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Aql X-1 back in outburst: multi-wavelength observations

ATel #2288; M. Linares (MIT-MKI, Amsterdam), J. Miller-Jones (NRAO) , D. Altamirano (Amsterdam), G. Sivakoff (University of Virginia), D. Maitra (University of Michigan), D. Russell (Amsterdam), F. Lewis (Open University, University of Glamorgan), C. Markwardt (U. Maryland & NASA/GSFC), R. Remillard (MIT) and the JACPOT XRB collaboration.
on 10 Nov 2009; 17:51 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Diego Altamirano (diego@science.uva.nl)

Subjects: Request for Observations, Binary, Neutron Star, Transient

Referred to by ATel #: 2299, 2302, 2317, 2871

On November 1st. RXTE-PCA scans of the Aql region detected X-ray emission consistent with the neutron star transient and atoll source Aql X-1. Further scans and pointed PCA observations show a rising X-ray flux; the most recent flux is about 40 mCrab.

Swift observed the field for about 2.5 ksec on November 8th, 2009, and detected a bright X-ray source at a position consistent with Aql X-1. The XRT (wt mode) spectrum is well fitted by an absorbed blackbody plus power law model (nH~0.5E22 cm^{-2}; kTbb=0.9+/-0.1 keV; p.l. index 2.1+/-0.1), giving an 0.5--10 keV unabsorbed flux of (3.6+/-0.2)E-10 erg/cm2/s, which corresponds to a luminosity of 1.2E36 erg/s for a distance of 5.2 kpc (Jonker & Nelemans 2004).

The Ultra-Violet Optical Telescope (UVOT) on board Swift also took two long integrations in the UVW1 filter. From the combined 2.5 ksec exposure we detect Aql X-1 at 8.8 sigma significance; its UVW1 magnitude was 20.05 +/- 0.17 (using a 3 arcsec aperture centered on the position of Aql X-1). This is > 2 magnitudes above the quiescent flux level; from archival UVOT observations we measure UVW1 mag > 21.90 on 2007-07-26 and > 22.05 on 2007-07-30 (at 5 sigma level).

Radio observations are ongoing. We are monitoring the source at an observing frequency of 8.4 GHz with the VLA, under program code AM991. The array is in its most compact D configuration. To date, the source has not been detected, with the derived upper limits being:
Date MJD 3-sigma upper limit (mJy)
2009 Nov. 5 55140.12 0.20
2009 Nov. 6 55141.19 0.21
2009 Nov. 7 55142.99 0.12
2009 Nov. 9 55144.97 0.13


Prior to the reported X-ray outburst, Faulkes Telescope monitoring of Aql X-1 in optical V and i'-band (see, e.g. ATel #1970) did not detect any new outburst; the last observation was on 23rd October, 9 days before the first RXTE-PCA scan detection. Up until 23rd October the measured magnitudes of V ~ 19.1 - 19.5 and i' ~ 17.7 - 17.9 are consistent with the quiescent levels.

Further observations at all wavelengths are encouraged. We thank the Swift, RXTE and VLA teams for scheduling the observations. The Faulkes Telescope Project is an educational and research arm of the Las Cumbres Observatory Global Telescope Network (LCOGTN).

PCA Bulge scan light curve can be seen here:

http://lheawww.gsfc.nasa.gov/users/craigm/galscan/html/Aql_X-1.html