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Unusual X-ray Activity from LS I +61 303 on 2008 August 21

ATEL # 1730; Paul S. Ray (NRL), Jacob M. Hartman (NRL, NRC) for a larger collaboration
on 23 Sep 2008; 14:05 UT
Password Certification: Paul S. Ray (paul.ray@nrl.navy.mil)

Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Soft Gamma-ray Repeaters, Transients

We have been monitoring LS I +61 303 with the PCA on RXTE, making 1.5 ks observations about every three days since 2008 February. We report on a search of our data for unusual behavior that may be related to the recently reported SGR-like outburst seen by the Swift BAT from a direction consistent with LS I +61 303 (see ATel #1715 for a summary) on 2008 September 10 at 12:52:21 UT.

The RXTE observation closest to the Swift burst began on 2008 September 10 at 18:17 UT, about 6 hours after the burst. The flux and spectrum observed were completely consistent with our normal monitoring observations with no unusual flaring or bursting activity apparent.

However, inspection of all of our monitoring observations revealed extremely unusual activity in one observation that began on 2008 August 21 at 15:23 UT (ObsId 93102-01-29-01), about 20 days before the Swift burst. First of all, the average flux was 5E-11 erg/cm^2/s (2-10 keV) which is about 4 times the typical flux. In addition, the spectrum was best fit with a powerlaw of photon index about 1.5, which is considerably harder than normal. Most astonishingly, the variability observed during this observation was completely different than we have ever seen from LS I +61 303. There is strong flaring and variability observed in the time domain. Power spectral analysis reveals a strong red-noise component and an apparent QPO at 2 Hz, reminiscent of the power spectra of LMXB Z-sources. There is also a significant narrow peak in the power spectrum at a frequency of 0.1 Hz, but most of the significance comes during a short period of flaring at about 1000 seconds into the observation so it is not clear whether this is a coherent pulsation or just a chance alignment of several flares around that time.

We note that our observations immediately preceding and succeeding (2008 August 17 at 04:57 and 2008 August 24 at 17:01) show none of these unusual characteristics.

It is important to understand that the PCA field of view is about 1 degree (FWHM), so we can not be certain that this unusual X-ray behavior is associated with LS I +61 303. If it is from LS I +61 303, it has important implications. For example, the presence of red noise and the QPO are strong indications that the observed X-rays are being produced in an accretion disk. It is also very hard to understand how this X-ray behavior could be related to the SGR-like outburst observed with Swift.

We request observations and archival searches at other wavelengths to constrain the possibility of a transient LMXB very close to LS I +61 303 or to find other changes in the system around 2008 Aug 21 that would help clarify the situation.


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