Exceptionally bright TeV flares from the binary LS I +61 303 PDF Print



Average and flare differential photon spectra of LS I +61o 303 from the VERITAS 2014 observations. For details, see Figure 2 below.

Reference: S. Archambault et al. (The VERITAS Collaboration), Astrophysical Journal Letters 817: L7, 2016

Full text version

ArXiv: ArXiV:1601.01812

Contacts: Anna O'Faolain de Bhroithe

The high-mass X-ray binary LS I +61o 303 consists of a compact object (either a black hole or a neutron star) and a B0 Ve stellar companion. The multiwavelength emission from the binary is variable at all energies and is modulated with a period of 26.5 days. This period is believed to be associated with the orbital motion of the binary system. Shorter timescale variability has also been detected in X-rays and hinted at in the very-high-energy (VHE; E > 100 GeV) regime. Additionally, the binary exhibits a periodic superorbital modulation with a period of about 4.5 years in Hα, radio, X-ray, and GeV emission. This long-term modulation could be attributed to precessing relativistic jets if the compact object is a black hole or to cyclic variations of the stellar envelope if the compact object is a neutron star.

LS I +61o 303 typically presents elevated VHE emission around apastron passage (the point in the orbit at which the compact object and the star are furthest apart). VERITAS observed LS I +61o 303 in late 2014 and detected bright VHE flares around apastron at flux levels peaking at levels a factor of 2-3 times higher than ever previously observed, making these flares the brightest VHE activity ever seen from this binary. The strong outbursts have rise and fall times of less than a day. The short timescale of the flares, combined with the detection of photons with energies of 10 TeV during the flares, provides constraints of the properties of the accelerator in the source. These observations hint in favour of the compact object being a black hole, however, the possibility that it could be a neutron star cannot be ruled out.

There are a number of insights that could help to distinguish between a black hole and neutron star compact object. High-resolution radio imaging could reveal extended jet-like structures that would suggest a black hole. Refined measurements of the orbital parameters of the system constraining the inclination angle and hence the system's mass could clearly identify the compact object as a black hole if its mass were found to be more than three solar masses. The detection of pulsed emission from the binary at any wavelength would unambiguously identify the compact object as a pulsar, but it is also possible that the dense environment around the companion star could hinder such a detection. A correlation between the electromagnetic emission and Hα variations would indicate that the emission is related to changes in the circumstellar disk of the optical star. It is clear that further observations of LS I +61o 303 are necessary across all wavelengths to fully understand the nature of its varying emission..


FITS files: Light curve (Figure 1); spectra (Figure 2).


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


Figure 1:  Light curve of LS I +61o 303 during the 2014 observation season shown as a function of orbital phase in nightly bins. The phase range is shown from 0.45 to 0.45 as the VERITAS observations commenced around a phase of φ = 0.5 in each orbit. The data for the first orbit (October) are shown with orange circles, while the second orbit (November) is represented by purple diamonds, and the third (December) by blue squares. Flux upper limits at the 99% confidence level (using the unbounded approach of Rolke et al. 2005) are shown for points with significance < 3 σ and are represented by arrows. 
Figure 2: Average and flare differential photon spectra of LS I +61o 303 from the VERITAS 2014 observations, shown in comparison with the average spectra from Acciari et al. (2008) and Aleksic et al. (2012).

Last Updated on Friday, 27 May 2016 13:00

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