A 0535+26: Back in business

In May/June 2005, after 10 years of inactivity, the Be/X-ray binary system A 0535+26 underwent a major X-ray outburst. In this paper data are presented from 10 years of optical, IR and X-ray monitoring showing the behaviour of the system during the quiescent epoch and the lead up to the new outburst. The results show the system going through a period when the Be star in the system had a minimal circumstellar disk and then a dramatic disk recovery leading, presumably, to the latest flare up of X-ray emission. The data are interpreted in terms of the state of the disk and its interaction with the neutron star companion.Comment: Accepted for publication in MNRA

Turbulent plane Couette flow at moderately high Reynolds number

A new set of numerical simulations of turbulent plane Couette flow in a large box of dimension (20 pi h; 2h; 6 pi h /at Reynolds number. (Re-tau) = 125, 180, 250 and 550 is described and compared with simulations at lower Reynolds numbers, Poiseuille flows and experiments. The simulations present a logarithmic near-wall layer and are used to verify and revise previously known results. It is confirmed that the fluctuation intensities in the streamwise and spanwise directions do not scale well in wall units. The scaling failure occurs both near to and away from the wall. On the contrary, the wall-normal intensity scales in inner units in the near-wall region and in outer units in the core region. The spectral ridge found by Hoyas & Jimenez (Phys. Fluids, vol. 18, 2003, 011702) for the turbulent Poiseuille flow can also be seen in the present flow. Away from the wall, very large-scale motions are found spanning through all the length of the channel. The statistics of these simulations can be downloaded from the webpage of the Chair of Fluid Dynamics.This work was partially supported by the German Science Foundation (DFG) under the grant number KH 257/2-1 (2010). The computations of the new simulations were made possible by a generous grant of computer time from the FUCHS cluster at the University of Frankfurt-am-Main, the SuperMUC Petascale System at Leibniz Supercomputing Centre (LRZ) and the supercomputation center of the Universitat Politecnica de Valencia. We are grateful to Messrs Tsukuhara, Kawamura, Shingai, Bernardini, Pirozzoli and Orlandi for providing us with copies of their original data and to Mr P. Raga for his help in the preparation of figure 7. S. H. is indebted to J. Jimenez.Avsarkisov, V.; Hoyas Calvo, S.; Oberlack, M.; García Galache, JP. (2014). Turbulent plane Couette flow at moderately high Reynolds number. Journal of Fluid Mechanics. 751:1-10. https://doi.org/10.1017/jfm.2014.323S110751Tillmark, N. 1995 Experiments on transition and turbulence in plane Couette flow. PhD thesis, KTH, Royal Institute of Technology.Moser, R. D., Kim, J., & Mansour, N. N. (1999). Direct numerical simulation of turbulent channel flow up to Reτ=590. Physics of Fluids, 11(4), 943-945. doi:10.1063/1.869966KITOH, O., NAKABYASHI, K., & NISHIMURA, F. (2005). Experimental study on mean velocity and turbulence characteristics of plane Couette flow: low-Reynolds-number effects and large longitudinal vortical structure. Journal of Fluid Mechanics, 539(-1), 199. doi:10.1017/s0022112005005641Hoyas, S., & Jiménez, J. (2008). Reynolds number effects on the Reynolds-stress budgets in turbulent channels. Physics of Fluids, 20(10), 101511. doi:10.1063/1.3005862Tsukahara, T., Kawamura, H., & Shingai, K. (2006). DNS of turbulent Couette flow with emphasis on the large-scale structure in the core region. Journal of Turbulence, 7, N19. doi:10.1080/14685240600609866Busse, F. H. (1970). Bounds for turbulent shear flow. Journal of Fluid Mechanics, 41(1), 219-240. doi:10.1017/s0022112070000599Hoyas, S., & Jiménez, J. (2006). Scaling of the velocity fluctuations in turbulent channels up to Reτ=2003. Physics of Fluids, 18(1), 011702. doi:10.1063/1.2162185Mansour, N. N., Kim, J., & Moin, P. (1988). Reynolds-stress and dissipation-rate budgets in a turbulent channel flow. Journal of Fluid Mechanics, 194(-1), 15. doi:10.1017/s0022112088002885Lund, K. O., & Bush, W. B. (1980). Asymptotic analysis of plane turbulent Couette-Poiseuille flows. Journal of Fluid Mechanics, 96(01), 81. doi:10.1017/s0022112080002030JIMÉNEZ, J., & HOYAS, S. (2008). Turbulent fluctuations above the buffer layer of wall-bounded flows. Journal of Fluid Mechanics, 611, 215-236. doi:10.1017/s0022112008002747Lele, S. K. (1992). Compact finite difference schemes with spectral-like resolution. Journal of Computational Physics, 103(1), 16-42. doi:10.1016/0021-9991(92)90324-rJiménez, J. (2013). Near-wall turbulence. Physics of Fluids, 25(10), 101302. doi:10.1063/1.4824988Hamilton, J. M., Kim, J., & Waleffe, F. (1995). Regeneration mechanisms of near-wall turbulence structures. Journal of Fluid Mechanics, 287, 317-348. doi:10.1017/s0022112095000978Bernardini, M., Pirozzoli, S., & Orlandi, P. (2013). The effect of large-scale turbulent structures on particle dispersion in wall-bounded flows. International Journal of Multiphase Flow, 51, 55-64. doi:10.1016/j.ijmultiphaseflow.2012.11.007PIROZZOLI, S., BERNARDINI, M., & ORLANDI, P. (2011). Large-scale motions and inner/outer layer interactions in turbulent Couette–Poiseuille flows. Journal of Fluid Mechanics, 680, 534-563. doi:10.1017/jfm.2011.186Komminaho, J., Lundbladh, A., & Johansson, A. V. (1996). Very large structures in plane turbulent Couette flow. Journal of Fluid Mechanics, 320(-1), 259. doi:10.1017/s0022112096007537Melnikov, K., Kreilos, T., & Eckhardt, B. (2014). Long-wavelength instability of coherent structures in plane Couette flow. Physical Review E, 89(4). doi:10.1103/physreve.89.043008Spalart, P. R., Moser, R. D., & Rogers, M. M. (1991). Spectral methods for the Navier-Stokes equations with one infinite and two periodic directions. Journal of Computational Physics, 96(2), 297-324. doi:10.1016/0021-9991(91)90238-gKitoh, O., & Umeki, M. (2008). Experimental study on large-scale streak structure in the core region of turbulent plane Couette flow. Physics of Fluids, 20(2), 025107. doi:10.1063/1.2844476Bech, K. H., Tillmark, N., Alfredsson, P. H., & Andersson, H. I. (1995). An investigation of turbulent plane Couette flow at low Reynolds numbers. Journal of Fluid Mechanics, 286, 291-325. doi:10.1017/s0022112095000747Del Álamo, J. C., & Jiménez, J. (2003). Spectra of the very large anisotropic scales in turbulent channels. Physics of Fluids, 15(6), L41. doi:10.1063/1.157083

