arXiv subjects

Phil Charles

Publications and source records attributed to Phil Charles.

24 records · Page 2Linked to original sources

arXiv2010

Interpretation of the 115 Day Periodic Modulation in the X-ray Flux of NGC 5408 X-1

We comment on the recent observation of a 115-day modulation in the X-ray flux of the ultraluminous X-ray source (ULX) NGC 5408 X-1, and in particular, the interpretation of this modulation as the orbital period. We suggest that this modulation may instead be due to a precessing jet, and is thus superorbital in nature. Comparing the properties of this ULX with those of the prototypical microquasar SS 433, we argue that NGC 5408 X-1 is very similar to SS 433: a hyper-accreting stellar mass black hole in a shorter-period binary. If the analogy holds, the 115-day modulation is best explained by the still poorly-understood physics of inner-disc/jet precession and a longer observing baseline would be able to reveal an intrinsic phase jitter that is associated with such a precession.

astro-ph.HE↗

arXiv2009

GX 9+9: Variability of the X-Ray Orbital Modulation

Results of observations of the Galactic bulge X-ray source GX 9+9 by the All-Sky Monitor (ASM) and Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer are presented. The ASM results show that the 4.19 hour X-ray periodicity first reported by Hertz and Wood in 1987 was weak or not detected for most of the mission prior to late 2004, but then became strong and remained strong for approximately 2 years after which it weakened considerably. When the modulation at the 4.19 hour period is strong, it appears in folded light curves as an intensity dip over slightly less than 30% of a cycle and is distinctly nonsinusoidal. A number of PCA observations of GX 9+9 were performed before the appearance of strong modulation; two were performed in 2006 during the epoch of strong modulation. Data obtained from the earlier PCA observations yield at best limited evidence of the presence of phase-dependent intensity changes, while the data from the later observations confirm the presence of flux minima with depths and phases compatible with those apparent in folded ASM light curves. Light curves from a Chandra observation of GX 9+9 performed in the year 2000 prior to the start of strong modulation show the possible presence of shallow dips at the predicted times. Optical observations performed in 2006 while the X-ray modulation was strong do not show an increase in the degree of modulation at the 4.19 hour period. Implications of the changes in modulation strength in X-rays and other observational results are considered.

astro-ph.HE↗

arXiv2008

An Overview of the Bowen Survey; detecting donor star signatures in Low Mass X-ray Binaries

In this paper we give a review of the Bowen fluorescence survey, showing that narrow emission lines (mainly NIII and CIII lines between 4630 and 4660 A) appear to be universally present in the Bowen blend of optically bright low mass X-ray binaries. These narrow lines are attributed to reprocessing in the companion star giving the first estimates of K_2, and thereby providing the first constraints on their system parameters. We will give an overview of the constraints on the masses of the compact objects and briefly highlight the most important results of the survey. Furthermore, we will point out the most promising systems for future follow-up studies and indicate how we think their estimates of the component masses can be improved.

astro-ph↗

arXiv2005

Fundamental Parameters of Low Mass X-ray Binaries II: X-Ray Persistent Systems

The determination of fundamental parameters in X-ray luminous (persistent) X-ray binaries has been classically hampered by the large optical luminosity of the accretion disc. New methods, based on irradiation of the donor star and burst oscillations, provide the opportunity to derive dynamical information and mass constraints in many persistent systems for the first time. These techniques are here reviewed and the latest results presented.

astro-ph↗

arXiv1999

WZ Sagittae as a DQ Herculis star

We argue that quiescent WZ Sge is a rapidly spinning magnetic rotator in which most of the matter transfered from the secondary is ejected from the system. Assuming that the observed 27.87 s oscillation period is due to the spinning white dwarf we propose that the other observed principal period of 28.96 s is a beat due to reprocessing of the rotating white dwarf beam on plasma blobs in Keplerian rotation at the outer disc rim. The weaker, transient, 29.69 s period is identified as a beat with the Keplerian period of the magnetosphere. WZ Sge evolves through a cycle of spin-up and spin-down phases. During the spin-down phase it is a DQ Her star, during the spin-up phase it should be a ER UMa star.

astro-ph↗

arXiv1997

The mass of the neutron star in Cyg X-2 (V1341 Cyg)

Cygnus X-2 is one of the brightest and longest known X-ray sources. We present high resolution optical spectroscopy of Cyg X-2 obtained over 4 years which gives an improved mass function of 0.69 +/- 0.03 Msun (1 sigma error). In addition, we resolve the rotationally broadened absorption features of the secondary star for the first time, deriving a rotation speed of vsin(i) = 34.2 +/- 2.5 km per s (1 sigma error) which leads to a mass ratio of q = M_c/M_x = 0.34 +/- 0.04 (1 sigma error), assuming a tidally-locked and Roche lobe-filling secondary). Hence with the lack of X-ray eclipses (i.e. i <~ 73 degrees) we can set firm 95% confidence lower limits to the neutron star mass of M_x > 1.27 Msun and to the companion star mass of M_c > 0.39 Msun. However, by additionally requiring that the companion must exceed 0.75 Msun (as required theoretically to produce a steady low-mass X-ray binary), then M_x > 1.88 Msun and i < 61 degrees (95% confidence lower and upper limit, respectively), thereby making Cyg X-2 the highest mass neutron star measured to date. If confirmed this would set significant constraints on the equation of state of nuclear matter.

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