Browsing by Author "Gendreau, K. C."

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    NICER Observes the Effects of an X-Ray Burst on the Accretion Environment in Aql X-1
    (2018) Keek, L.; Arzoumanian, Z.; Bult, P.; Cackett, E. M.; Chakrabarty, D.; Chenevez, J.; Fabian, A. C.; Gendreau, K. C.; Guillot, S.; Guver, T.; Homan, J.; Jaisawal, G. K.; Lamb, F. K.; Ludlam, R. M.; Mahmoodifar, S.; Markwardt, C. B.; Miller, J. M.; Prigozhin, G.; Soong, Y.; Strohmayer, T. E.; Wolff, M. T.
    Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type-I X-ray bursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impact on the disk during one of the first X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hard spectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits an excess of soft X-ray photons below 1 keV, where NICER's sensitivity peaks. We interpret the excess as a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burst persistent flux, possibly due to Poynting-Robertson drag or coronal reprocessing. This is the first such detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER will be able to study how X-ray bursts affect the disk and corona for a range of accreting neutron star systems and disk states.
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    Joint ALMA/X-ray monitoring of the radio-quiet type 1 active galactic nucleus IC 4329A
    (2024) Shablovinskaya, E.; Ricci, C.; Chang, C. -s.; Tortosa, A.; del Palacio, S.; Kawamuro, T.; Aalto, S.; Arzoumanian, Z.; Balokovic, M.; Bauer, F. E.; Gendreau, K. C.; Ho, L. C.; Kakkad, D.; Kara, E.; Koss, M. J.; Liu, T.; Loewenstein, M.; Mushotzky, R.; Paltani, S.; Privon, G. C.; Smith, K.; Tombesi, F.; Trakhtenbrot, B.
    The origin of a compact millimeter (mm, 100-250 GHz) emission in radio-quiet active galactic nuclei (RQ AGN) remains debated. Recent studies propose a connection with self-absorbed synchrotron emission from the accretion disk X-ray corona. We present the first joint ALMA (similar to 100 GHz) and X-ray (NICER/XMM-Newton/Swift; 2-10 keV) observations of the unobscured RQ AGN, IC 4329A (z = 0.016). The time-averaged mm-to-X-ray flux ratio aligns with recently established trends for larger samples, but with a tighter scatter (similar to 0.1 dex) compared to previous studies. However, there is no significant correlation on timescales of less than 20 days. The compact mm emission exhibits a spectral index of -0.23 +/- 0.18, remains unresolved with a 13 pc upper limit, and shows no jet signatures. Notably, the mm flux density varies significantly (by factor of 3) within four days, exceeding the contemporaneous X-ray variability and showing the largest mm variations ever detected in RQ AGN over daily timescales. The high amplitude variability rules out scenarios of heated dust and thermal free-free emission, pointing toward a synchrotron origin for the mm radiation in a source of similar to 1 light day (similar to 120 gravitational radii) size. While the exact source is not yet certain, an X-ray corona scenario emerges as the most plausible compared to a scaled-down jet or outflow-driven shocks.