Monthly Notices of the Royal Astronomical Society Letters

Jin, Chichuan; Done, Chris; Ward, Martin; Gierliński, Marek; Mullaney, James

doi: 10.1111/j.1745-3933.2009.00697.xpmid: N/A

We have carried out a survey of long 50 ks XMM–Newton observations of a sample of bright, variable active galactic nuclei (AGN). We found a distinctive energy dependence of the variability in RX J0136.9−3510 where the fractional variability increases from 0.3 to 2 keV, and then remains constant. This is in sharp contrast to other AGN where the X-ray variability is either flat or falling with energy, sometimes with a peak at ∼2 keV superimposed on the overall trend. Intriguingly, these unusual characteristics of the variability are shared by one other AGN, namely RE J1034+396, which is so far unique showing a significant X-ray quasi-periodic oscillation (QPO). In addition, the broad-band spectrum of RX J0136.9−3510 is also remarkably similar to that of RE J1034+396, being dominated by a huge soft excess in the Extreme-UV (EUV) to soft X-ray bandpass. The bolometric luminosity of RX J0136.9−3510 gives an Eddington ratio of about 2.7 for a black hole mass (from the H beta line width) of 7.9 × 107 M⊙. This mass is about a factor of 50 higher than that of RE J1034+396, making any QPO undetectable in this length of observation. None the less, its X-ray spectral and variability similarities suggest that RE J1034+396 is simply the closest representative of a new class of AGN spectra, representing the most extreme mass accretion rates.

Sahayanathan, S.

doi: 10.1111/j.1745-3933.2009.00707.xpmid: N/A

The high-resolution image of the jet of the BL Lac object MKN501 in radio shows a limb-brightened feature. An explanation of this feature as an outcome of the differential Doppler boosting of jet spine and jet boundary due to transverse velocity structure of the jet requires large viewing angle. However, this inference contradicts with the constraints derived from the high-energy γ-ray studies unless the jets bend over a large angle immediately after the γ-ray zone (close to the central engine). In this Letter, we propose an alternate explanation to the limb-brightened feature of MKN501 by considering the diffusion of electrons accelerated at the boundary shear layer into the jet medium, and this consideration does not require large viewing angle. Also, the observed difference in the spectral index at the jet boundary and jet spine can be understood within the framework of shear acceleration.

Clarke, C. J.; Lodato, G.

doi: 10.1111/j.1745-3933.2009.00695.xpmid: N/A

In this Letter, we show that if planetesimals form in spiral features in self-gravitating discs, as previously suggested by the idealized simulations of Rice et al., then in realistic protostellar discs, this process will be restricted to the outer regions of the disc (i.e. at radii in excess of several tens of au). This restriction relates to the requirement that dust has to be concentrated in spiral features on a time-scale that is less than the (roughly dynamical) lifetime of such features, and that such rapid accumulation requires spiral features whose fractional amplitude is not much less than unity. This in turn requires that the cooling time-scale of the gas is relatively short, which restricts the process to the outer disc. We point out that the efficient conversion of a large fraction of the primordial dust in the disc into planetesimals could rescue this material from the well-known problem of rapid inward migration at an approximate metre-size scale and that in principle the collisional evolution of these objects could help to resupply small dust to the protostellar disc. We also point out the possible implications of this scenario for the location of planetesimal belts inferred in debris discs around main sequence stars, but stress that further dynamical studies are required in order to establish whether the disc retains a memory of the initial site of planetesimal creation.

Bolton, James S.; Becker, George D.

doi: 10.1111/j.1745-3933.2009.00700.xpmid: N/A

We use a large set of cosmological smoothed particle hydrodynamics (SPH) simulations to examine the effect of mass resolution and box size on synthetic Lyα forest spectra at 2 ≤z≤ 5. The mass resolution requirements for the convergence of the mean Lyα flux and flux power spectrum at z= 5 are significantly stricter than at lower redshift. This is because transmission in the high redshift Lyα forest is primarily due to underdense regions in the intergalactic medium (IGM), and these are less well resolved compared to the moderately overdense regions which dominate the Lyα forest opacity at z≃ 2–3. We further find that the gas density distribution in our simulations differs significantly from previous results in the literature at large overdensities (Δ > 10). We conclude that studies of the Lyα forest at z= 5 using SPH simulations require a gas particle mass of Mgas≤ 2 × 105h−1 M⊙, which is ≳8 times the value required at z= 2. A box size of at least 40 h−1 Mpc is preferable at all redshifts.

Soker, Noam

doi: 10.1111/j.1745-3933.2009.00704.xpmid: N/A

I propose a feedback model to explain the correlation between the supermassive black hole (SMBH) mass and the host galaxy bulge mass. The feedback is based on narrow jets that are launched by the central SMBH, and expel large amounts of mass to large distances. The condition is that the jets do not penetrate through the inflowing gas, such that they can deposit their energy in the inner region where the bulge is formed. For that to occur, the SMBH must move relative to the inflowing gas, such that the jets continuously encounter fresh gas. Taking into account the relative motion of the SMBH and the inflowing gas I derive a relation between the mass accreted by the SMBH and the mass that is not expelled, and is assumed to form the bulge. This relation is not linear, but rather the SMBH to bulge mass ratio increases slowly with mass. The same mechanism was applied to suppress star formation in cooling flow clusters, making a tighter connection between the feedback in galaxy formation and cooling flows.

