| 1. |
- Barthelmy, S D, et al.
(författare)
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An origin for short gamma-ray bursts unassociated with current star formation
- 2005
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 438, s. 994-996
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Tidskriftsartikel (refereegranskat)abstract
- Two short (< 2 s) gamma-ray bursts (GRBs) have recently been localized(1-4) and fading afterglow counterparts detected(2-4). The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual gamma-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations(6-8). The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 ( refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2 - 3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts - the coalescence of binary systems of neutron stars or a neutron star-black hole pair - are the progenitors of short bursts.
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| 2. |
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| 3. |
- Chapman, R., et al.
(författare)
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Soft Gamma Repeaters and Short Gamma Ray Bursts: Making Magnetars from WD -- WD Mergers
- 2007
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Ingår i: ASP Conference Proceedings. ; 372, s. 415-415
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Konferensbidrag (refereegranskat)abstract
- Recent progress on the nature of short duration Gamma Ray Bursts (GRBs) has shown that a fraction of them originate in the local universe. These systems may well be the result of giant flares from Soft Gamma Repeaters (SGRs) believed to be magnetars (neutron stars with extremely large magnetic fields ≥1014G). If these magnetars are formed via the core collapse of massive stars, then it would be expected that the bursts should originate from predominantly young stellar populations. However, correlating the positions of BATSE short bursts with structure in the local universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars forming via the core collapse of massive stars, they also form via accretion induced collapse following the merger of two white dwarfs (WDs), one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via WD--WD mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of magnetar production by core collapse is proportional to the star formation rate, the rate of production via WD--WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD--WD mergers may produce SGR giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short duration GRB population at low redshift.
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| 4. |
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| 5. |
- Church, Ross P., et al.
(författare)
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Detailed models of the binary pulsars J1141-6545 and B2303+46
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:2, s. 715-727
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Tidskriftsartikel (refereegranskat)abstract
- We have modelled the formation of the eccentric double-degenerate binaries J1141-6545 and B2303+46 using the Henyey-type full stellar evolution code STARS and the population synthesis code BSE. We find that the outcome depends strongly on the common envelope (CE) evolution efficiency parameter alpha(CE) and show that both systems can be modelled with a single value of alpha(CE). The final orbit of the system depends critically on the order of Roche lobe filling events. The phase space of progenitors and the different evolutionary pathways followed by binary stars that form eccentric double-degenerate binaries in the two codes are compared. We show that the pathways are similar between the codes and that the distribution of progenitors in mass and separation phase space is qualitatively the same, thus validating the use of BSE-like population synthesis for simulations of this type. The phase space of initial parameters is very different to that obtained using ad hoc arguments about the evolution, which shows that such arguments are insufficient to model evolutionary pathways of this complexity. There are some differences associated with the prescription adopted for CE evolution but these are not qualitatively significant. We investigate the dependence of the formation mechanism on wind mass loss and the CE efficiency parameter alpha(CE), showing that it depends strongly on the latter but rather less on the former.
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| 6. |
- Church, Ross P., et al.
(författare)
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Mass transfer in eccentric binaries: the new oil-on-water smoothed particle hydrodynamics technique
- 2009
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 395:2, s. 1127-1134
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Tidskriftsartikel (refereegranskat)abstract
- To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a 0.6 M-circle dot donor star and a 1 M-circle dot white dwarf; such binaries are thought to form cataclysmic variable ( CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of q = 0.6 and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity.
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| 7. |
- Dale, JE, et al.
(författare)
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Collisions and close encounters involving massive main-sequence stars
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 366:4, s. 1424-1436
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Tidskriftsartikel (refereegranskat)abstract
- We study close encounters involving massive main-sequence stars and the evolution of the exotic products of these encounters as common-envelope systems or possible hypernova progenitors. We show that parabolic encounters between low- and high-mass stars and between two high-mass stars with small periastrons result in mergers on time-scales of a few tens of stellar free-fall times (a few tens of hours). We show that such mergers of unevolved low-mass stars with evolved high-mass stars result in little mass-loss (similar to 0.01 M-circle dot) and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common-envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of similar to 100. If these remnants exist in very densely populated environments (n greater than or similar to 10(7) pc(-3)), they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma-ray burst progenitors and that similar to 0.1 per cent of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma-ray bursts.
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| 8. |
- Dale, James E., et al.
(författare)
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Red giant stellar collisions in the Galactic Centre
- 2009
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 393:3, s. 1016-1033
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Tidskriftsartikel (refereegranskat)abstract
- We show that collisions with stellar-mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al. We show that the missing objects are low-mass giants and asymptotic giant branch stars in the range 1-3 M-circle dot. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of smoothed particle hydrodynamic calculations, restricted three-body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main-sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the 10.5 < K < 12 band. However, depleting the brighter (K < 10.5) objects out to the required radius would require a large population of black hole impactors which would in turn deplete the 10.5 < K < 12 giants in a region much larger than is observed. We conclude that collisions with stellar-mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar-mass black holes in the Galactic Centre.
