We present results from a long-term monitoring campaign on the TeV binary LSI +61 303 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 binary 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.
Deep Dive into Multiwavelength Observations of LS I +61 303 with VERITAS, Swift and RXTE.
We present results from a long-term monitoring campaign on the TeV binary LSI +61 303 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 binary 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^-
• 303 is one of the most extensively studied binary star systems in the Milky Way and, although it has been the subject of many observational campaigns, the true nature (i.e. microquasar or binary pulsar) of the system remains unclear. The system can be classified as a high mass X-ray binary (HMXB) located at a distance of ∼2 kpc; the components of the system consisting of a compact object in a 26.496 (±0.003) day orbit around a massive BO Ve main sequence star (Hutchings andCrampton 1981, Casares et al. 2005). The motion of the compact object around its main sequence companion is traditionally characterized by the orbital phase, φ, ranging from 0.0 to 1.0. φ = 0 is set at JD 2443366.775 (Gregory and Taylor 1978), with periastron passage believed to occur at φ=0.23±0.02 (Casares et al. 2005) or φ=0.30±0.01 (Grundstrom et al. 2006), and apastron passage between φ=0.65 and φ=0.85. Historically, LS I +61 • 303 has been an object of interest due to its periodic outbursts at radio (Paredes et al. 1998, Gregory 2002) and X-ray energies (Leahy et al. 1997, Taylor et al. 1996, Greiner and Rau 2001, Harrison et al. 2000).
The radio outbursts are well correlated with the orbital phase (Gregory 2002), although the phase of maximum emission can vary between φ=0.45 and φ=0.95. LS I +61 • 303 was first identified at gamma-ray energies with the COS-B source 2CG 135 +01 (Hermsen et al. 1977) and has also been identified with the EGRET source 3EG J0241+6103 which also shows evidence for 26.5 day modulation in the GeV band (Massi 2004). More recently, LS I +61 • 303 has been detected as a variable TeV gamma-ray source (Albert et al. 2006, Acciari et al. 2008) with maximum emission observed near apastron.
• 303 is one of only three reliably detected TeV binaries: the other two being LS 5039 (Aharonian et al. 2005a) and PSR B1259-63 (Aharonian et al. 2005b). PSR 1259-63 is a confirmed binary pulsar (Johnston et al. 1992a,b) whereas the nature of both LS 5039 and LS I +61 • 303 is still under debate. The two main competing scenarios which can explain these systems are microquasar (i.e. non-thermal emission powered by accretion and jet ejection) or binary pulsar (i.e. non-thermal emission powered by the interaction between the stellar and pulsar winds). The microquasar model used to describe LS I +61 • 303 is supported by evidence for strong jet outflows (Massi 2001). However, this model suffers from the failure to detect blackbody X-ray spectra expected in an accretion scenario.
The microquasar scenario has not been ruled out and is still the subject of much theoretical work, for example, see Romero et al. (2007). The binary pulsar model is most strongly supported by VLBA data (Dhawan 2006) which reveal a cometary radio structure around LS I +61 • 303 that is interpreted as due to the interaction between the pulsar and Be star wind structures. However, there is currently no detection of pulsed radio or X-ray emission confirming the presence of a pulsar. Possible models for LS I +61 • 303 will be discussed further in Section 4.
X-ray monitoring campaigns conducted with RXTE (Harrison et al. 2000, Greiner andRau 2001), ROSAT (Taylor et al. 1996), Chandra (Paredes et al. 2007), Beppo-Sax and XMM-Newton (Sidoli et al. 2006) show that LS I +61 • 303 is a highly variable hard X-ray source with flux levels modulated with the 26.5 day orbital period, the highest flux usually appearing between orbital phases 0.4 and 0.9. The XMM-Newton observations also detail very fast changes of flux, with fluxes doubling over the span of 1000 seconds (Sidoli et al. 2006). This result of kilosecond scale variability in the X-ray band has also been shown in Esposito et al. (2007) Chandra observations (Paredes et al. 2007) detail fast variability of the flux levels, while also showing evidence for extended X-ray emission reaching between 5" and 12.5" to the north of LS I +61 • 303. This provides an indication that particle acceleration may be taking place as far away as 0.05-0.12 parsecs from LS I +61 • 303. Recent RXTE observations (Smith et al. 2009), which cover a total of six orbital cycles, show no strong orbital modulation of the 2-10 keV X-ray flux, but a highly significant correlation between spectral index and flux levels. These observations (which are used in this work) show the presence of three large flares, the largest peaking at a flux value of 7.2 ( +0.1 -0.2 ) ×10 -11 ergs s -1 cm -2 . Closer examination of these flaring states shows that the X-ray flux from LS I +61 • 303 doubles within timescales of <2 s, indicating that the X-ray emission region is less than 10 11 cm in extent.
The MAGIC collaboration first detected LS I +61 • 303 as a variable TeV source above 200 GeV using observations made in 2005/2006 (Albert et al. 2006). This dataset covered six orbital cycles in the phase range φ=0.1-0.8 and a strong gamma-ray flux was detected during orbital phases φ=0.4-0.7, with the observed flux peaking at 16% of the Crab Nebula flux
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