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  • 1
    Electronic Resource
    Electronic Resource
    Low-mass relics of early star formation (2003)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 422 (2003), S. 869-871 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The earliest stars to form in the Universe were the first sources of light, heat and metals after the Big Bang. The products of their evolution will have had a profound impact on subsequent generations of stars. Recent studies of primordial star formation have shown that, in the absence of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature01579
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    Paper (German National Licenses)
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  • 2
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    The contribution of high-redshift galaxies to the near-infrared background (2013)
    Oxford University Press
    Details
    Publication Date: 2013-04-03
    Description: Several independent measurements have confirmed the existence of fluctuations ( F obs  ~ 0.1 nW m –2 sr –1 at 3.6 μm) up to degree angular scales in the source-subtracted near-infrared background (NIRB), but their origin is unknown. By combining high-resolution cosmological N -body/hydrodynamical simulations with an analytical model, and by matching galaxy luminosity functions (LFs) and the constraints on reionization simultaneously, we predict the NIRB absolute flux and fluctuation amplitude produced by high-redshift ( z  〉 5) galaxies (some of which harbour Population III stars, shown to provide a negligible contribution). This strategy also allows us to make an empirical determination of the evolution of the ionizing photon escape fraction: we find f esc  = 1 at z  ≥ 11, decreasing to ~0.05 at z  = 5. In the wavelength range 1.0–4.5 μm, the predicted cumulative flux is F  = 0.2–0.04 nW m –2 sr –1 . However, we find that the radiation from high-redshift galaxies (including those undetected by current surveys) is insufficient to explain the amplitude of the observed fluctuations: at l  = 2000, the fluctuation level attributable to z  〉 5 galaxies is F  = 0.01–0.002 nW m –2 sr –1 , with a wavelength-independent relative amplitude F / F  = 4 per cent. The source of the missing power remains unknown. This might indicate that an unknown component/foreground, with a clustering signal very similar to that of high-redshift galaxies, dominates the source-subtracted NIRB fluctuation signal.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Gamma-ray bursts from massive Population-III stars: clues from the radio band (2016)
    Oxford University Press
    Details
    Publication Date: 2016-05-12
    Description: Current models suggest gamma-ray bursts could be used as a way of probing Population-III stars – the first stars in the early Universe. In this paper, we use numerical simulations to demonstrate that late-time radio observations of gamma-ray burst afterglows could provide a means of identifying bursts that originate from Population-III stars, if these were highly massive, independently from their redshift. We then present the results from a pilot study using the Australia Telescope Compact Array at 17 GHz, designed to test the hypothesis that there may be Population-III gamma-ray bursts amongst the current sample of known events. We observed three candidates plus a control gamma-ray burst, and make no detections with upper limits of 20–40 μJy at 500–1300 d post-explosion.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    PopIII signatures in the spectra of PopII/I GRBs (2015)
    Oxford University Press
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    Publication Date: 2015-04-10
    Description: We investigate signatures of Population III (PopIII) stars in the metal-enriched environment of gamma-ray bursts (GRBs) originating from Population II-I (PopII/I) stars by using abundance ratios derived from numerical simulations that follow stellar evolution and chemical enrichment. We find that at z  〉 10 more than 10 per cent of PopII/I GRBs explode in a medium previously enriched by PopIII stars (we refer to them as GRBII-〉III). Although the formation of GRBII-〉III is more frequent than that of pristine PopIII GRBs (GRBIIIs), we find that the expected GRBII-〉III observed rate is comparable to that of GRBIIIs, due to the usually larger luminosities of the latter. GRBII-〉III events take place preferentially in small protogalaxies with stellar masses M * ~ 10 4.5 -10 7 M , star formation rates $\rm SFR \sim 10^{-3}{\rm -}10^{-1}\,\rm M_{\odot }\,yr^{-1}$ and metallicities Z ~ 10 – 4 -10 – 2 Z . On the other hand, galaxies with Z 〈 10 – 2.8 Z are dominated by metal enrichment from PopIII stars and should preferentially host GRBII-〉III. Hence, measured GRB metal content below this limit could represent a strong evidence of enrichment by pristine stellar populations. We discuss how to discriminate PopIII metal enrichment on the basis of various abundance ratios observable in the spectra of GRBs’ afterglows. By employing such analysis, we conclude that the currently known candidates at redshift z ~= 6 – i.e. GRB 050904 and GRB 130606A – are likely not originated in environments pre-enriched by PopIII stars. Abundance measurements for GRBs at z ~= 5 – such as GRB 100219A and GRB 111008A – are still poor to draw definitive conclusions, although their hosts seem to be dominated by PopII/I pollution and do not show evident signatures of massive PopIII pre-enrichment.