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| 9. |
- Davies, Melvyn B
(författare)
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ASTROPHYSICS Stellar revival in old clusters
- 2009
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 462:7276, s. 991-992
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 10. |
- Davies, Melvyn B, et al.
(författare)
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Is our Sun a singleton?
- 2008
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Ingår i: Physica Scripta. - 0031-8949. ; T130
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Tidskriftsartikel (refereegranskat)abstract
- All stars are formed in some form of cluster or association. These environments can have a much higher number density of stars than the field of the galaxy. Such crowded places are hostile environments: a large fraction of initially single stars will undergo close encounters with other stars or exchange into binaries. We describe how such close encounters and exchange encounters will affect the properties of a planetary system around a single star. We define singletons as single stars which have never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, leading to strong planet-planet interactions, often leaving planets on tighter and more eccentric orbits. Thus, planetary systems which initially resembled our own solar system may later more closely resemble the observed exoplanetary systems.
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| 11. |
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| 12. |
- Davies, Melvyn B, et al.
(författare)
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Stellar encounters involving massive stars in young clusters
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:4, s. 2038-2046
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Tidskriftsartikel (refereegranskat)abstract
- We model collisions between pre-main-sequence stars using a smoothed particle hydrodynamics method. Assuming that all collisions lead to simple mergers, we use derived merger cross-sections to calculate the time-scale to make a 50-M-circle dot star by collisions within the core of a stellar cluster as a function of stellar number density. We show that a 50-M-circle dot star may be produced in this manner within 106 yr beginning with a cluster core of 200 1-M-circle dot stars within a radius of 0.0025 pc. Encounters between one high-mass star and one low-mass star tend to result in the tidal shredding of the latter, producing a massive disc around the former. This disc spreads viscously and provided a much larger target than any star for subsequent collisions. If a star strikes the disc, it is likely to be captured, and so forms a binary with the other star. Subsequent encounters between the binary and single stars lead either to exchanges or to the formation of merged objects. The inclusion of this effect leads to a significant reduction in the time taken to produce a 50-M-circle dot star. We also consider the role played by primordial binaries. We show that the time-scale required to produce a 50-M-circle dot star decreases with increasing binary fraction. We find that the number of primordial binaries is reduced by encounters. The core of a cluster must therefore contain a very high binary fraction initially if a large fraction of the massive stars are to be contained within binaries when the 50-M-circle dot star is produced.
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| 13. |
- Davies, Melvyn B, et al.
(författare)
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The Astrophysics of Crowded Places
- 2005
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Ingår i: Advances in Astronomy: from Big Bang to the Solar System. - 1860945775
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 14. |
- Davies, Melvyn B, et al.
(författare)
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The MODEST questions: Challenges and future directions in stellar cluster research
- 2006
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Ingår i: New Astronomy. - : Elsevier BV. - 1384-1092 .- 1384-1076. ; 12:3, s. 201-214
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Forskningsöversikt (refereegranskat)abstract
- We present a review of some of the current major challenges in stellar cluster research, including young clusters, globular clusters, and galactic nuclei. Topics considered include: primordial mass segregation and runaway mergers, expulsion of gas from clusters, the production of stellar exotica seen in some clusters (e.g., blue stragglers and extreme horizontal-branch stars), binary populations within clusters, the black-hole population within stellar clusters, the final parsec problem, stellar dynamics around a massive black hole, and stellar collisions. The Modest Questions posed here are the outcome of discussions which took place at the Modest-6A workshop held in Lund, Sweden, in December, 2005. Modest-6A was organised as part of the activities of the Modest Collaboration (see www.manybody.org for further details).
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| 15. |
- Davies, Melvyn B, et al.
(författare)
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The stars of the Galactic center
- 2005
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 624:Part 2, s. 25-27
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Tidskriftsartikel (refereegranskat)abstract
- We consider the origin of the so-called S stars orbiting the supermassive black hole at the very center of the Galaxy. These are usually assumed to be massive main-sequence stars. We argue instead that they are the remnants of low-to-intermediate mass red giants that have been scattered into near-radial orbits and tidally stripped as they approach the central black hole. Such stars retain only low-mass envelopes and thus have high effective temperatures. Our picture simultaneously explains why S stars have tightly bound orbits and the observed depletion of red giants in the very center of the Galaxy.
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| 16. |
- Davies, Melvyn B, et al.
