Astrophysical jets are ubiquitous phenomena in our universe, linked to a wide range of objects, from young stars to black holes. These powerful, highly collimated outflows deposit significant amounts of energy and matter into the surrounding medium, affecting star formation, galaxy evolution, and even the distribution of matter in the universe. However, despite decades of research, our current knowledge of the physics that gives rise to and governs the behaviour of jets is extremely limited, where for example jet launching and acceleration mechanisms are still widely unknown.

Of all the systems that launch jets, X-ray binaries (XBs; binary systems containing a stellar mass compact object, such as a black hole or neutron star, accreting matter from a companion star, see Fig. 1) are particularly excellent testbeds for studying jet phenomena. These systems are typically transient in nature, evolving from periods of inactivity into a bright out-bursting state on timescales of days to months, in turn providing a real time view of how jets evolve and interact with their environment. While XBs emit across the electromagnetic spectrum, the mm/sub-mm frequency bands uniquely allow us to isolate radiation produced by the jet from radiation produced in the accretion flow near the compact object, where the jet is launched. My current research focuses on utilizing these critical mm/sub-mm frequency bands to develop new ways to study XB jets in the (i) spectral domain, (ii) temporal domain, and (iii) through interactions with the surrounding environment.

Fig.1 - Schematic of an XB system and its local environment (where the compact object is a black hole). The components of the XB (and the frequencies at which their emission dominates), as well as the zones where the jets collide with the surrounding medium (jet working surfaces), are labelled. An electromagnetic spectrum, mapping out the frequency bands that we can sample with different telescopes, is shown at the top of the panel (background image credit: R. Hynes)

Spectral Domain

XB jets display a characteristic broad-band spectrum consisting of optically thick synchrotron emission extending from radio up to sub-mm or infrared frequencies, which breaks to optically thin synchrotron emission at higher frequencies (Fender, 2001, MNRAS, 322, 31; Corbel et al., 2002, ApJ, 573, L35). The spectral break is a key observational tracer of jet physics, as this portion of the spectrum probes emission from the most compact region of the jet, where particles are first accelerated up to high energies (Markoff, et al., 2005, ApJ, 635, 1203). The exact spectral shape (i.e., spectral index, location of the spectral break) depends on jet properties such as geometry, magnetic field structure, and particle density profiles, as well as the plasma conditions in the region where the jet is first accelerated. My team's recent work has revealed that the jet spectrum varies dramatically during an outburst (Russell et al., 2014, MNRAS, 439, 139; Russell et al., 2015, MNRAS, 450, 1745). We have shown that through identifying and tracking how the spectral break changes with time, we can track the evolution of the physical conditions in the jet acceleration region (e.g., radius, magnetic field). By coupling these changes to variations in the accretion flow, we aim to determine which accretion flow properties govern the launching and quenching of XB jets. Further, the scaling of the break with luminosity provides a key physical anchor needed to test and guide the development of relativistic jet simulations, ultimately allowing us to understand how the jet we observe is produced (Polko et al., 2014, MNRAS, 438, 559; Ceccobello, et al., 2018, MNRAS, 473, 4417). Obtaining data in the mm/sub-mm regime is crucial in tracking the spectral break (see Fig. 2), as these bands fill a 2 orders of magnitude gap in frequency in our broad-band coverage (Tetarenko, et al., 2015, ApJ, 805, 300; Tetarenko, et al., 2019, MNRAS, 482, 2950).

Fig.2 - Broad-band spectra of the compact jet launched from MAXI J1836-194 during its 2011 outburst (Russell et al., 2014, MNRAS, 439, 139). Lines indicate fits to the data, where the spectral breaks (i.e., where the power-laws meet) vary dramatically, with the range indicated by the horizontal bars. The grey lines indicate the overlapping frequency range covered by my teams current mm/sub-mm programs with ALMA, SMA, JCMT, and NOEMA. There is a clear need for mm/sub-mm data; without mm/sub-mm data the break frequency of 2011 Sep 17 is uncertain to an order of magnitude.

Temporal Domain

The emission observed from XBs can be strongly variable over a range of timescales (minutes to months). While broad-band spectral measurements and high resolution radio imaging studies with Very Long Baseline Interferometry (VLBI; e.g. Stirling et al., 2001, MNRAS, 327, 1273) are traditionally used to constrain jet properties, time domain observations offer a promising new way to address the key open questions in jet research (Uttley et al., 2015, SSRv, 83, 453). Detecting and characterizing rapid flux variability in jet emission from multiple BHXBs can allow us to probe detailed jet properties that are difficult, if not impossible, to measure by other means (e.g., jet geometry, speed, the sequence of events leading to jet launching; Casella et al., 2010, MNRAS, 404, L21; Gandhi et al., 2017, Nature Astronomy, 1, 859; Vincentelli et al., 2018, MNRAS, 447, 4524; Malzac et al., 2018, MNRAS, 480, 2054). While time-resolved observations are a staple for XB studies at higher frequencies (optical, X-ray; probing the accretion flow), there are many challenges that accompany such studies at low frequencies (radio, sub-mm; probing the jet). In particular, it can be difficult to disentangle intrinsic source variations from atmospheric variations, and the need to cycle between observing a target source and a calibrator prevents continuous data recording. Further, until recently, most telescopes were not sensitive enough, nor capable of taking the data to probe rapid (second) timescales. My team and I have been working on developing new observational techniques and computational tools (e.g., see CASA Variability Measurment Scripts) to overcome such limitations. This work allows us to accurately sample XB jets in the time domain at radio/sub-mm frequencies for the first time (see Fig. 3; Tetarenko et al., 2017, MNRAS, 469, 314), and connect jet variability properties with internal jet physics (Tetarenko et al., 2019, MNRAS, 484, 2987).

