MAXI J1820+070: A powerful outburst launching superluminal jets in a Black Hole Binary

August 2020

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Fig. 1: Artist's impression of a Black Hole Binary (Image Credit: ESO/L. Calcada/M. Kornmesser).

MAXI J1820+070 is a Black Hole Binary (BHB) system consisting of a black hole with an estimated mass of 7-8 solar masses [1] and a companion star of less than one solar mass ([2], see figure 1). Its distance to the Solar System is about 3.5 kpc (i.e. about 10000 light-years) [3]. In such a BHB system, matter is transferred from the star onto the black hole via an accretion disk. Most BHB systems, including MAXIJ1820+070, spend the vast majority of their time in a so-called quiescent state: the accretion rate is very low, and there is no significant X-ray emission. If the accretion rate increases, more matter is transferred and the system enters into an outburst. This leads to a succession of different phases of increased emission over a wide range of wavelengths from the radio to the X-ray regime. At some point during the outburst, a jet is launched from the inner region of the accretion disk. This inspired the term of 'Microquasar' for these systems by analogy with quasars, albeit with a one-million-fold down-scaling of typical sizes [4]. The jet velocity can reach high values, sometimes even mimicking superluminal speeds [5]. Non-thermal radio and X-ray emission is believed to originate in the jets. It has long been speculated that the energies of the particles responsible for the non-thermal emission might be high enough for them to also emit gamma-radiation. Beyond the - usually brief - jet launching episode, X-rays also provide a good picture of the evolution of the system throughout the outburst. The interpretation of the phenomenology of microquasar behaviour is a very active research domain, with a variety of models proposed (e.g. [6], [7])

Fig. 2: Hardness-Intensity Diagram (HID) of X-ray emission and the inferred phenomenology of an eruption in a Black Hole Binary (reproduced from [6]). This sketch presents a simplified model for the relationship between the disk and the jet in BHBs, corresponding to the HID representation (top panel). Left to right corresponds to increasing hardness, bottom to top corresponds to increasing intensity. The states are labelled from i to iv. Gamma is the Lorentz factor of the jet. The jet is represented in blue, the accretion disk in red, and the corona (plasma surrounding the black hole) in yellow. The bottom panel shows the inner radius of the accretion disc and the jet Lorentz factor as functions of hardness.
A Hardness-Intensity Diagram (HID, see fig. 2) of X-ray emission illustrates the main concepts of the underlying physics. The "Hardness" is the ratio of X-ray fluxes in two adjacent energy bands, e.g. derived via [ 4, 10 ] / [ 2, 4 ] keV, while "Intensity" is the total flux in both bands. The canonical path followed by most of the BHBs in outburst is related to the states depicted on either side of the HID. In phases i and ii, a steady compact and mildly relativistic jet grows: it is the hard X-ray state. At some point the jet velocity increases rapidly, shocking the matter previously expelled ("jet line" on HID). This occurs at the transition from the hard to the soft state. Then the jet vanishes, and the disc dominates the emission (soft X-ray state, iv), and finally the system returns to quiescence. The dashed arrows indicate possibly more complex behaviour as seen in some systems, but MAXIJ1820+070 followed a single path as described by the solid arrows. The emergence of a jet, emitting non-thermal (synchrotron) radio emission has been followed with radio interferometers that provide high angular resolution. In the case of MAXIJ1820+070 a superluminal jet ejection has been seen [8]. The jet is thought to provide the adequate conditions to accelerate charged particles (electrons, protons) via first order Fermi processes within the internal shocks occurring when the jet velocity increases. These energetic particles then produce high energy (HE) and very high energy (VHE, >tens of GeV) gamma rays, e.g. via Inverse Compton scattering of low energy photons by the accelerated electrons. VHE instruments hence hunt the strong ejection events during the outbursts with the aim to find VHE photons which would confirm these scenarios and shed new light on the physics at play for these objects. BHBs are extensively studied at lower energies, but have not yet been detected in the VHE domain (with a possible hint for a transient signal at a satistical significance of 4.1 sigma for the microquasar Cygnus-X1 [9] and steady emission of the microquasar SS433 [10]).

