EVIDENCE FOR QUASI-PERIODIC X-RAY DIPS FROM AN ULTRALUMINOUS X-RAY SOURCE: IMPLICATIONS FOR THE BINARY MOTION

Journal Article · Sun Feb 10 04:00:00 EST 2013 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/764/1/93· OSTI ID:22167785

Pasham, Dheeraj R. ^[1]

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1. Astronomy Department, University of Maryland, College Park, MD 20742 (United States)

We report results from long-term ( Almost-Equal-To 1240 days) X-ray (0.3-8.0 keV) monitoring of the ultraluminous X-ray source NGC 5408 X-1 with the Swift/X-Ray Telescope. Here we expand on earlier work by Strohmayer (2009) who used only a part of the present data set. Our primary results are: (1) the discovery of sharp, quasi-periodic, energy-independent dips in the X-ray intensity that recur on average every 243 days, (2) the detection of an energy dependent (variability amplitude decreases with increasing energy), quasi-sinusoidal X-ray modulation with a period of 112.6 {+-} 4 days, the amplitude of which weakens during the second half of the light curve, and (3) spectral evidence for an increase in photoelectric absorption during the last continuous segment of the data. We interpret the X-ray modulations within the context of binary motion in analogy to that seen in high-inclination accreting X-ray binaries. If correct, this implies that NGC 5408 X-1 is in a binary with an orbital period of 243 {+-} 23 days, in contrast to the 115.5 day quasi-sinusoidal period previously reported by Strohmayer (2009). We discuss the overall X-ray modulation within the framework of accretion via Roche-lobe overflow of the donor star. In addition, if the X-ray modulation is caused by vertically structured obscuring material in the accretion disk, this would imply a high value for the inclination of the orbit. A comparison with estimates from accreting X-ray binaries suggests an inclination {approx}>70 Degree-Sign . We note that, in principle, a precessing accretion disk could also produce the observed X-ray modulations.

• OSTI ID: 22167785
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 764; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ACCRETION DISKS
AMPLITUDES
COMPARATIVE EVALUATIONS
DATA ANALYSIS
ENERGY DEPENDENCE
INCLINATION
KEV RANGE 01-10
MODULATION
PERIODICITY
ROCHE EQUIPOTENTIALS
STARS
TELESCOPES
X RADIATION
X-RAY SOURCES