Fig.3 - High time resolution light curves of V404 Cygni, taken simultaneously in an unprecedented 8 different bands (Tetarenko et al., 2017, MNRAS, 469, 314). We detect rapid flux variability, in the form of multiple flaring events, which coincide with the ejection of discrete jet ejecta (shown in inset panels). This work represents the first time-resolved mm/sub-mm study of XB jets, and demonstrates the paramount importance of the mm/sub-mm bands. Detecting detailed substructure, uniquely visible in the mm/sub-mm light curves (which are smoothed out, and of lower amplitude in the radio regime), allows us to separate out emission from different ejection events, and in turn is essential in understanding the rapidly evolving jets in V404 Cygni.

Jet-ISM Interactions

My team and I have developed a novel technique to identify and probe the regions where XB jets are colliding with the surrounding ISM (Tetarenko et al. 2018, MNRAS, 475, 448). Encoded within the physical conditions in these zones are valuable information on unknown jet properties, such as, the total jet power, jet composition, duty cycles, and the efficiency of jet feedback (Castor et al. 1975, ApJ, 200, L107; Heinz 2006, ApJ, 6363, 316). To take full advantage of these jet interaction regions to probe jet physics, we need to be able to efficiently identify these zones near multiple XBs. However, prior to our work there were only two BHXBs in our Galaxy with confirmed jet interaction sites (in the form of jet-blown cavities coincident with excited atomic/molecular gas) in the surrounding ISM (see Fig. 4; Gallo et al. 2005, Nature, 436, 819; Dubner et al. 1998, AJ, 116, 1842). Previous methods used to identify interaction zones near XBs involved a cumbersome multi-wavelength approach (radio + optical/X-ray), that hinges on detecting extended radio emission. This technique is expensive in terms of telescope time, and also very observationally difficult, due to the expected low surface brightness of such radio emission structures. Our new technique, using molecular line emission visible in the sub-mm bands to trace density, temperature, and the presence of jet driven shocks in these interaction zones, overcomes these complications; we can efficiently identify interaction sites, and accurately characterize the physical conditions of the ISM in these regions (needed to infer jet properties), with a short observation on a single telescope.

Fig.4 - Radio frequency images of known black hole jet interaction zones. Panels (a) and (b) display AGN sources in other galaxies (image credits: NRAO), where large scale radio lobe structures are observed, while panels (c) and (d) display BHXBs in our Galaxy (Gallo et al. 2005, Nature, 436, 819; Dubner et al. 1998, AJ, 116, 1842). The location of the black hole (black star), and the direction in which the jets propagate (purple cones) are indicated. The background color map represents the intensity of the radio emission (blue is faint, red is bright). In all cases, the jet carves out some form of cavity in the intervening medium.

Disc-Jet Coupling

I have also done some work exploring the empirical disc-jet coupling relationship, relating radio and X-ray luminosity, in the XB population (see Fig. 5; Gallo et al. 2003, MNRAS, 344, 60; Corbel et al. 2013, MNRAS, 428, 2500). In particular, I focused on analyzing the mechanisms that govern radio luminosity in neutron star systems, where this coupling relationship had not previously been well studied. I worked to quantify the similarities and differences between this disc-jet coupling relationship in different accreting systems, to understand the key factors that may govern jet production and evolution (e.g., mass, spin, magnetic fields; Tetarenko et al. 2016, MNRAS, 460, 345; Tetarenko et al. 2018, ApJ, 854, 125).

Fig.5 - Disc-jet coupling correlation for XBs, relating radio luminosity (at 5 GHz) to X-ray luminosity (in the 1-10 keV band). Different types of accreting stellar-mass compact objects are shown; black holes (BHs; binaries harboring black holes), neutron stars (NS; binaries harboring non-pulsating neutron stars), accreting milli-second X-ray pulsars (AMXPs; accreting neutron star binaries where X-ray pulsations at the spin period of the neutron star are observed), transitional milli-second pulsars (tMSPs; accreting neutron star binaries that switch between a rotation-powered pulsar state and an accretion-powered state), and out-bursting cataclysmic variables (CVs; binaries harboring white dwarfs). The best-fit relation for black holes (grey dashed; Gallo et al. 2014, MNRAS, 445, 290) is also shown. This plot is produced using the repository of Bahramian et al., 2018

A large part of my work involves analyzing data from different telescopes around the world. The telescopes I rountinely use are shown in the right panel; the Karl G. Jansky Very Large Array (VLA), the Submillimeter Array (SMA), the James Clerk Maxwell Telescope (JCMT), the Northern Extended Millimeter Array (NOEMA), the IRAM 30m telescope, and the Atacama Large Millimetre Array (ALMA).