The Jets of Microquasars during Giant Flares and Quiet State

Abstract

We report on the radio properties of jets of the following microquasars, as determined from daily multi-frequency monitoring observations with the RATAN-600 radio telescope during 2010–2017: V404 Cyg, SS433, Cyg X-1, GRS1915+105 and LSI+61${}^{\circ }$303. We have detected many giant flares from SS433, a powerful flare from V404 Cyg in June 2015, an active state of Cyg X-1 in 2017 and fifty periodic flares from LSI+61${}^{\circ }$303. We describe the properties of massive ejections based on multi-band (radio, X-ray and $\gamma$-ray) studies. The general properties of the light curves are closely connected with the processes of jet formation in microquasars.

1. Microquasars Studies with the RATAN-600 Telescope

The bright microquasars GRS1915+105, SS433, Cyg X-1, Cyg X-3, LSI+61${}^{\circ }$303 and LS5039 are well-known X-ray binaries (XRBs) with relativistic jets, detected directly in the VLBI mapping. The radio emission of XRBs is mostly a variable synchrotron radiation from these jets, formed as a mass of ejections from central regions of an accretion disk within an intensive wind of the optical star. The inner shock waves in relativistic jets are the main generator of the relativistic electrons [1]. We have carried out a long-term monitoring of XRBs in almost daily measurements with RATAN-600 at 1.3–30 GHz during the last seven years [2,3]. During 52 orbital periods ($P_1=26.5$ d) we studied the super-orbital modulation ($P_2=1667$ d) of the flaring radio emission from LSI+61${}^{\circ }$303. The mean orbital light curves (usually for ten orbits) depend strongly on a phase $P_2$. We detected two giant flares from Cyg X-3 in September 2016 and in April 2017 at 1.3–30 GHz. We discuss the properties of these flares in the paper of the Proceedings of [4].

2. Results

2.1. V404 Cyg

V404 Cygni is a low-mass XRB consisting of a black hole of mass 9.0 M${}_{\odot }$, accreting matter from a low-mass (<1 M${}_{\odot }$), late-type companion star. The orbital period $P_1=6.5$ d is the longest of all known black hole XRBs. V404 Cyg is the closest microquasar, $d=2.39$ kpc [5]. We observed V404 Cyg in June 2015 [6] and have detected the bright flare [7] simultaneously with the huge X-ray flare (40 Crabs at 15–50 keV, [8]) at MJD57198.933 (Figure 1). This flare had the characteristic synchrotron spectrum with a low-frequency turn-over. This is probably due to a synchrotron self-absorption or an absorption due to thermal electrons mixed with the relativistic ones [9]. During four days, the flaring flux decreased with time as a power-law: $S\propto t^{-1.5}$.

2.2. SS433

SS433 is a bright microquasar that we have observed with RATAN-600 at 1.3–22 GHz [3]. There are multiple powerful flares on the RATAN light curves (Figure 2). Most of them are optically thin synchrotron flares with spectral indices between $-0.3$ and $-0.8$. By comparing VLBA images of SS433 with photometric radio monitoring from the RATAN-600 telescope, we explored the properties of these radio flares [11]. Usually, the active states continue for about 100 days, while we can see single bright flares within 10–15 days. In the quiet states, lasting up to 200 days, we have detected a weak modulation with an orbital period of 13.1 days or with a nutation period of 6.3 days. We did not find any correlation between the appearance of flares and the jet precession of the 164-day period. All the local peaks on the light curves can be associated with the appearance of new bright components in the VLBI maps, pointing to a connection with the ejection of new blobs.

2.3. Cyg X-1

The persistent black hole XRB Cyg X-1 had a variable flux density of 5–40 mJy at 4.7 GHz during the daily measurements in 2017 (Figure 3). We have detected a significant correlation ($\rho =0.7$) between the smoothed hard X-ray emission at 15–50 keV (Swift/BAT) and the radio flux density at 4.7 GHz.

2.4. GRS1915+105

The prototypical microquasar GRS1915+105 showed very bright flares up to 1.5 Jy at 5 GHz in the 1990th years. These flares are definitely related to changes in the X-ray states. According to recent VLBA observations [12] and our studies [13], the X-ray luminosity increased in the last 2–4 h preceding the ejection in May 2013, which we later detected as the 80-mJy flare at 4.7–11 GHz. In the long-term monitoring of GRS1915+105 with RATAN-600, the flares have been detected commonly at the level of 0.6 Jy or lower. They occurred 2–3-times per year, usually when the low/soft X-ray state changed to the high/hard state. We plotted in Figure 4 one example of such behavior, where the light curves at 2–20 keV (MAXI) and at 4.7 GHz (RATAN) are shown.

2.5. LS5039

The high-mass microquasar XRB LS 5039, comprising the O6.5-type star and probably a black hole ([14]) (or NS), has been detected in $\gamma$-rays (see [15,16] (EGRET source 3EG J1824-1514 and the Fermi catalog source 3FGL J1826.2-1450). It has been monitored with the Green Bank Interferometer in the 1990th at 2.3 and 8.3 GHz. We measured the light curve at 4.7–4.8 GHz during 2014–2017 (Figure 5). Usually, the flux density varied from 10 to 100 mJy, while the mean flux density for 10–15 days was 25–30 mJy. In 2017, we measured the average (over 10–15 days) flux density at 8.2 GHz at the level of 5–20 mJy; thus, the spectrum is non-thermal with spectral indices (here, always $S_{\nu }\propto {\nu }^{\alpha }$) between $-0.5$ and $-1.2$, with the median value of $-0.7$. We did not detect any significant modulation associated with the orbit period ($P_1=3.9$ d) in these data at 4.7 GHz, while such modulation could be related to the measured eccentricity >0.25, which can influence the accretion rate, and therefore the jet formation, as we found in XRB LSI+61${}^{\circ }$303.

2.6. LSI+61${}^{\circ }$303

We continued our monitoring of the periodical flaring activity in radio and $\gamma$-rays XRB LSI+61${}^{\circ }$303 [17,18] during 1400 days, i.e., 52 orbits ($P_1=26.5$ d) at frequencies of 2.3, 4.7 and 11.2 GHz (Figure 6). The aim was to trace the enigmatic super-orbital ($P_2=1667$ d) period [19] in full to study the evolution of the radio properties. Indeed, we found that the maximal flaring flux density and their $P_1$-phases notably changed with the $P_2$ phases (Figure 7).

3. Discussion

We have conducted an intensive monitoring of the bright microquasars during the last 4–7 years and have obtained an unprecedented spectral and temporal set of data. It serves as a good tracer of the jet activity and could be used for comparison with the optical, X-ray and $\gamma$-ray data. A search for the correlations or associations of processes in the accretion disks and jets (the so-called ‘disk-jets coupling’) is the key task in the building of the physical picture for microquasars [20].