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    There is a short gamma-ray burst prompt phase at the beginning of each long one (2015)
    Oxford University Press
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    Publication Date: 2015-02-07
    Description: We compare the prompt intrinsic spectral properties of a sample of short gamma-ray bursts (GRBs) with the first 0.3 s (rest frame) of long GRBs observed by Fermi /GBM (Gamma Burst Monitor). We find that short GRBs and the first part of long GRBs lie on the same E p – E iso correlation, that is parallel to the relation for the time-averaged spectra of long GRBs. Moreover, they are indistinguishable in the E p – L iso plane. This suggests that the emission mechanism is the same for short and for the beginning of long events, and both short and long GRBs are very similar phenomena, occurring on different time-scales. If the central engine of a long GRB would stop after ~0.3 x (1 + z ) s, the resulting event would be spectrally indistinguishable from a short GRB.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Radio afterglows of a complete sample of bright Swift GRBs: predictions from present days to the SKA era (2013)
    Oxford University Press
    Details
    Publication Date: 2013-10-08
    Description: Radio observations of Gamma-Ray Bursts (GRBs) afterglows are fundamental in providing insights into their physics and environment, and in constraining the true energetics of these sources. Nonetheless, radio observations of GRB afterglows are presently sparse in the time/frequency domain. Starting from a complete sample of 58 bright Swift long bursts (BAT6), we constructed a homogeneous sub-sample of 38 radio detections/upper limits which preserves all the properties of the parent sample. One half of the bursts have detections between 1 and 5 d after the explosion with typical fluxes F 100 μJy at 8.4 GHz. Through a Population SYnthesis Code coupled with the standard afterglow Hydrodynamical Emission model, we reproduce the radio flux distribution of the radio sub-sample. Based on these results, we study the detectability in the time/frequency domain of the entire long GRB population by present and future radio facilities. We find that the GRBs that typically trigger Swift can be detected at 8.4 GHz by Jansky Very Large Array within few days with modest exposures even at high redshifts. The final Square Kilometre Array (SKA) can potentially observe the whole GRB population provided that there will be a dedicated GRB gamma-ray detector more sensitive than Swift . For a sizeable fraction (50 per cent) of these bursts, SKA will allow us to perform radio calorimetry, after the trans-relativistic transition (occurring ~100 d), providing an estimate of the true (collimation corrected) energetics of GRBs.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    A magnetar powering the ordinary monster GRB 130427A? (2014)
    Oxford University Press
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    Publication Date: 2014-03-02
    Description: We present the analysis of the extraordinarily bright gamma-ray burst (GRB) 130427A under the hypothesis that the GRB central engine is an accretion-powered magnetar. In this framework, initially proposed to explain GRBs with precursor activity, the prompt emission is produced by accretion of matter on to a newly born magnetar, and the observed power is related to the accretion rate. The emission is eventually halted if the centrifugal forces are able to pause accretion. We show that the X-ray and optical afterglow is well explained as the forward shock emission with a jet break plus a contribution from the spin-down of the magnetar. Our modelling does not require any contribution from the reverse shock, that may still influence the afterglow light curve at radio and mm frequencies, or in the optical at early times. We derive the magnetic field ( B  ~ 10 16  G) and the spin period ( P  ~ 20 ms) of the magnetar and obtain an independent estimate of the minimum luminosity for accretion. This minimum luminosity results well below the prompt emission luminosity of GRB 130427A, providing a strong consistency check for the scenario where the entire prompt emission is the result of continuous accretion on to the magnetar. This is in agreement with the relatively long spin period of the magnetar. GRB 130427A was a well-monitored GRB showing a very standard behaviour and, thus, is a well-suited benchmark to show that an accretion-powered magnetar gives a unique view of the properties of long GRBs.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Luminosity function and jet structure of Gamma-Ray Burst (2015)
