknowledgements
N. Christensen is supported by the NSF grant PHY-1806990. The
Fermi
GBM Collaboration acknowledgesthe support of NASA in the United States under grant xtragalactic MGFs as SGRBs
Aasi, J., Abbott, B., Abbott, R., et al. 2014, Physicalreview letters, 113, 011102Abadie, J., Abbott, B., Abbott, T., et al. 2012, Astrophys.J., 755, 2Abbott, B., Abbott, R., Adhikari, R., et al. 2008,Astrophys. J., 681, 1419Abbott, B. P., Abbott, R., Abbott, T., et al. 2017, TheAstrophysical Journal Letters, 848, L13Aptekar, R., Frederiks, D., Golenetskii, S., et al. 1995,Space Science Reviews, 71, 265Ashton, G., Burns, E., Dal Canton, T., et al. 2018,Astrophys. J., 860, 6Barat, C., Chambon, G., Hurley, K., et al. 1979, Astronomyand Astrophysics, 79, L24Barat, C., Hayles, R. I., Hurley, K., et al. 1983, Astronomy& Astrophysics, 126, 400Barthelmy, S. D., Barbier, L. M., Cummings, J. R., et al.2005, Space Science Reviews, 120, 143Bauke, H. 2007, The European Physical Journal B, 58, 167Beniamini, P., Hotokezaka, K., van der Horst, A., &Kouveliotou, C. 2019, Mon. Not. R. Astron. Soc., 487,1426Bochenek, C. D., Ravi, V., Belov, K. V., et al. 2020, arXivpreprint arXiv:2005.10828Botticella, M., Smartt, S., Kennicutt, R., et al. 2012,Astronomy & Astrophysics, 537, A132Briggs, M. S., Pendleton, G. N., Kippen, R. M., et al. 1999,Astrophys. J. Supp., 122, 503Cheng, B., Epstein, R. I., Guyer, R. A., & Young, A. C.1996, Nature, 382, 518Cline, T., Desai, U., Pizzichini, G., et al. 1980, Astrophys.J., 237, L1Cline, T., Desai, U., Teegarden, B., et al. 1982, TheAstrophysical Journal, 255, L45Cook, D. O., Kasliwal, M. M., Van Sistine, A., et al. 2019,Astrophys. J., 880, 7Dessart, L., Burrows, A., Livne, E., & Ott, C. D. 2007,Astrophys. J., 669, 585Eichler, D., Livio, M., Piran, T., & Schramm, D. N. 1989,Nature, 340, 126Evans, W., Klebesadel, R., Laros, J., et al. 1980,Astrophys. J., 237, L7 Fishman, G., Meegan, C., Wilson, R., et al. 1989, in Proc.GRO Science Workshop, GSFC, Vol. 2Fong, W.-f., Berger, E., Margutti, R., & Zauderer, B. A.2015, Astrophys. J., 815, 102Frederiks, D. D., Golenetskii, S. V., Palshin, V. D., et al.2007a, Astron. Lett., 33, 1Frederiks, D. D., Palshin, V. D., Aptekar, R. L., et al.2007b, Astron. Lett., 33, 19Galama, T. J., Vreeswijk, P., Van Paradijs, J., et al. 1998,Nature, 395, 670Gehrels, N., Norris, J., Barthelmy, S., et al. 2006, Nature,444, 1044Goldstein, A., Preece, R. D., Mallozzi, R. S., et al. 2013,Astrophys. J. Supp., 208, 21Goldstein, A., et al. 2017, Astrophys. J. Lett., 848Goldstein, A., Fletcher, C., Veres, P., et al. 2020,Astrophys. J., 895, 40Gorski, K. M., Hivon, E., Banday, A. J., et al. 2005,Astrophys. J., 622, 759G¨otz, D., Mereghetti, S., Molkov, S., et al. 2006,Astronomy & Astrophysics, 445, 313Grupe, D., Gronwall, C., Wang, X.-Y., et al. 2007, TheAstrophysical Journal, 662, 443Gull´on, M., Pons, J. A., Miralles, J. A., et al. 2015,MNRAS, 454, 615Hakkila, J., Horv´ath, I., Hofesmann, E., & Lesage, S. 2018,Astrophys. J., 855, 101Hurley, K., Cline, T., Mazets, E., et al. 1999a, Nature, 397,41—. 