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Register a new userMAGIC observations of MWC 656, the only known Be/BH system
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Added by
Alicia L\\'opez-Oramas
Publication date
2015
fields
Physics
and research's language is
English
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Context: MWC 656 has recently been established as the first observationally detected high-mass X-ray binary system containing a Be star and a black hole (BH). The system has been associated with a gamma-ray flaring event detected by the AGILE satellite in July 2010. Aims: Our aim is to evaluate if the MWC 656 gamma-ray emission extends to very high energy (VHE > 100 GeV) gamma rays. Methods. We have observed MWC 656 with the MAGIC telescopes for $sim$23 hours during two observation periods: between May and June 2012 and June 2013. During the last period, observations were performed contemporaneously with X-ray (XMM-Newton) and optical (STELLA) instruments. Results: We have not detected the MWC 656 binary system at TeV energies with the MAGIC Telescopes in either of the two campaigns carried out. Upper limits (ULs) to the integral flux above 300 GeV have been set, as well as differential ULs at a level of $sim$5% of the Crab Nebula flux. The results obtained from the MAGIC observations do not support persistent emission of very high energy gamma rays from this system at a level of 2.4% the Crab flux.
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Using TESS photometry and Rozhen spectra of the Be/gamma-ray binaries MWC 148 and MWC 656, we estimate the projected rotational velocity ($ {v} sin i$), the rotational period (P$_{rm rot}$), radius (R$_{rm 1}$), and inclination ($i$) of the mass donor. For MWC 148 we derive P$_{rm rot} = 1.10 pm 0.03$~d, R$_{rm 1}= 9.2 pm 0.5$~R$_odot$, $i = 40^circ pm 2^circ$, and $ {v} sin i =272 pm 5$~km~s$^{-1}$. For MWC 656 we obtain P$_{rm rot} = 1.12 pm 0.03$~d, R$_{rm 1}= 8.8 pm 0.5$~R$_odot$, $i = 52^circ pm 3^circ$, and $ {v} sin i =313 pm 3$~km~s$^{-1}$. For MWC 656 we also find that the rotation of the mass donor is coplanar with the orbital plane.
We find that the formation of MWC 656 (the first Be binary containing a black hole) involves a common envelope phase and a supernova explosion. This result supports the idea that a rapidly rotating Be star can emerge out of a common envelope phase, which is very intriguing because this evolutionary stage is thought to be too fast to lead to significant accretion and spin up of the B star. We predict $sim 10-100$ of B BH binaries to currently reside in the Galactic disk, among which around $1/3$ contain a Be star, but there is only a small chance to observe a system with parameters resembling MWC 656. If MWC 656 is representative of intrinsic Galactic Be BH binary population, it may indicate that standard evolutionary theory needs to be revised. This would pose another evolutionary problem in understanding BH binaries, with BH X-ray Novae formation issue being the prime example. The future evolution of MWC 656 with a $sim 5$ M$_{odot}$ black hole and with a $sim 13$ M$_{odot}$ main sequence companion on a $sim 60$ day orbit may lead to the formation of a coalescing BH-NS system. The estimated Advanced LIGO/Virgo detection rate of such systems is up to $sim 0.2$ yr$^{-1}$. This empirical estimate is a lower limit as it is obtained with only one particular evolutionary scenario, the MWC 656 binary. This is only a third such estimate available (after Cyg X-1 and Cyg X-3), and it lends additional support to the existence of so far undetected BH--NS binaries.
The LIGO/Virgo collaboration has reported 50 BH-BH mergers and 8 additional candidates recovered from digging deeper into the detectors noise. Majority of these mergers have low effective spins pointing toward low BH spins and efficient angular momentum transport in massive stars as proposed by several models (e.g., Tayler-Spruit magnetic dynamo or Fuller model). However, out of these 58 mergers, 7 are consistent with having high effective spin parameter (chi_eff>0.3). Additionally, 2 out of these 7 events seem to have high effective spins sourced from the high spin of a primary (more massive) BH. The most extreme merger has very high primary BH dimensionless spin (a_1=0.9). These particular observations may be potentially used to discriminate between the isolated binary and dynamical globular cluster BH-BH formation channels. It may seem that high BH spins point to the dynamical origin if stars have efficient angular momentum transport and form low-spinning BHs. Then dynamical formation is required to produce second and third generations of BH-BH mergers that typically produce high-spinning BHs. Here we show that isolated binary BH-BH formation channel can naturally reproduce such highly spinning BHs. Our models start with efficient angular momentum transport in massive stars that is needed to reproduce majority of BH-BH mergers with low effective spins. However, some massive binaries are subject to strong tidal spin-up allowing for the formation of moderate fraction (~10%) of BH-BH mergers with high effective spins (chi_eff>0.4-0.5). Moreover, binary evolution can produce small fraction (~1%) of BH-BH mergers with almost maximally spinning primary BHs ($a_1>0.9$). Therefore, the formation scenario of those unusual BH-BH mergers remains unresolved.
MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes sensitive above ~60 GeV, and located on the Canary Island of La Palma at the height of 2200 m.a.s.l. Since Autumn 2009 both telescopes are working together in stereoscopic mode. We use both Crab Nebula observations and Monte Carlo simulations to evaluate the performance of the system. Advanced stereo analysis allows MAGIC to achieve a sensitivity better than 0.8% of the Crab Nebula flux in 50 h of observations in the medium energy range (around a few hundred GeV). At those energies the angular resolution is better than 0.07{circ}, and the energy resolution is as good as 16%. We perform also a detailed study of possible systematics effects for the MAGIC telescopes.
Microquasars, X-ray binaries displaying relativistic jets driven by accretion onto a compact object, are some of the most efficient accelerators in the Galaxy. Theoretical models predict Very High Energy (VHE) emission at the base of the jet where particles are accelerated to multi-TeV energies. This emission could be detected by present IACTs. %Moreover, gamma-ray fluxes should increase during flaring events when accretion rates are enhanced. The MAGIC telescope observed the microquasars GRS 1915+105, Cyg X-3, Cyg X-1 and SS433 for ~ 150 hours in total from 2005 to 2008. We triggered our observations by using multi wavelength information through radio flaring alerts with the RATAN telescope as well as by ensuring the low/hard state of the source through RXTE/ASM and Swift/BAT monitoring data. We report on the upper limits on steady and variable emission from these sources over this long period.
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