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X-ray, UV, Optical and NIR Observations of Aql X-1

ATel #2744; Dipankar Maitra, Mark Reynolds, Jon M. Miller, and Kayhan Gultekin (Univ. of Michigan)
on 18 Jul 2010; 07:50 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dipankar Maitra (maitra@astro.yale.edu)

Subjects: Infra-Red, Optical, Ultra-Violet, X-ray, Binary, Neutron Star, Transient

Referred to by ATel #: 2748, 2850, 2871, 5129

Aql X-1 was observed using Swift between 2010 July 16.8965 and 2010 July 16.9083. The source was recently found to start a new outburst as inferred from Swift/BAT and MAXI/GSC monitoring data (ATEL #2742).

Preliminary analyses of the XRT (WT mode) "quicklook" data suggests that the 0.6-10 keV spectrum can be well modeled (chi^2/dof=565.2/400) by a phenomenological absorbed power law model with column density of (3.4 +/- 0.02) E21 per cm^2 and photon index 1.65 +/- 0.05. The unabsorbed flux in this energy range is (1.05 +/- 0.02) E-9 erg/s/cm^2 which translates to an isotropic luminosity of ~3E36 (d/5 kpc)^2 erg/s or about 2 percent of the Eddington luminosity for a 1.4 Solar mass neutron star at a distance of 5 kpc. We note that the fit-derived column density is consistent with the Dickey and Lockman (1990) value in the direction of Aql X-1. The average count-rate over the XRT bandpass was 16.2 counts/s and the r.m.s. variability in the one-second binned XRT light curve is 11.6 +/- 1.5 percent. No thermonuclear bursts were seen during this pointed Swift observation. The relatively hard photon index, absence of any thermal component, and high r.m.s. variability suggests that the source was in "hard" state during the Swift observation.

The source was also unambiguously detected in the UVOT U-band filter at 18.59 +/- 0.22 mag (>5 sigma detection). Due to high extinction in the UV, the detections at bluer bands are marginal: 20.15 +/- 0.62 mag in the UW1 filter (2 sigma) and 20.78 +/- 0.78 mag in the UW2 filter (1.4 sigma). It was not detected in the UVM2 filter.

Aql X-1 is regularly observed in R- and J-bands simultaneously using the SMARTS 1.3m telescope, as part of our long-term XRB monitoring campaign. As of the latest observations using the CTIO/SMARTS 1.3m telescope on July 16, 6.8h UTC (mid-exposure), the source was at R=17.65 +/- 0.05 mag and J=16.16 +/- 0.1 mag, compared to mean quiescent values of R=18.8 +/- 0.09 mag and J=16.7 +/- 0.10 mag. All magnitudes quoted above include the unrelated interloper 0.5" away (Chevalier et al. 1999; Maitra and Bailyn 2008) of magnitude V=19.42 +/- 0.06; i'=18.02 +/- 0.06 mag.

It is a pleasure to thank the Neil Gehrels and the Swift planning team for their prompt scheduling of our target of opportunity request.