The decline and fall of GRS 1915+105: the end is nigh?Truss, Michael; Done, Chris
doi: 10.1111/j.1745-3933.2006.00149.xpmid: N/A
The Galactic microquasar GRS 1915+105 has been in a continuous state of outburst since 1992, over 20 times longer than any other black hole X-ray transient. Assuming that the outburst is powered via accretion of an irradiated gaseous disc, we calculate how the predicted outburst duration varies according to the efficiency of the self-irradiation mechanism. At least one current model leads to the conclusion that the end of the outburst is imminent. The timing of the decline of GRS 1915+105, whenever it arrives, will be an excellent discriminator of the self-irradiation mechanism in X-ray transients, allowing us to infer the fraction of the disc that is heated by the incident X-rays and the magnitude of the mass loss rate in the form of a wind.
QPOs during magnetar flares are not driven by mechanical normal modes of the crustLevin, Yuri
doi: 10.1111/j.1745-3933.2006.00155.xpmid: N/A
QPOs have been observed during three powerful magnetar flares, from SGR 0526−66, SGR 1806−20 and SGR 1900+14. These QPOs have been commonly interpreted as being driven by the mechanical modes of the magnetar's solid crust which are excited during the flare. Here we show that this interpretation is in sharp contradiction with the conventional magnetar model. Firstly, we show that a magnetar crustal mode decays on the time-scale of at most 1 s due to the emission of Alfvén waves into the neutron star interior. A possible modification is then to assume that the QPOs are associated with the magnetars' global modes. However, we argue that at the frequencies of the observed QPOs, the neutron star core is likely to support a continuum of magnetohydrodynamic normal modes. We demonstrate this on a completely solvable toy model which captures the essential physics of the system. We then show that the frequency of the global mode of the whole star is likely to have a significant imaginary component, and its amplitude is likely to decay on a short time-scale. This is not observed. Thus we conclude that either (i) the origin of the QPO is in the magnetar's magnetosphere, or (ii) the magnetic field has a special configuration: either it is expelled from the magenta's core prior to the flares, or its poloidal component has very small coherence length.
Upper limits on the central black hole masses of 47 Tuc and NGC 6397 from radio continuum emissionDe Rijcke, S.; Buyle, P.; Dejonghe, H.
doi: 10.1111/j.1745-3933.2006.00153.xpmid: N/A
We present upper limits on the masses of the putative central intermediate-mass black holes in two nearby Galactic globular clusters: 47 Tuc (NGC 104), the second brightest Galactic globular cluster, and NGC 6397, a core-collapse globular cluster and, with a distance of 2.7 kpc, quite possibly the nearest globular cluster. These upper limits are obtained using a technique suggested by T. Maccarone. These mass estimates have been derived from 3σ upper limits on the radio continuum flux at 1.4 GHz, assuming that the putative central black hole accretes the surrounding matter at a rate of between 0.1 and 1 per cent of the Bondi accretion rate. For 47 Tuc, we find a 3σ upper limit of 2060–670 M⊙, depending on the actual accretion rate of the black hole and the distance to 47 Tuc. For NGC 6397, which is closer to us, we derive a 3σ upper limit of 1290–390 M⊙. While estimating mass upper limits based on radio continuum observations requires making assumptions about the gas density and the accretion rate of the black hole, their derivation does not require complex and time-consuming dynamical modelling. Thus, this method offers an independent way of estimating black hole masses in nearby globular clusters. If, generally, central black holes in stellar systems accrete matter faster than 0.1 per cent of the Bondi accretion rate, then these results indicate the absence of black holes in these globular clusters with masses as predicted by the extrapolated M•–σc relation.
Hard TeV spectra of blazars and the constraints to the infrared intergalactic backgroundKatarzyński, K.; Ghisellini, G.; Tavecchio, F.; Gracia, J.; Maraschi, L.
doi: 10.1111/j.1745-3933.2006.00156.xpmid: N/A
Recent gamma-ray observations of the blazar 1ES 1101−232 (redshift z= 0.186) reveal that the unabsorbed TeV spectrum is hard, with spectral index α≲ 0.5 [F(ν) ∝ν−α]. We show that simple one-zone synchrotron self-Compton model can explain such hard spectra if we assume a power law energy distribution of the emitting electrons with a relatively high minimum energy. In this case the intrinsic TeV spectrum can be as hard as F(ν) ∝ν1/3, while the predicted X-ray spectrum can still be much softer. The observations of 1ES 1101−232 can therefore be reconciled with relatively high intensities of the infrared background, even if some extreme background levels can indeed be excluded. We show that the other TeV sources (Mrk 421, Mrk 501 and PKS 2155−304) can be interpreted in the same framework, with a somewhat larger minimum energy.
