Viscous timescale in high mass X-ray binaries

Context: Low mass X-ray binaries were found to have very low frequency breaks in their power density spectra below which the power density spectra are nearly in white noise structure and at higher frequencies they approximately follow the $P_\nu \propto \nu^{-1.3}$ law. Aims: In 2005, Gilfanov and Arefiev studied X-ray variability of persistent LMXBs in the $10^{-8}-10^{-1}$ Hz frequency range and To determine whether high mass X-ray binary power density spectra have similar properties and the findings for low mass X-ray binaries are also valid for high mass binaries, we analyzed the time series of high mass X-ray binary sources produced by All Sky Monitor of Rossi X-ray Timing Explorer. Method: We obtained the power density spectra of the high mass X-ray binaries using the cosine transform of autocorrelation function. Results: We identified break frequencies for seven sources, namely OAO 1657-415, SS 433, Vela X-1, SMC X-1, 4U 1700-377, GX 301-2, and LMC X-1. The normalized break frequencies with respect to the orbital frequency ($f_{break}/f_{orbit}$) for sources OAO 1657-415, SS 433, SMC X-1 and LMC X-1 are consistent with those of Roche lobe overflow systems. The other high mass X-ray binary systems, Vela X-1, GX 301-2, and 4U 1700-377, however, have larger break frequency ratios, $f_{break}/f_{orb}$, which are indicative of short viscous times. These are all wind-accreting sources and the stellar winds in the systems allow the formation of only short radius discs. Consequently, we qualitatively distinguished the Roche lobe overflow binaries from the wind accreting system by comparing their normalized break frequencies.Comment: 9 pages, 5 figures, accepted by A&

An Anti-Glitch in a Magnetar

Magnetars are neutron stars showing dramatic X-ray and soft $\gamma$-ray outbursting behaviour that is thought to be powered by intense internal magnetic fields. Like conventional young neutron stars in the form of radio pulsars, magnetars exhibit "glitches" during which angular momentum is believed to be transferred between the solid outer crust and the superfluid component of the inner crust. Hitherto, the several hundred observed glitches in radio pulsars and magnetars have involved a sudden spin-up of the star, due presumably to the interior superfluid rotating faster than the crust. Here we report on X-ray timing observations of the magnetar 1E 2259+586 which we show exhibited a clear "anti-glitch" -- a sudden spin down. We show that this event, like some previous magnetar spin-up glitches, was accompanied by multiple X-ray radiative changes and a significant spin-down rate change. This event, if of origin internal to the star, is unpredicted in models of neutron star spin-down and is suggestive of differential rotation in the neutron star, further supporting the need for a rethinking of glitch theory for all neutron stars

Observations of the magnetars 4U 0142+61 and 1E 2259+586 with the MAGIC telescopes (Research Note)

Context. Magnetars are an extreme, highly magnetized class of isolated neutron stars whose large X-ray luminosity is believed to be driven by their high magnetic field. Aims. We study for the first time the possible very high energy gamma-ray emission above 100 GeV from magnetars, observing the sources 4U 0142+ 61 and 1E 2259+586. Methods. We observed the two sources with atmospheric Cherenkov telescopes in the very high energy range (E > 100 GeV). 4U0142+61 was observed with the MAGIC I telescope in 2008 for about 25 h and 1E 2259+586 was observed with the MAGIC stereoscopic system in 2010 for about 14 h. The data were analyzed with the standard MAGIC analysis software. Results. Neither magnetar was detected. Upper limits to the differential and integral flux above 200 GeV were computed using the Rolke algorithm. We obtain integral upper limits to the flux of 1.52 x 10(-12) cm(-2) s(-1) and 2.7 x 10(-12) cm(-2) s(-1) with a confidence level of 95% for 4U 0142+ 61 and 1E 2259+586, respectively. The resulting differential upper limits are presented together with X-ray data and upper limits in the GeV energy range