LS I +61 303 - Multiwavelength studies of a binary star system PDF Print

LS I +61 303
The multiwavelenght data obtained by VERITAS (>500 GeV) and Swift/RXTE (2-10 keV) on LS I +61 303. Although there exist many instances of closely contemporaneous data in both TeV and X-ray, a clear picture of the relation between emission in these bands has yet to emerge.


LS I +61 303 - Multiwavelength studies of a binary star system



Reference:  V. A. Acciari et al. (The VERITAS Collaboration), The Astrophysical Journal, 700:1034, 2009

Full text version

ArXiv version: ArXiV:0904.4422

Contact person: Andy Smith



LS I +61 303 is one of only a handful of binary star systems which are known TeV emitters. It is known to be a pairing of a massive main sequence star and a compact object of unknown nature (most likely a neutron star , but the presence of a black hole has not been ruled out). The system is known to exhibit large outbursts of radio, X-ray, and TeV electromagnetic radiation; these bursts highly correlated with the ~26.5 day orbital system. The most distinct feature of the TeV gamma-ray emission from this system is that it only significantly appears at "apastron " passage when the two objects in the system are furthest apart. This feature of the system has fueled extensive modeling of the fundamental nature of LS I +61 303, a nature which is not clearly known at this point.

The two principal competing models are known as the "microquasar " and "binary pulsar " models which rely on particle acceleration around an accretion disk or particle acceleration at the interfact shock between a pulsar and stellar wind respectively. Although it is not clear which of these models best fits the observed data from LS I +61 303, each of these models to some degree makes predictions about correlations (or lack thereof) between the TeV and X-ray emission. Along these lines, VERITAS has completed an extensive, long-term monitoring campaign of the system in the TeV energy regime taken in conjunction with X-ray observations taken by the Swift and RXTE satellites.

Observations were taken with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between September 2006 and February 2008. The system was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8 sigma (2006/2007) and 7.3 sigma (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5 to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6 sigma significance level) for TeV emission outside of the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0 *10^-11 ergs cm^-2 s^-1 over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is shown that the TeV sampling is not dense enough to detect a correlation between the two bands.

While this work does not shed any light on the fundamental nature of the system, it demonstrates the need for further multiwavelength campaigns of this kind to increase the sampling density in both bands. In addition, the marginal evidence for periastron TeV emission in the system is enticing and deserved to be followed up, as this emission may play a key role in our ability to distinguish between the models which seek to explain this curious and confusing galactic source.



Figures from paper (click to get full size image):



Figure 2: Differential photon spectra of LS I +61◦ 303 as measured by VERITAS during orbital phases 0.5–0.8 from both the 2006/2007 (Acciari et al. 2008) and 2007/2008 seasons, along with fitted Crab Nebula spectrum as measured by VERITAS. See Section 3.1 for a description of the data and power-law fits.
Figure 3 (shown above): Results of the 2006/2007 TeV and X-ray observations. The vertical scale on the right (blue) represents the X-ray flux only, with the left vertical scale representing TeV flux.
Figure 4: Results of the 2007/2008 TeV and X-ray observations. Scales are as in Figure 3.
Figure 5: SED of LS I +61◦ 303 compared to the models of Gupta & Boettcher (2006) shown by a dashed line and Zdziarski et al. (2008) shown by a solid line. The Gupta and Boettcher spectrum is themodel’s prediction at orbital phase 0.5, whereas the ZNC spectrum is that model’s prediction at a general high-emission state which is not defined in terms of orbital positio



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