| 1. |
- Tanvir, N. R., et al.
(författare)
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A γ-ray burst at a redshift of z~8.2
- 2009
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 461, s. 1254-1257
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Tidskriftsartikel (refereegranskat)abstract
- Long-duration γ-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z>20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-α emitting galaxy. Here we report that GRB090423 lies at a redshift of z~8.2, implying that massive stars were being produced and dying as GRBs ~630Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.
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| 2. |
- Ahmed, Tanvir, 1970, et al.
(författare)
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Enhanced immunogenicity of an oral inactivated cholera vaccine in infants in Bangladesh obtained by zinc supplementation and by temporary withholding breast-feeding.
- 2009
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Ingår i: Vaccine. - : Elsevier BV. - 0264-410X. ; 27:9, s. 1433-9
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Tidskriftsartikel (refereegranskat)abstract
- The killed oral cholera vaccine Dukoral is recommended for adults and only children over 2 years of age, although cholera is seen frequently in younger children and there is an urgent need for a vaccine for them. Since decreased immunogenicity of oral vaccines in children in developing countries is a critical problem, we tested interventions to enhance responses to Dukoral. We evaluated the effect on the immune responses by temporarily withholding breast-feeding or by giving zinc supplementation. Two doses of Dukoral consisting of killed cholera vibrios and cholera B subunit were given to 6-18 months old Bangladeshi children (n=340) and safety and immunogenicity studied. Our results showed that two doses of the vaccine were safe and induced antibacterial (vibriocidal) antibody responses in 57% and antitoxin responses in 85% of the children. Immune responses were comparable after intake of one and two doses. Temporary withholding breast-feeding for 3 h before immunization or supplementation with 20 mg of zinc per day for 42 days resulted in increased magnitude of vibriocidal antibodies (77% and 79% responders, respectively). Administration of vaccines without buffer or in water did not result in reduction of vibriocidal responses. This study demonstrates that the vaccine is safe and immunogenic in children under 2 years of age and that simple interventions can enhance immune responses in young children.
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| 3. |
- Ferguson, A M N, et al.
(författare)
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The stellar populations of the M31 halo substructure
- 2005
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 622:2, s. 109-112
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Tidskriftsartikel (refereegranskat)abstract
- We present the first results from our survey of stellar substructure in the outskirts of M31 using the Advanced Camera for Surveys on board the Hubble Space Telescope. We discuss the stellar populations associated with five prominent stellar overdensities discovered during the course of our panoramic ground-based imaging survey with the Isaac Newton Telescope Wide-Field Camera; a sixth pointing targets a region of "clean" halo. The color-magnitude diagrams, which contain between approximate to 10,000 and 90,000 stars and reach 4 mag below the horizontal branch, reveal clear variations in morphology between most fields, indicating that the age and/or metallicity mix of stars is not constant at large radius. This directly confirms the existence of large-scale population inhomogeneities within the halo of M31 and lends further support to the notion that M31 has formed, at least in part, through satellite accretions. We find a striking similarity between the populations of the giant stellar stream and those of another overdensity, the NE shelf, which lies northeast of the galaxy center. If these overdensities are associated with the same population, then the difference in their red clump magnitudes implies that the NE shelf lies in front of the stream by several tens of kiloparsecs, in good agreement with recent orbit calculations for the stream progenitor.
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| 4. |
- Fynbo, J. P. U., et al.
