Despite being a staggering 23,000 light-years away from us, and further obscured by a massive amount of interstellar matter and gas, Cygnus X-3 is a galactic source extensively studied by astronomers, extremely fascinating and unique in its kind. It is a microquasar, a typical binary system where a star feeds a black hole that, in turn, expels enormous amounts of particles in opposite directions, known as “plasma jets” or “relativistic jets.”
These jets occasionally become the protagonists of sudden and thunderous expansions in the surrounding space, which, if cyclical and weak (up to 10 Jansky in power), are called flares. In the case of sudden and more powerful flares (over 10 Jansky), they are referred to as giant flares. Cygnus X-3 is the only known example to date that possesses both. From time to time, it bubbles with giant flares whose origin remains unknown to this day. The last one was observed a full five and a half years ago.
The research of Elise Egron
The observation of this rare and peculiar phenomenon originated from the work of Elise Egron, a researcher at the INAF Astronomical Observatory of Cagliari and the principal investigator of the study, recently published in the journal Monthly Notices of the Royal Astronomical Society, titled, as we mentioned earlier, “Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode.” Among the main co-authors are other INAF researchers: Alberto Pellizzoni, Marcello Giroletti, Simona Righini, and Matteo Stagni. The first is based in Cagliari, the last three in Bologna.
But how is it possible to detect these giant flares if they are so random and unpredictable? In reality, there is a minimal level of predictability. Here’s how it went. It was the end of summer 2016, and Elise Egron, like almost every day, was checking a series of astronomical “alerts” on the specialized site Astronomer’s Telegram, a sort of “social network” for astronomers where small and large discoveries made around the world by researchers from various astronomical fields and specializations can be published in real-time. No waiting lists, no official publications, no peer review by colleagues.
Among the many news items on the site, the French researcher noticed a report related to Cygnus X-3. A Russian colleague warned that the source’s emission, observed in radio, was too low, and this, for those who know Cygnus X-3 well, meant only one thing: after over five years of silence, a “giant flare” would soon appear. In these cases, the exact moment is impossible to pinpoint, but it usually ranges from two weeks to two or three months.
For astronomers, this is a big hunt because these are almost unique events called “transient phenomena” that require quick reactions and communications among scholars. Italy has the “fortune” (which would be better called “scientific foresight”) of possessing a system of three antennas managed by INAF suitable for joint observation in VLBI mode and, above all, capable of managing recorded data through a powerful computer called a correlator, located in Bologna and developed about two years ago by Matteo Stagni.
[SRT (Cagliari), Medicina (Bologna), and Noto: the three antennas of the Italian VLBI]
Simona Righini in Medicina (Bologna) and Elise Egron and Alberto Pellizzoni in San Basilio (Cagliari) thus immersed themselves in long hours of joint observation, and the three Italian antennas began observations in both single dish and VLBI modes. It’s not easy to explain simply the difference between joint observation (VLBI) and single (single dish). “The VLBI and single dish observation modes are very complementary, because in VLBI you can see the structure and morphology of the observed region, while in single dish you can accurately measure the flow,” observes Egron, highlighting the great difference between resolution, guaranteed by many antennas capable of seeing practically in three dimensions, and the measurability of movements and spaces guaranteed by single observation which – continues the researcher – “also allows for a quick frequency change. In VLBI, you have to observe for up to 10 hours straight, while in single dish we can observe in multifrequency by changing receivers in a few minutes to understand, for example, the change in the spectral index. The combination of these observations produces exceptional results, but this is just the beginning.”
Meanwhile, thanks to the work of Marcello Giroletti, at the time the Italian representative on the program committee of the European VLBI Network (EVN), other European researchers were also alerted to support the research. Within a few days, the antennas of Onsala (Sweden), Yebes (Spain), and Torun (Poland) accepted the invitation and thus monitored Cygnus X-3 during a “mini flare” phase, a phenomenon limited to the “heart” of the source but still important and revealing of the great potential of this method.
The discovery
The giant flare arrived ten days after the mini one. It was possible to observe it with an “emergency” call only four days after its peak phase. The scientific results were nonetheless surprising: for the first time, it was possible to observe a variation in the spectral index of Cygnus X-3 in just 5 hours, thus offering new elements that can help better understand the mechanisms of the non-uniformity of plasma jets that emerge from black holes (for which there are several theories not summarized here).
The VLBI observations of this “descending” phase of the giant flare led to the observation of jets (nonexistent just four days earlier) about 10 billion km long. This means that the black hole at the center of the giant flare suddenly expelled two plasma jets at an enormous speed, equal to about one hundred thousand km per second, or one-third of the speed of light.
The Italian VLBI network leading in Europe
Even more importantly, which goes far beyond this study, is Italy’s undisputed leadership role in the global context of VLBI, large baseline interferometry. “Only the United States is capable of doing what the Italian antennas have done, and will increasingly do,” says Giroletti. “In the past, for VLBI projects, from conception to publication, it took 5 years between conception, observations, and correlations. This time, within 2 days, the correlator gave us the feedback we needed to produce concrete results. Almost a revolution. Now INAF is able to conduct VLBI observations independently and even lead research projects in Europe.”
To learn more:
Read the article “Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode” by Elise Egron et al. in the Monthly Notices of the Royal Astronomical Society.
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