Time lags of the kilohertz quasi-periodic oscillations in the low-mass X-ray binaries 4U 1608−52 and 4U 1636−53

We studied the energy and frequency dependence of the Fourier time lags and intrinsic coherence of the kilohertz quasi-periodic oscillations (kHz QPOs) in the neutron-star lowmass X-ray binaries 4U 1608−52 and 4U 1636−53, using a large data set obtained with the Rossi X-ray Timing Explorer. We confirmed that, in both sources, the time lags of the lower kHz QPO are soft and their magnitude increases with energy. We also found that: (i) In 4U 1636−53, the soft lags of the lower kHz QPO remain constant at∼30 μs in the QPO frequency range 500–850 Hz, and decrease to ∼10 μs when the QPO frequency increases further. In 4U 1608−52, the soft lags of the lower kHz QPO remain constant at 40 μs up to 800 Hz, the highest frequency reached by this QPO in our data. (ii) In both sources, the time lags of the upper kHz QPO are hard, independent of energy or frequency and inconsistent with the soft lags of the lower kHz QPO. (iii) In both sources the intrinsic coherence of the lower kHz QPO remains constant at ∼0.6 between 5 and 12 keV, and drops to zero above that energy. The intrinsic coherence of the upper kHz QPO is consistent with being zero across the full energy range. (iv) In 4U 1636−53, the intrinsic coherence of the lower kHz QPO increases from ∼0 at ∼600 Hz to ∼1, and it decreases to ∼0.5 at 920 Hz; in 4U 1608−52, the intrinsic coherence is consistent with the same trend. (v) In both sources the intrinsic coherence of the upper kHz QPO is consistent with zero over the full frequency range of the QPO, except in 4U 1636−53 between 700 and 900 Hz where the intrinsic coherence marginally increases. We discuss our results in the context of scenarios in which the soft lags are either due to reflection off the accretion disc or up-/down-scattering in a hot medium close to the neutron star. We finally explore the connection between, on one hand the time lags and the intrinsic coherence of the kHz QPOs, and on the other the QPOs’ amplitude and quality factor in these two sources.

Charles P. Daly's Gendered Geography, 1860-1890

The American Geographical Society (AGS) serves as a case study for considering the nature of “gendered geography” in the nineteenth-century United States. This article links the ideals and programmatic interests of the society—which were fundamentally commercial in nature—with the personal subjectivity of its chief protagonist, Charles P. Daly, AGS president from 1864 until his death in 1899. Daly is presented as an “armchair explorer” who shifted the focus of the society away from statistical representations of the world toward the action packed narrative descriptions of the world supplied by embodied explorers in the field. The gender dynamics associated with the center versus the field provide a useful way to contrast both sides of Daly’s persona—as a scholar performing detached, careful study yet someone who also derived a great deal of personal authority by staging popular and dramatic spectacles in New York City, speechifying and presenting himself on stage at geographical society meetings with returning heroic explorers. Daly not only served as New York’ smost influential access point to the Arctic at the time, he also served as an important node in the reproduction of masculine culture in promotion of a particularly masculinist commercial geography. Key Words: American Geographical Society, Charles Patrick Daly, gender and geography, history of geography, masculinity.