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Results from the binaries LS I +61{\deg}303 and LS 5039 after 2.5 years of Fermi monitoring

The Fermi Large Area Telescope (LAT) has made the first definitive GeV detections of the binaries LS I +61^{\circ}303 and LS5039 in the first year after its launch in August 2008. These detections were unambiguous because, apart from a reduced positional uncertainty, the gamma-ray emission in each case was orbitally modulated with the corresponding orbital period. The LAT results posed new questions about the nature of these objects, after the unexpected observation of an exponential cutoff in the GeV gamma-ray spectra of both LS I +61^{\circ}303 and LS5039, at least along part of their orbital motion. We present here the analysis of new data from the LAT, comprising 2.5 years of observations through which LS I +61^{\circ}303 continues to provide some surprises. We find an increase in flux in March 2009 and a steady decrease in the flux fraction modulation. The LAT now detects emission up to 30 GeV, where prior datasets led to upper limits only. At the same time, contemporaneous TeV observations either no longer detected the source, or found it -at least in some orbits- close to periastron, far from the usual phases in which the source usually appeared at TeV energies. The on-source exposure of LS 5039 has also drastically increased along the last years, and whilst our analysis shows no new behavior in comparison with our earlier report, the higher statistics of the current dataset allows for a deeper investigation of its orbital and spectral evolution.

The early LAT reports of GeV emission from LS 5039 and LS I +61 • 303 were based upon 6-9 months of survey observations [3,5]. Both sources were detected at high significance and were unambiguously identified with the binaries by their flux modulation at the corresponding orbital periods, 26.4960 for LS I +61 • 303 [11] and 3.90603 days for LS 5039 [12]. The modulation patterns were consistent with expectations from inverse Compton scattering models, and were anti-correlated in phase with pre-existing TeV measurements (e.g., [13,14]). The anti-correlation of GeV-TeV fluxes is in fact a generic feature embedded in inverse Compton models describing the TeV fluxes, where the GeV emission is enhanced (reduced) when the highly relativistic electrons seen by the observer encounter the seed photons head-on (rear-on); (e.g., [15,16]). Fermi measurements provided a generic confirmation for these inverse Compton models.
Both sources presented exponential cutoffs in their high-energy spectra, at least along part of the orbit.
To be precise, an exponential cutoff was a better fit to the SED -as compared with a pure power-law-in phases surrounding the inferior conjunction (INFC) of LS 5039, and when taking into account the average spectrum of LS I +61 • 303 along its whole orbit. Statistical limitations with the used amount of data prevented the determination or the ruling out of an exponential cutoff in any orbital cut of LS I +61 • 303 or in the superior conjunction (SUPC) of LS 5039. The spectral energy distributions with the exponential cutoffs that were reported were reminiscent of the many pulsars the LAT has discovered since launch [17], although this was far from a proof of an LS 5039 or LS I +61 • 303 pulsar nature. No pulsations has been found at GeV energies.
In this work we present the results of the analysis of 2.5 years of LAT survey observations of both LS I +61 • 303 and LS 5039. We investigate any longterm flux variations of the sources and explore too the possible spectral variability for both systems. We expect that our analysis will constrain future theoretical studies on these interesting objects.

Observations and data reduction
The Fermi Gamma-ray Space Telescope, launched on 2008 June 11, carries onboard the Large Area Telescope (LAT). The LAT is an electron-positron pair production telescope, featuring solid state silicon trackers and cesium iodide calorimeters, sensitive to photons from ∼20 MeV to >300 GeV [18]. It has a large field of view with ∼2.4 sr (at 1 GeV) and an effective area of ∼8000 cm 2 for >1 GeV.
The Fermi survey mode operations began on 2008 August 4. The full dataset used for this analysis spans 2008 August 4, through 2010 December 4. The data were reduced and analyzed using the Fermi Science Tools v9r20 package. 1 The standard onboard filtering, event reconstruction, and classification were applied to the data [18]. The highquality "diffuse" event class was used. Throughout the analysis the "Pass 6 v3 Diffuse" instrument response functions (IRFs) were applied. Where required in the analysis, models for the Galactic diffuse emission (gll iem v02.fit ) and isotropic backgrounds (isotropic iem v02.txt ) were used. 2 The binned maximum-likelihood method of gtlike, which is part of the ScienceTools, was applied to determine the intensities and spectral parameters presented in this paper. We used all photons with energy >100 MeV in a circular region of interest (ROI) of 10 • radius centered at the position of LS I +61 • 303 and LS 5039, respectively. For source modeling, the 1FGL catalog [19], derived from 11 months of survey data, was taken.