(författare)
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The ultimate outcome of black hole - neutron star mergers
- 2005
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 356:1, s. 54-58
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Tidskriftsartikel (refereegranskat)abstract
- We present a simple, semi--analytical description for the final stagesof mergers of black hole (BH) -- neutron star (NS) systems. Such systemsare of much interest as gravitational wave sources and gamma--ray burstprogenitors. Numerical studies show that in general the neutron star isnot disrupted at the first phase of mass transfer. Instead, what remainsof the neutron star is left on a wider, eccentric, orbit. We considerthe evolution of such systems as they lose angular momentum viagravitational radiation and come into contact for further phases of masstransfer. During each mass transfer event the neutron star mass isreduced until a critical value where mass loss leads to a rapid increasein the stellar radius. At this point Roche lobe overflow shreds whatremains of the neutron star, most of the mass forming a disc around theblack hole. Such a disc may be massive enough to power a gamma--rayburst. The mass of the neutron star at the time of disruption (andtherefore the disc mass) is largely independent of the initial masses ofthe black hole and neutron star, indicating that BH--NS star mergers maybe standard candles.
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| 17. |
- De Marchi, F., et al.
(författare)
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Search and analysis of blue straggler stars in open clusters
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 459:2, s. 489-497
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Tidskriftsartikel (refereegranskat)abstract
- Aims. This paper presents a new homogeneous catalogue of blue straggler stars ( BSS) in Galactic open clusters. Methods. Photometric data for 216 clusters were collected from the literature and 2782 BSS candidates were extracted from 76 of them. Results. We found that the anticorrelation of BSS frequency vs. total magnitude identified in similar studies conducted on Galactic globular clusters extends to the open cluster regime: clusters with smaller total magnitude tend to have higher BSS frequencies. Moreover, a clear correlation between the BSS frequency ( obtained normalising the total number of BSS either to the total cluster mass or, for the older clusters, to the total number of clump stars) and the age of the clusters was found. A simple model is developed here to try to explain this last and new result. The model allows us to ascertain the important effect played by mass loss in the evolution of open clusters.
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| 18. |
- Dray, L. M., et al.
(författare)
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Young stars in the Galactic Centre: a potential intermediate-mass star origin
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:1, s. 31-44
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Tidskriftsartikel (refereegranskat)abstract
- There has been recent speculation that the cores of intermediate-mass stars stripped of their envelopes by tidal interaction with the supermassive black hole in the Galactic Centre could form a population observationally similar to the so-called Sgr A* cluster or 'S' stars, which have close eccentric orbits around the hole. We model the evolution of such stars, and show that the more luminous end of the population may indeed appear similar to young B stars within the observational limits of the Galactic Centre region. Whether some or all of these cluster stars can be accounted for in this manner depends strongly on the assumed initial mass function (IMF) of the loss cone stars and the scattering rate. If most of the observed stars are in fact scattered from the Galactic Centre inner cusp region itself then the population of similar to 20 to current observational limits may be reproduced. However, this only works if the local relaxation time is small and relies on the cusp stars themselves being young, i.e. it is dependent on some star formation being possible in the central few parsecs. Conversely, we obtain a possible constraint on the tidal stripping rate of 'normal' IMF stars if there are not to be red stars visible in the Sgr A* cluster.
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| 19. |
- Haehnelt, MG, et al.
(författare)
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Possible evidence for the ejection of a supermassive black hole from an ongoing merger of galaxies
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 1745-3925 .- 1745-3933. ; 366:1, s. 22-25
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Tidskriftsartikel (refereegranskat)abstract
- Attempts of Magain et al. to detect the host galaxy of the bright quasi-stellar object (QSO) HE0450-2958 have not been successful. We suggest that the supermassive black hole (SMBH) powering the QSO was ejected from the observed ultraluminous infrared galaxy (ULIRG) at the same redshift and at 1.5 arcsec distance. Ejection could have been caused either by recoil due to gravitational wave emission from a coalescing binary of SMBHs or the gravitational slingshot of three or more SMBHs in the ongoing merger of galaxies which triggered the starburst activity in the ULIRG. We discuss implications for the possible hierarchical build-up of SMBHs from intermediate and/or stellar mass black holes, and for the detection of coalescing supermassive binary black holes by LISA.
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| 20. |
- Haggard, Daryl, et al.