1999b, Nature, 397, 41Hurley, K., Boggs, S., Smith, D., et al. 2005, Nature, 434,1098Hurley, K., Rowlinson, A., Bellm, E., et al. 2010, Mon. Not.R. Astron. Soc., 403, 342Israel, G., Belloni, T., Stella, L., et al. 2005, Astrophys. J.Lett., 628, L53Karachentsev, I. D., & Kaisina, E. I. 2013, TheAstronomical Journal, 146, 46Kennicutt Jr, R. C. 1998, Annual Review of Astronomyand Astrophysics, 36, 189Klebesadel, R. W., Strong, I. B., & Olson, R. A. 1973,Astrophys. J., 182, L85 Kouveliotou, C., Meegan, C. A., Fishman, G. J., et al.1993, Astrophys. J. Lett., 413, L101Lee, J. C., De Paz, A. G., Kennicutt Jr, R. C., et al. 2010,Astrophys. J. Supp., 192, 6Lehmer, B., Tyler, J., Hornschemeier, A., et al. 2015,Astrophys. J., 806, 126Leroy, A. K., Sandstrom, K. M., Lang, D., et al. 2019, TheAstrophysics Journal Supplement Series, 244, 24Li, C., Lin, L., Xiong, S., et al. 2020, arXiv preprintarXiv:2005.11071Li, W., Chornock, R., Leaman, J., et al. 2011a, Mon. Not.R. Astron. Soc., 412, 1473Li, W., Leaman, J., Chornock, R., et al. 2011b, Mon. Not.R. Astron. Soc., 412, 1441Licquia, T. C., & Newman, J. A. 2015, Astrophys. J., 806,96Lien, A., Sakamoto, T., Barthelmy, S. D., et al. 2016,Astrophys. J., 829, 7Mandhai, S., Tanvir, N., Lamb, G., Levan, A., & Tsang, D.2018, Galaxies, 6, 130Marcote, B., Nimmo, K., Hessels, J., et al. 2020, Nature,577, 190Mattila, S., Dahl´en, T., Efstathiou, A., et al. 2012,Astrophys. J., 756, 111Mazets, E., Aptekar, R., Butterworth, P., et al. 1999a,Astrophys. J. Lett., 519, L151Mazets, E., Cline, T., Aptekar, R., et al. 1999b, arXivpreprint astro-ph/9905196Mazets, E., Golenetskii, S., Gurian, I. A., & Ilinskii, V.1982, Astrophysics and Space Science, 84, 173Mazets, E., Golenetskii, S., Il’Inskii, V., Guryan, Y. A.,et al. 1979, Nature, 282, 587Mazets, E. P., Aptekar, R. L., Cline, T. L., et al. 2008,Astrophys. J., 680, 545Meegan, C., Fishman, G., Wilson, R., et al. 1992, Nature,355, 143Meegan, C., Lichti, G., Bhat, P. N., et al. 2009, Astrophys.J., 702, 791Messick, C., Blackburn, K., Brady, P., et al. 2017, PhysicalReview D, 95, 042001Metzger, M., Djorgovski, S., Kulkarni, S., et al. 1997,Nature, 387, 878Morrissey, P., Conrow, T., Barlow, T. A., et al. 2007,Astrophys. J. Supp., 173, 682Newman, M. E. 2005, Contemporary physics, 46, 323Nicholl, M., Guillochon, J., & Berger, E. 2017, Astrophys.J., 850, 55Ofek, E., Muno, M., Quimby, R., et al. 2008, Astrophys. J.,681, 1464Ofek, E. O. 2007, Astrophys. J., 659, 339 Ofek, E. O., Kulkarni, S., Nakar, E., et al. 2006, Astrophys.J., 652, 507Olausen, S., & Kaspi, V. 2014, Astrophys. J. Supp., 212, 6Paciesas, W. S., Meegan, C. A., Pendleton, G. N., et al.1999, Astrophys. J. Supp., 122, 465Palmer, D. M., Barthelmy, S., Gehrels, N., et al. 2005,Nature, 434, 1107Pal’Shin, V., Hurley, K., Svinkin, D., et al. 2013,Astrophys. J. Supp., 207, 38Paturel, G., Petit, C., Prugniel, P., et al. 2003, Astronomy& Astrophysics, 412, 45Perley, D. A., Meyers, J., Hsiao, E., et al. 2010, GRBCoordinates Network, 10429, 1Popov, S. B., & Stern, B. 2006, Mon. Not. R. Astron. Soc.,365, 885Price, D. J., & Rosswog, S. 2006, Science, 312, 719Ridnaia, A., Svinkin, D., Frederiks, D., et al. 2020, arXivpreprint arXiv:2005.11178Roberts, O. J., Veres, P., Baring, M. G., et al. 2021,Nature, 589, 207Siegel, D. M., Barnes, J., & Metzger, B. D. 2019, Nature,569, 241Strohmayer, T. E., & Watts, A. L. 2005, Astrophys. J.Lett., 632, L111Svinkin, D., Frederiks, D., Aptekar, R., et al. 2016,Astrophys. J. Supp., 224, 10Svinkin, D., Frederiks, D., Hurley, K., et al. 2021, Nature,589, 211Svinkin, D. S., Hurley, K., Aptekar, R. L., Golenetskii,S. V., & Frederiks, D. D. 2015, Mon. Not. R. Astron.Soc., 447, 1028Taggart, K., & Perley, D. 2019, arXiv e-prints,arXiv:1911.09112Tendulkar, S. P., Kaspi, V. M., & Patel, C. 2016,Astrophys. J., 827, 59Tikhomirova, Y. Y., Pozanenko, A., & Hurley, K. 