Fig. 3: Evolution of MAXI J1820+070 in its 2018 outburst. Top panel: Swift/BAT X-ray light curve in the 15–50 keV range [17]. The estimated time of jet launch is represented as an orange dashed vertical line. The red vertical lines show the times at which H.E.S.S. observations were taken. Middle panel: MAXI/GSC hardness ratio of the 4–10 to 2–4 keV X-ray fluxes [18]: HS, IM, and SS abbreviate Hard State, Intermediate state, and Soft State, respectively. The corresponding time ranges, highlighted here with colours, are proposed in [19]. The periods coloured in yellow ('IM') correspond to the state transitions from hard to soft (for the first one) and soft to hard (for the second one). Bottom panel: Fermi-LAT integral flux upper limits above 100 MeV with a 99% confidence level and a power-law index of -2.5 (adapted from [20]).
Detection and evolution of MAXIJ1820+070

On March 11, 2018, the previously inconspicuous Binary System MAXIJ1820+070 underwent a major X-ray outburst, resulting in an early identification with the MAXI X-ray observatory located on the International Space Station [11]. It has been associated a posteriori with the bright optical transient ASASSN-18ey discovered March 6, 2018 [12]. The source was very bright, reaching up to four times the flux of the reference source (the Crab Nebula) in X-rays. An extensive follow-up campaign started, involving Swift XRT, Swift BAT, NICER and Integral conducting X-ray observations. Radio emission, typical of the compact jet present in the hard state, has been measured as well [13]. After spending about four months in the hard X-ray state, MAXIJ1820+070 transitioned from the hard to soft state, as announced by NICER [14], on July 5, 2018 within about ten days. An isolated radio flare was detected by AMI-LA [15]. Subsequent interferometric observations with high angular resolution radio observations revealed that the flare was associated with the launch of bi-polar relativistic jets [8]. Discrete ejections with superluminal velocities have been resolved by radio-telescopes (eMERLIN, MeerKAT, VLA) and followed up for about 200 days. The estimated date of their launch is July 7, 2018, during the X-ray state transition from hard to soft. These superluminal jets have also been observed in the X-ray band using the Chandra satellite [16]. After 70 days in the soft state, MAXI J1820+070 transitioned back to the hard state with very low X-ray luminosity, closing this major outburst. The top and middle panels of the figure 3 show the X-ray behaviour of the system during the whole outburst. In the top panel, the date of the jet launch, esitmated from radio observations [8], is indicated with an orange, dashed vertical line and H.E.S.S observation dates are indicated with red vertical lines. We also studied the GeV domain with public data of the Large Area Telescope on board of the Fermi satellite: no evidence for a GeV signal has been found, and upper limits have been computed.

The extraordinary outburst of MAXI J1820+070 was observed with the H.E.S.S. telescopes over the course of half a year, probing the hard state as well as transitions into and out of the soft state. The coverage and the actual HID of the outburst of MAXI J1820+070 are shown in Figure 4. This diagram shows the MAXI dataset that covers the time of jet launch estimated using radio data [8]. It matches well with the typical location of the "Jet line" shown in figure 2. MAXI data points contemporaneous to H.E.S.S. observations are encircled in red. Preliminary analysis of the H.E.S.S. observations have been combined with data obtained by our colleagues of the MAGIC and VERITAS collaborations and have been presented in Reference [20].

No significant signal has been detected from MAXIJ1820+070, leading to calculation of upper limits in the VHE band. Integrating the H.E.S.S. observations over the full cycle, the absence of signal implies that there is only a 1% chance that the photon flux at Earth from MAXIJ1820+070 exceeds ~0.5% of the flux of the Crab Nebula, the "standard candle" for VHE gamma-ray astronomy (assuming a power-law spectral shape of index -2.5).

Fig. 4: MAXIJ1820+070 HID for the 2018 outburst (adapted from [20]). Error bars are omitted for better visibility.

In its 2018 outburst, MAXIJ1820+070 has joined the handful of microquasars exhibiting superluminal ejections. It has emitted at all wavelengths up to X-rays: radio, infra-red, optical and X-rays. It has evolved in a canonical way, going through the full expected path on the X-ray hardness intensity diagram. At higher energies, the H.E.S.S., MAGIC and VERITAS telescopes have followed the event throughout the cycle, focusing on the more promising phases. While no VHE signal has been found, the modelling of underlying physics will benefit from the ensuing constraints.

The outburst observed in 2018 event was spectacular, but not the first recorded flare of MAXIJ1820+070. Digital archival data of optical astronomy revealed two outbursts that occurred in 1898 and 1934 ([21]), predating the era of radio, X-ray, and gamma-ray astronomy.


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