(författare)
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A Chandra Study Of The Galactic Globular Cluster Omega Centauri
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X. ; 697:1, s. 224-236
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Tidskriftsartikel (refereegranskat)abstract
- We analyze a similar to 70 ks Chandra Advanced CCD Imaging Spectrometer exposure of the globular cluster omega Cen (NGC 5139). The similar to 17' x 17' field of view fully encompasses three core radii and almost twice the half-mass radius. We detect 180 sources to a limiting flux of similar to 4.3 x 10(-16) erg cm(-2) s(-1) (L-x = 1.2 x 10(30) erg s(-1) at 4.9 kpc). After accounting for the number of active galactic nuclei and possible foreground stars, we estimate that 45-70 of the sources are cluster members. Four of the X-ray sources have previously been identified as compact accreting binaries in the cluster-three cataclysmic variables (CVs) and one quiescent neutron star. Correlating the Chandra positions with known variable stars yields eight matches, of which five are probable cluster members that are likely to be binary stars with active coronae. Extrapolating these optical identifications to the remaining unidentified X-ray source population, we estimate that 20-35 of the sources are CVs and a similar number are active binaries. This likely represents most of the CVs in the cluster, but only a small fraction of all the active binaries. We place a 2 sigma upper limit of L-x < 3 x 10(30) erg s(-1) on the integrated luminosity of any additional faint, unresolved population of sources in the core. We explore the significance of these findings in the context of primordial versus dynamical channels for CV formation. The number of CVs per unit mass in. Cen is at least 2-3 times lower than in the field, suggesting that primordial binaries that would otherwise lead to CVs are being destroyed in the cluster environment.
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| 21. |
- Hannikainen, D C, et al.
(författare)
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The X-ray source population of the globular cluster M15: Chandra high-resolution imaging
- 2005
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 357:1, s. 325-332
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Tidskriftsartikel (refereegranskat)abstract
- The globular cluster M15 was observed on three occasions with the High Resolution Camera on- board Chandra in 2001 in order to investigate the X- ray source population in the cluster centre. After subtraction of the two bright central sources, four faint sources were identified within 50 arcsec of the core. One of these sources is probably the planetary nebula K648, making this the first positive detection of X- rays from a planetary nebula inside a globular cluster. Another two are identified with UV variables (one previously known), which we suggest are cataclysmic variables (CVs). The nature of the fourth source is more difficult to ascertain, and we discuss whether it is possibly a quiescent soft X- ray transient or also a CV.
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| 22. |
- King, Andrew, et al.
(författare)
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A new type of long gamma-ray burst
- 2007
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 1745-3925 .- 1745-3933. ; 374:1, s. 34-36
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Tidskriftsartikel (refereegranskat)abstract
- We consider gamma-ray bursts (GRBs) produced by the merger of a massive white dwarf with a neutron star. We show that these are likely to produce long-duration GRBs, in some cases definitely without an accompanying supernova, as observed recently. This class of burst would have a strong correlation with star formation, and occur close to the host galaxy. However, rare members of the class need not be near star-forming regions, and could have any type of host galaxy. Thus a long-duration burst far from any star-forming region would also be a signature of this class. Estimates based on the existence of a known progenitor suggest that our proposed class may be an important contributor to the observed GRB rate.
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| 23. |
- Larsson, Josefin, et al.
(författare)
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A new constraint for gamma-ray burst progenitor mass
- 2007
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 376:3, s. 1285-1290
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Tidskriftsartikel (refereegranskat)abstract
- Recent comparative observations of long-duration gamma-ray bursts (LGRBs) and core collapse supernova (cc SN) host galaxies demonstrate that these two, highly energetic transient events are distributed very differently upon their hosts. LGRBs are much more concentrated on their host galaxy light than cc SN. Here we explore the suggestion that this differing distribution reflects different progenitor masses for LGRBs and cc SN. Using a simple model we show that, assuming cc SN arise from stars with main-sequence masses > 8 M-circle dot, GRBs are likely to arise from stars with initial masses > 20 M-circle dot. This difference can naturally be explained by the requirement that stars which create a LGRB must also create a black hole.
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| 24. |
- Levan, Andrew J., et al.
(författare)
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Neutron star binaries and long-duration gamma-ray bursts
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:3, s. 1351-1356
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Tidskriftsartikel (refereegranskat)abstract
- Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes (BHs) of stripped massive stars. Those with sufficient rotation form a centrifugally supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50 per cent of the observed systems. As these systems created a neutron star and not a BH, they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity.
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| 25. |
- Levan, Andrew, et al.
(författare)
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Short gamma-ray bursts in old populations: magnetars from white dwarf-white dwarf mergers
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 1745-3925 .- 1745-3933. ; 368:1, s. 1-5
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress on the nature of short-duration gamma-ray bursts has shown that a fraction of them originate in the local Universe. These systems may well be the result of giant flares from soft gamma-repeaters (highly magnetized neutron stars commonly known as magnetars). However, if these neutron stars are formed via the core collapse of massive stars then it would be expected that the bursts should originate from predominantly young stellar populations, while correlating the positions of BATSE short bursts with structure in the local Universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars formed via the core collapse of massive stars, they also form via accretion-induced collapse following the merger of two white dwarfs, one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via white dwarf-white dwarf (WD-WD) mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of production of magnetars by core collapse is proportional to the star formation rate, the rate of production via WD-WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD-WD mergers may produce soft gamma-repeater giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short-duration burst population at higher redshift.
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