On the origin of the iron Kα line cores in active galactic nucleiNandra, K.
doi: 10.1111/j.1745-3933.2006.00158.xpmid: N/A
X-ray observations made with Chandra and XMM–Newton have shown that there are relatively narrow cores to the iron Kα emission lines in active galactic nuclei (AGN). Plausible origins for this core emission include the outer regions of an accretion disc, a parsec-scale molecular torus, and the optical broad-line region (BLR). Using data from the literature it is shown that no correlation exists between the Fe Kα core width and the BLR (specifically Hβ) line width. This shows that in general the iron Kα core emission does not arise from the BLR. There is a similar lack of correlation between the width of the Fe Kα core and black hole mass. The average Kα width is about a factor of 2 lower than the Hβ width. It therefore seems likely that, in many cases, the narrow core arises in the torus. There is a very wide range of observed Fe Kα core widths, however, and this argues for multiple origins. The simplest explanation for the observed line profiles in AGN is that they are due to a mixing of very narrow emission from the inner edge of the torus, and broadened emission from the accretion disc, in varying proportions from object to object.
Relativistic force-free electrodynamic simulations of neutron star magnetospheresMcKinney, Jonathan C.
doi: 10.1111/j.1745-3933.2006.00150.xpmid: N/A
The luminosity and structure of neutron star magnetospheres are crucial to our understanding of pulsar and plerion emission. A solution found using the force-free approximation would be an interesting standard with which any model with more physics could be compared. Prior quasi-analytic force-free solutions may not be stable, while prior time-dependent magnetohydrodynamic models used unphysical model parameters. We use a time-dependent relativistic force-free electrodynamics code with no free parameters to find a unique stationary solution for the axisymmetric rotating pulsar magnetosphere in a Minkowski space–time in the case of no surface currents on the star. The solution is similar to the force-free quasi-analytic solution found in 1999 by Contopoulos, Kazanas & Fendt and the numerical magnetohydrodynamic solution found in 2006 by Komissarov. The magnetosphere structure and the usefulness of the classical y-point in the general dissipative regime are discussed. The pulsar luminosity is found to be L≈ 0.99 ± 0.01 μ2Ω4★/c3 for a dipole moment μ and stellar angular frequency Ω★.
On the correlation of short gamma-ray bursts and clusters of galaxiesGhirlanda, G.; Magliocchetti, M.; Ghisellini, G.; Guzzo, L.
doi: 10.1111/j.1745-3933.2006.00147.xpmid: N/A
We cross-correlate gamma-ray bursts (GRBs) and X-ray selected clusters of galaxies at z≤ 0.45. We find a positive 2σ signal for the angular cross-correlation function wbc(θ) on scales θ≤ 3° between short GRBs and clusters. Conversely, no correlation is found between clusters and the population of long GRBs. The comparison with the cluster autocorrelation function shows that short GRBs do not trace the cluster distribution, as not all short GRBs are found in clusters. A higher signal in wbc(θ) is found if we only consider the cluster population up to z= 0.1. By comparing the short-burst autocorrelation function with model predictions, we then constrain short bursts to mostly originate within ∼270 Mpc (i.e. z≤ 0.06). Our analysis also reveals that short GRBs are better correlated with ‘normal’ galaxies. The double compact-object merger model for short GRBs would associate them preferentially with early-type galaxies, but the present statistics do not allow us to exclude that at least a fraction of these events might also take place in late-type galaxies, in agreement with recent evidence.
The infrared glow of the first starsSalvaterra, Ruben; Magliocchetti, Manuela; Ferrara, Andrea; Schneider, Raffaella
doi: 10.1111/j.1745-3933.2006.00145.xpmid: N/A
Kashlinsky et al. find a significant cosmic infrared background fluctuation excess on angular scales ≳50 arcsec that cannot be explained by instrumental noise or local foregrounds. The excess has been tentatively attributed to emission from primordial, very massive (Population III, hereafter PopIII) stars formed ≤ 200 Myr after the Big Bang. Using an evolutionary model motivated by independent observations and including various feedback processes, we find that PopIII stars can contribute <40 per cent of the total background intensity (νJν∼ 1–2 nW m−2 sr−1 in the 0.8–8 μm range) produced by all galaxies (hosting both PopIII and PopII stars) at z≥ 5. The infrared fluctuation excess is instead very precisely accounted for by the clustering signal of galaxies at z≥ 5, predominantly hosting PopII stars with masses and properties similar to the present ones.
Short gamma-ray bursts in old populations: magnetars from white dwarf—white dwarf mergersLevan, Andrew J.; Wynn, Graham A.; Chapman, Robert; Davies, Melvyn B.; King, Andrew R.; Priddey, Robert S.; Tanvir, Nial R.
doi: 10.1111/j.1745-3933.2006.00144.xpmid: N/A
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.