    Oxford University Press
    Details
    Publication Date: 2015-01-07
    Description: The structure of gamma-ray burst (GRB) jets impacts on their prompt and afterglow emission properties. The jet of GRBs could be uniform , with constant energy per unit solid angle within the jet aperture, or it could be structured , namely with energy and velocity that depend on the angular distance from the axis of the jet. We try to get some insight about the still unknown structure of GRBs by studying their luminosity function. We show that low (10 46–48  erg s –1 ) and high (i.e. with L  ≥ 10 50  erg s –1 ) luminosity GRBs can be described by a unique luminosity function, which is also consistent with current lower limits in the intermediate luminosity range (10 48–50  erg s –1 ). We derive analytical expressions for the luminosity function of GRBs in uniform and structured jet models and compare them with the data. Uniform jets can reproduce the entire luminosity function with reasonable values of the free parameters. A structured jet can also fit adequately the current data, provided that the energy within the jet is relatively strongly structured, i.e. E – k with k  ≥ 4. The classical E –2 structured jet model is excluded by the current data.
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    Topics: Physics
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  • 9
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    Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra (2014)
    Oxford University Press
    Details
    Publication Date: 2014-06-05
    Description: We investigate the scaling relation between the observed amount of absorption in the X-ray spectra of gamma-ray burst afterglows and the absorber redshift. Through dedicated numerical simulations of an ideal instrument, we establish that this dependence has a power-law shape with index 2.4. However, for real instruments, this value depends on their low-energy cut-off, spectral resolution and on the detector spectral response in general. We thus provide appropriate scaling laws for specific instruments. Finally, we discuss the possibility to measure the absorber redshift from X-ray data alone. We find that 10 5 -10 6 counts in the 0.3–10 keV band are needed to constrain the redshift with 10 per cent accuracy. As a test case, we discuss the XMM–Newton observation of GRB 090618 at z  = 0.54. We are able to recover the correct redshift of this burst with the expected accuracy.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    The brief era of direct collapse black hole formation (2014)
    Oxford University Press
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    Publication Date: 2014-04-15
    Description: It has been proposed that the first, intermediate-mass ( 10 5-6 M ) black holes might form through direct collapse of unpolluted gas in atomic-cooling haloes exposed to a strong Lyman–Werner (LW) or near-infrared (NIR) radiation. As these systems are expected to be Compton thick, photons above 13.6 eV are largely absorbed and reprocessed into lower energy bands. It follows that direct collapse black holes (DCBHs) are very bright in the LW/NIR bands, typically outshining small high-redshift galaxies by more than 10 times. Once the first DCBHs form, they then trigger a runaway process of further DCBH formation, producing a sudden rise in their cosmic mass density. The universe enters the ‘DCBH era’ at z 20 when a large fraction of atomic-cooling haloes are experiencing DCBH formation. By combining the clustering properties of the radiation sources with Monte Carlo simulations, we show that in this scenario the DCBH mass density rises from ~5 M Mpc –3 at z ~ 30 to the peak value ~5 10 5 M Mpc –3 at z ~ 14 in our fiducial model. However, the abundance of active (accreting) DCBHs drops after z ~ 14, as gas in the potential formation sites (unpolluted haloes with virial temperature slightly above 10 4 K) is photoevaporated. This effect almost completely suppresses DCBH formation after z ~ 13. The DCBH formation era lasts only 150 Myr, but it might crucially provide the seeds of the supermassive black holes powering z ~ 6 quasars.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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