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Aql X-1 transition towards the soft (banana) state accompanied by radio/NIR detection

ATel #2302; G. R. Sivakoff (University of Virginia), J. Miller-Jones (NRAO), O. Fox (University of Virginia), M. Linares (MIT-MKI, Amsterdam), D. Altamirano (Amsterdam), D. Russell (Amsterdam), and the JACPOT XRB collaboration
on 18 Nov 2009; 04:01 UT
Credential Certification: Gregory R Sivakoff (grs8g@virginia.edu)

Subjects: Radio, Infra-Red, X-ray, Request for Observations, Binary, Neutron Star, Transient, Variables

Referred to by ATel #: 2303, 2317

The currently active neutron star transient and atoll source Aql X-1 (Linares et al., ATEL #2288) has begun the transition from the hard (extreme island) state to the soft (banana) state (Rodriguez et al. ATEL #2299). This transition likely began around 2009 Nov 15 (MJD = 55150). The latest RXTE PCA observation (2009 Nov 17, MJD=55152.17+/-0.02, 2-60 keV fractional rms variability amplitude of ~11% for 0.1-10 Hz) indicates that the source is in the intermediate (island) state.

VLA observations taken on 2009 Nov 17 (MJD=55152.072 +/- 0.009) at 8.4 GHz, with the array in its D configuration detected radio emission from Aql X-1 at 0.62 +/- 0.05 mJy. The uncertainty on the absolute flux density scale is of order 5%. This appears to be the brightest recorded flux density for Aql X-1. Prior to this detection, VLA observations set 3-sigma upper limits of 0.32 mJy on 2009 Nov 14 (55149.0796 +/- 0.009) and 0.13 mJy prior to 2009 Nov 11 (MJD < 55146.1). The source is consistent with being unresolved by the Nov 17 observation, which has a beamsize of 15.7 x 8.2 arcseconds, oriented 42 degrees E of N. VLA and VLBA observations are ongoing.

NIR observations with FanCam and PAIRITEL are ongoing. In the table below, we report on FanCam observations over the period of 2009 Nov 16 - Nov 17. Marginal evidence is seen for an increase in the H and Ks bands between the two days, which could be consistent with a rise in the flux from a jet in Aql X-1. The NIR magnitudes are ~0.15, 0.05-0.15, and 0.20-0.30 magnitudes brighter, respectively in J/H/Ks, than the magnitudes 2MASS observed (1999 Aug 11) in the middle of the extended 1999 outburst. Errors listed below do not include a systematic error of 0.07 mag (7.6%):

Date MJD J (mag) H (mag) Ks (mag) J (mJy) H (mJy) Ks (mJy)
2009 Nov 16 55151.01 +/- 0.03 15.64 +/- 0.03 15.25 +/- 0.02 14.85 +/- 0.04 0.93 +/- 0.03 0.78 +/- 0.02 0.71 +/- 0.03
2009 Nov 17 55152.00 +/- 0.04 15.66 +/- 0.03 15.15 +/- 0.02 14.76 +/- 0.04 0.91 +/- 0.03 0.85 +/- 0.02 0.77 +/- 0.02

Further observations at all wavelengths are encouraged.

We thank the VLA, VLBA, RXTE, FanCam, and PAIRITEL teams for scheduling the observations. FanCam is operated by the University of Virginia and was made possible by a grant from the NSF Major Research Instrumentation Program. The Peters Automated Infrared Imaging Telescope (PAIRITEL) is operated by the Smithsonian Astrophysical Observatory (SAO) and was made possible by a grant from the Harvard University Milton Fund, the camera loan from the University of Virginia, and the continued support of the SAO, UC Berkeley, and NASA/Swift Guest Investigator Program.

JACPOT XRB Collaboration