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Possible 6.6-mHz modulation in IGR J18245-2452

ATel #4934; S. Campana (INAF-OAB), P. Esposito (INAF-IASF Mi), G. L. Israel (INAF-OAR), P. Romano (INAF-IASF Pa)
on 1 Apr 2013; 21:48 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)

Subjects: X-ray, Globular Cluster, Transient

Referred to by ATel #: 4960, 4961, 4981, 5031, 5068, 5069, 5086

The source IGR J18245-2452 is a recently discovered transient (Eckert et al., ATel #4925), with a position consistent with the core of the globular cluster M28 (Heinke et al., ATel #4927). It repeatedly triggered the Swift/BAT on 2013-03-30 (Romano et al., ATel #4929). We carried out a search for timing signatures in the emission of IGR J18245-2452 using the Swift/XRT data collected in Windowed Timing (WT) mode (the exposures in Photon Counting mode are shorter and the PC data are heavily affected by pile up). In the first observation, obs.ID 00552336000 (start date: 2013-03-30 02:25:40, end date: 2013-03-30 02:58:17; net exposure time: 1.35 ks), we found a possible modulation at a frequency of approximately 6.6 mHz (~150 s). Both the folded profile and the central frequency of the modulation are somewhat variable across the observation (elapsed time: ~1.9 ks). This variability may indicate a quasi-periodic signal and/or orbital Doppler shifts. Further observations are needed to confirm this signal (and assess its nature).