(författare)
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Low-resolution Spectroscopy of Gamma-ray Burst Optical Afterglows : Biases in the Swift Sample and Characterization of the Absorbers
- 2009
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 185:2, s. 526-573
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Tidskriftsartikel (refereegranskat)abstract
- We present a sample of 77 optical afterglows (OAs) of Swift detected gamma-ray bursts (GRBs) for which spectroscopic follow-up observations have been secured. Our first objective is to measure the redshifts of the bursts. For the majority (90%) of the afterglows, the redshifts have been determined from the spectra. We provide line lists and equivalent widths (EWs) for all detected lines redward of Lyα covered by the spectra. In addition to the GRB absorption systems, these lists include line strengths for a total of 33 intervening absorption systems. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray-selected statistical sample of Swift bursts with optimal conditions for ground-based follow-up from the period 2005 March to 2008 September; 146 bursts fulfill our sample criteria. We derive the redshift distribution for the statistical (X-ray selected) sample and conclude that less than 18% of Swift bursts can be at z > 7. We compare the high-energy properties (e.g., γ-ray (15-350 keV) fluence and duration, X-ray flux, and excess absorption) for three subsamples of bursts in the statistical sample: (1) bursts with redshifts measured from OA spectroscopy; (2) bursts with detected optical and/or near-IR afterglow, but no afterglow-based redshift; and (3) bursts with no detection of the OA. The bursts in group (1) have slightly higher γ-ray fluences and higher X-ray fluxes and significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fractions of dark bursts, defined as bursts with an optical to X-ray slope βOX < 0.5, is 14% in group (1), 38% in group (2), and >39% in group (3). For the full sample, the dark burst fraction is constrained to be in the range 25%-42%. From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight lines. This should be taken into account when determining, e.g., the redshift or metallicity distribution of GRBs and when using GRBs as a probe of star formation. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular the damped Lyα absorbers (DLAs). On average GRB absorbers are characterized by significantly stronger EWs for H I as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. However, the distribution of line strengths is very broad and several GRB absorbers have lines with EWs well within the range spanned by QSO-DLAs. Based on the 33 z > 2 bursts in the sample, we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programs 275.D-5022 (PI: Chincarini), 075.D-0270 (PI: Fynbo), 077.D-0661 (PI: Vreeswijk), 077.D-0805 (PI: Tagliaferri), 177.A-0591 (PI: Hjorth), 078.D-0416 (PI: Vreeswijk), 079.D-0429 (PI: Vreeswijk), 080.D-0526 (PI: Vreeswijk), 081.A-0135 (PI: Greiner), 281.D-5002 (PI: Della Valle), and 081.A-0856 (PI: Vreeswijk). Also based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Some of the data obtained herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck foundation.
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| 5. |
- 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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| 6. |
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| 7. |
- Faria, Daniel, et al.
(författare)
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Probing the nature of the G1 clump stellar overdensity in the outskirts of M31
- 2007
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Ingår i: The Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 133:4, s. 1275-1286
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Tidskriftsartikel (refereegranskat)abstract
- We present deep Hubble Space Telescope ACS observations of the G1 clump, a distinct stellar overdensity lying at similar to 30 kpc along the southwestern major axis of M31, close to the G1 globular cluster ( from the work of Ferguson and coworkers). Our well-populated color-magnitude diagram reaches similar to 7 mag below the red giant branch tip with 90% completeness, and allows the detection of various morphological features that can be used to derive detailed constraints on the age and metallicity of the constituent stellar population. We find that the color-magnitude diagram is best described spread in metallicity is constrained to be P = 0.5 dex. The star formation rate in this region has declined over time, with the bulk of the stellar mass having formed > 6 Gyr ago. Nonetheless, a nonnegligible mass fraction (approximate to 10%) of the population has formed in the last 2 Gyr. We discuss the nature of the G1 clump in light of these new stellar population constraints and argue that the combination of stellar content and physical size make it unlikely that the structure is the remnant of an accreted dwarf galaxy. Instead, the strong similarity between the stellar content of the G1 clump and that of the M31 outer disk suggests that the substructure is a fragment of the outer disk, perhaps torn off from the main body during a past accretion/merger event; this interpretation is consistent with extant kinematical data. If this interpretation is correct, our analysis of the stellar content provides further evidence that the outskirts of large disk galaxies have been in place for a significant time.
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| 8. |
- Jakobsson, P, et al.
(författare)
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A mean redshift of 2.8 for Swift gamma-ray bursts
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 447:3, s. 897-903
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Tidskriftsartikel (refereegranskat)abstract
- The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (z(mean)), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that z(mean) = 2.8 and that at least 7% of GRBs originate at z > 5. In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.
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| 9. |
- Malesani, D., et al.
(författare)
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Early Spectroscopic Identification of SN 2008D
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 692:2, s. L84-L87
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Tidskriftsartikel (refereegranskat)abstract
- SN 2008D was discovered while following up an unusually bright X-ray transient (XT) in the nearby spiral galaxy NGC 2770. We present early optical spectra ( obtained 1.75 days after the XT) which allowed the first identification of the object as a supernova ( SN) at redshift z = 0.007. These spectra were acquired during the initial declining phase of the light curve, likely produced in the stellar envelope cooling after shock breakout, and rarely observed. They exhibit a rather flat spectral energy distribution with broad undulations, and a strong, W-shaped feature with minima at 3980 and 4190 angstrom ( rest frame). We also present extensive spectroscopy and photometry of the SN during the subsequent photospheric phase. Unlike SNe associated with gamma-ray bursts, SN 2008D displayed prominent He features and is therefore of Type Ib.
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