LS 5039 Results
LS 5039 is located in a complicated region toward the inner Galaxy with high Galactic diffuse emission and many gamma-ray sources. In an earlier publication about this source [5] we derived that a power law plus an exponential cutoff describes best the data. The photon index was Γ = 1.9 ± 0.1 stat ± 0.3 syst ; the 100 -300 GeV flux was (4.9 ± 0.5 stat ± 1.8 syst ) × 10 −7 ph cm −2 s −1 and the cutoff energy was found to be 2.1 ± 0.3 stat ± 1.1 syst GeV.
Firstly, we analyzed the orbitally-averaged data of LS 5039. As spectral models for LS 5039, we used a power law as well as a power law with an exponential cutoff, and compared likelihood obtained to test the significance of a spectral cutoff. The likelihood ratio between the power-law and cutoff power-law cases clearly indicates that the power-law assumption is rejected.
Spectral points at each energy band were obtained by dividing the data set into each energy bin and performing maximum likelihood fits for each of them. Resulting spectral energy distribution (SED) is plotted in Figure 1.
Following the H.E.S.S. analysis by [14] and our previous analysis, the whole data set was divided into two orbital intervals: superior conjunction (SUPC; φ < 0.45 and 0.9 < φ) and inferior conjunction (INFC; 0.45 < φ < 0.9). The SUPC and INFC data were analyzed in the same way as the orbitally averaged data. Being consistent with our previous paper, the powerlaw assumption for the SUPC spectrum is rejected. Although a single power law was not rejected for INFC in our previous analysis using 10-month data, a cutoff power law is preferred also for INFC with the present data set. The corresponding SED is shown in Figure 1.

LS I +61 • 303 Results
LS I +61 • 303 is one of the brightest sources in the γ-ray sky and towers above other emitters in its neighborhood. This has allowed us to clearly detect, in March 2009, a ∼30% increase on the average flux. We graphically show the flux change in Figure 2, by plotting the folded lightcurves before and after the found transition in March 2009. Before the transition, the modulation was clearly seen and is compatible with the already published phasogram, whereas afterwards, the modulation gets fainter. Note that the datasets corresponding to the reported results [3] and what we here referred to as before the flux change span almost exactly the same time range, with the consequence of our current analysis essentially reproducing the one previously published [3]. The spectra derived before and after this flux change are also shown in Figure 2, where the increase in flux is obvious.

The multi-wavelength context
LS I +61 • 303 has also been monitored with the RXTE-Proportional Counter Array (PCA) and folded lightcurves were produced. In the left panel of Figure  3, we show a direct comparison between the phasograms in X-ray and in γ-rays, with simultaneously taken data. We divide the whole LAT dataset into 4 periods of 6 months each and compare them with correspondingly obtained PCA X-ray data. The division in periods of 6-months is justified in order to have enough statistics in γ-rays for each individual time bin, and such that orbit-to-orbit X-ray variability does not dominate the flux fraction changes. It is apparent that the X-ray modulation is always visible in each of these five panels, albeit with variable amplitude of flux modulation. Instead, at GeV energies, the LAT data indicates that the modulation fades away until the variability along the orbit is barely visible in the last 6 months of our data. In Figure 4 we show the average γ-and X-ray fluxes fitted in each of the periods considered. The X-ray flux is increasing, whereas the γ-ray flux seems to decrease again after the flux change in March 2009.

Summary
After two years of continuous data taking of the two binary systems LS 5039 and LS I +61 • 303 we confirm that their energy spectra are best described by a power law function with an exponential cutoff. For LS 5039 , in comparison with earlier publications of this source, with larger data set at hand we can extend the observed spectrum to higher energies. No significant emission of LS I +61 • 303 above 30 GeV has been detected with the F ermi-LAT yet. Furthermore, in the case of LS I +61 • 303 variable modulation in the folded light curves was observed. Before a flux change in March 2009, the modulation was clearly visible, whereas afterward it faints away. This behavior was not predicted by any theoretical model so far and has to be investigated further.