Qi, Shi; Lu, Tan; Wang, Fa-Yin

doi: 10.1111/j.1745-3933.2009.00713.xpmid: N/A

In the work of Qi, Wang & Lu, gamma-ray bursts (GRBs) are used together with an earlier Type Ia supernova (SN Ia) data set to constrain the dark energy equation of state (EOS) in a nearly model-independent way. The improvements made by including GRBs show a slight shift of the dark energy EOS toward w > −1 at redshifts z≳ 0.5. It is interesting that, when we have more SNe Ia, SNe Ia themselves also show the same trend. Motivated by the fact that both SNe Ia and GRBs seem to prefer a dark energy EOS greater than −1 at redshifts z≳ 0.5, we perform a careful investigation of this situation, including more careful treatments of measurement errors of GRBs and cross-checking the results by using different ways of including GRBs. We find that the deviation of dark energy from the cosmological constant at redshifts z≳ 0.5 is large enough that we should pay close attention to it with future observational data. Such a deviation may arise from some biasing systematic errors in the handling of SNe Ia and/or GRBs, or more interestingly from the nature of the dark energy itself.

Lai, X. Y.; Xu, R. X.

doi: 10.1111/j.1745-3933.2009.00701.xpmid: N/A

Quark clustering could occur in cold quark matter because of the strong coupling between quarks at realistic baryon densities of compact stars. Although one may still not be able to calculate this conjectured matter from the first principles, the intercluster interaction might be analogized to the interaction between inert molecules. Cold quark matter would then crystallize in a solid state if the intercluster potential is deep enough to trap the clusters in the wells. We apply the Lennard-Jones potential to describe the intercluster potential and derive the equations of state, which are stiffer than those derived in conventional models (e.g. MIT bag model). If quark stars are composed of the Lennard-Jones matter, they could have high maximum masses (>2 M⊙) as well as very low masses (<10−3 M⊙). These features could be tested by observations.

Sobral, D.; Best, P. N.; Geach, J. E.; Smail, Ian; Kurk, J.; Cirasuolo, M.; Casali, M.; Ivison, R. J.; Coppin, K.; Dalton, G. B.

doi: 10.1111/j.1745-3933.2009.00712.xpmid: N/A

New results are presented, as part of the Hi-z Emission Line Survey (HizELS), from the largest area survey to date (1.4 deg2) for Lyα emitters (LAEs) at z∼ 9. The survey, which is primarily targeting Hα emitters at z < 3, uses the Wide Field CAMera on the United Kingdom Infrared Telescope and a custom narrow-band filter in the J band and reaches a Lyα luminosity limit of ∼1043.8 erg s−1 over a co-moving volume of 1.12 × 106 Mpc3 at z= 8.96 ± 0.06. Only two candidates were found out of 1517 line emitters and those were rejected as LAEs after follow-up observations. The limit on the space density of bright LAEs is improved by three orders of magnitude, consistent with suppression of the bright end of the Lyα luminosity function beyond z∼ 6. Combined with upper limits from smaller but deeper surveys, this rules out some of the most extreme models for high-redshift LAEs. The potential contamination of future narrow-band Lyα surveys at z > 7 by Galactic brown dwarf stars is also examined, leading to the conclusion that such contamination may well be significant for searches at 7.7 < z < 8.0, 9.1 < z < 9.5 and 11.7 < z < 12.2.

Jarvis, Matt J.; Teimourian, Hanifa; Simpson, Chris; Smith, Daniel J. B.; Rawlings, Steve; Bonfield, David

doi: 10.1111/j.1745-3933.2009.00715.xpmid: N/A

In this Letter, we report the discovery of a z= 4.88 radio galaxy discovered with a new technique which does not rely on pre-selection of a sample based on radio properties such as steep-spectral index or small angular size. This radio galaxy was discovered in the Elais-N2 field and has a spectral index of α= 0.75, i.e. not ultra-steep spectrum. It also has a luminosity consistent with being drawn from the break of the radio luminosity function and can therefore be considered as a typical radio galaxy. Using the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) data over this field, we find that the host galaxy is consistent with being similarly massive to the lower redshift powerful radio galaxies (∼1–3L★). However, we note that at z= 4.88, the Hα line is redshifted into the IRAC 3.6 μm filter, and some of the flux in this band may be due to this fact rather than the stellar continuum emission. The discovery of such a distant radio source from our initial spectroscopic observations demonstrates the promise of our survey for finding the most distant radio sources.

Stern, Boris E.; Tikhomirova, Yana Y.

doi: 10.1111/j.1745-3933.2008.00611.xpmid: N/A

There is a robust upper limit on the energy of synchrotron radiation in high-energy astrophysics: ∼mec2/α, where α= 1/137 is the fine structure constant and the value refers to the comoving frame of the fluid. This is the maximal energy of synchrotron photons which can be emitted by an electron having an arbitrarily high initial energy after it turns by angle ∼π in the magnetic field. This upper limit can be naturally reached if the converter mechanism contributes to the jet radiation and can be imprinted in spectra of some blazars as a cut-off or a dip in the GeV range. We use numerical simulations to probe the range of parameters of a radiating jet where the ultimate synchrotron cut-off appears. We reproduce the variety of spectra depending on the source luminosity and on the scale of the emission site. We also compare our results with the EGRET blazar spectra in order to illustrate that the agreement is possible but still not statistically significant. The predicted feature, if it exists, should be observed by Fermi in spectra of some blazars.