Optical Follow-up of New SMC Wing Be/X-ray Binaries

We investigate the optical counterparts of recently discovered Be/X-ray binaries in the Small Magellanic Cloud. In total four sources, SXP101, SXP700, SXP348 and SXP65.8 were detected during the Chandra Survey of the Wing of the SMC. SXP700 and SXP65.8 were previously unknown. Many optical ground based telescopes have been utilised in the optical follow-up, providing coverage in both the red and blue bands. This has led to the classification of all of the counterparts as Be stars and confirms that three lie within the Galactic spectral distribution of known Be/X-ray binaries. SXP101 lies outside this distribution becoming the latest spectral type known. Monitoring of the Halpha emission line suggests that all the sources bar SXP700 have highly variable circumstellar disks, possibly a result of their comparatively short orbital periods. Phase resolved X-ray spectroscopy has also been performed on SXP65.8, revealing that the emission is indeed harder during the passage of the X-ray beam through the line of sight.Comment: 9 pages, 9 figures, 2 tables, accepted for publication in MNRA

INTEGRAL deep observations of the Small Magellanic Cloud

Deep observations of the Small Magellanic Cloud (SMC) and region were carried out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field of view of the instrument permitted simultaneous coverage of the entire SMC and the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven sources in the SMC and five in the Magellanic Bridge; the majority of the sources were previously unknown systems. Several of the new sources were detected undergoing bright X- ray outbursts and all the sources exhibited transient behaviour except the supergiant system SMC X-1. They are all thought to be High Mass X-ray Binary (HMXB) systems in which the compact object is a neutron star.Comment: 7 pages, 10 figures Accepted for publication in MNRA

The binary period and outburst behaviour of the SMC X-ray binary pulsar system SXP504

A probable binary period has been detected in the optical counterpart to the X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504 in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul 2002 and subsequently observed by XMM-Newton on 18 Dec 2003. The source is coincident with an Optical Gravitational Lensing (OGLE) object in the lightcurves of which several optical outburst peaks are visible at ~ 268 day intervals. Timing analysis shows a period of 268.6 +/- 0.1 days at > 99% significance. Archival Rossi X-ray Timing Explorer (RXTE) data for the 504s pulse-period has revealed detections which correspond closely with predicted or actual peaks in the optical data. The relationship between this orbital period and the pulse period of 504s is within the normal variance found in the Corbet diagram.Comment: Accepted by MNRAS. 1 LATEX page. 4 figure

Study of the cyclotron feature in MXB 0656-072

We have monitored a type II outburst of the Be/X-ray binary MXB 0656−072 in a series of pointed RXTE observations during October through December 2003. The source spectrum shows a cyclotron resonance scattering feature at 32.8 +0.5 −0.4 keV, corresponding to a magnetic field strength of 3.67 +0.06 −0.04 × 10 12 G and is stable through the outburst and over the pulsar spin phase. The pulsar, with an average pulse period of 160.4 ± 0.4s,shows a spin-up of 0.45 s over the duration of the outburst. From optical data, the source distance is estimated to be 3.9 ± 0.1 kpc and this is used to estimate the X-ray luminosity and a theoretical prediction of the pulsar spin-up during the outburst

Now you see it, now you don't - the circumstellar disk in the GRO J1008--57 system

Multiwavelength observations are reported here of the Be/X-ray binary pulsar system GRO J1008-57. Over ten years worth of data are gathered together to show that the periodic X-ray outbursts are dependant on both the binary motion and the size of the circumstellar disk. In the first instance an accurate orbital solution is determined from pulse periods, and in the second case the strength and shape of the Halpha emission line is shown to be a valuable indicator of disk size and its behaviour. Furthermore, the shape of the emission line permits a direct determination of the disk size which is in good agreement with theoretical estimates. A detailed study of the pulse period variations during outbursts determined the binary period to be 247.8, in good agreement with the period determined from the recurrence of the outbursts.Comment: Accepted for publication in MNRA