2010,Astronomy letters, 36, 231Tohuvavohu, A., Kennea, J. A., DeLaunay, J., et al. 2020,The Astrophysical Journal, 900Tsvetkova, A., Frederiks, D., Golenetskii, S., et al. 2017,Astrophys. J., 850, 161von Kienlin, A., Meegan, C., Paciesas, W., et al. 2020,Astrophys. J., 893, 46Watts, A. L., & Strohmayer, T. E. 2006, Astrophys. J.Lett., 637, L117Woods, P., & Thompson, C. 2004, arXiv preprintastro-ph/0406133Wright, E. L., Eisenhardt, P. R., Mainzer, A. K., et al.2010, The Astronomical Journal, 140, 1868 xtragalactic MGFs as SGRBs Younes, G., Baring, M. G., Kouveliotou, C., et al. 2017,Astrophys. J., 851, 17 APPENDIX AWe present rough estimates for the maximal detection distance of bright MGFs with representative active instru-ments. Konus-
Wind can detect bright MGFs to ∼ ≈ . ≈ ∼ x5 and thus a reduction in volume of > ∼ Swift
BAT has >
500 different rate trigger criteria running in real-time onboard, continuously sampling andtesting trigger timescales from 4ms up to 64 seconds, each of which is evaluated for 36 different combinations of energyranges and focal plane regions. While the BAT detector is sensitive to photons with energies up to 500 keV, thetransparency of the lead tiles in the mask above 200 keV limits its imaging energy range (necessary for a successfulautonomous 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 compute load on the BAT CPU, as well as the evolving state of the BAT detector array and changingoperational choices for trigger vetoes/thresholds, modelling the likelihood of an onboard autonomous trigger is quitedifficult. In addition, due to 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 vs ground searches, though thisis partly ameliorated by GUANO (Tohuvavohu et al. 2020). Under the assumed energetics and spectral values, weestimate that as of 2020 (averaging half of the original detector array online)
Swift /BAT should reliably trigger onMGFs out to ∼
25 Mpc in the highest coded region of its field of view. Ground analyses in the downlinked BATevent data can extend this, but the availability of this 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 sensitivityto extragalactic MGFs and local low-luminosity GRBs have been previously attempted. In 2012 the threshold for asuccessful trigger from an image was lowered from the usual value of 6.5 to 5.7, with the condition that triggers in thisrange be localized to within 12 arcminute projected offset from a local catalogued galaxy stored in the BAT onboardcatalog. No local GRB-like source was ever identified in this program. APPENDIX BAs GRB 070222 has not been reported elsewhere we describe its basic analysis here. The event was detected byKonus-
Wind , HEND on Mars Odyssey, and both SPI-ACS and PICsIT on INTEGRAL. Combination of the two bestannuli produce a localization with a 90% containment region of 0.004 deg . This location and its consistency with M83is shown in Figure 5.This burst is distinct from the separate candidates as having two separate pulses. Time-resolved analysis of thisburst is summarized in Table 2 while time integrated analysis is reported in the Second Konus GRB Catalog (Svinkinet al. 2016). xtragalactic MGFs as SGRBs W i n d ( K o n u s ) - I N T E G R A L ( S P I - A C S )M 8 3 ( N G C 5 2 3 6 )W i n d ( K o n u s ) - M a r s - O d y s s e y ( H E N D )
G R B 0 7 0 2 2 2 a , d e g Figure 5.
The localization of GRB 070222 compared to the position and angular size of M83. T Start T Stop
Index E Peak