The Birth of a Relativistic Outflow in the Unusual γ-ray Transient Swift J164449.3+573451

Abstract: Active galactic nuclei (AGN), powered by long-term accretion onto central supermassive black holes, produce relativistic jets with lifetimes of greater than one million yr that preclude observations at birth. Transient accretion onto a supermassive black hole, for example through the tidal disruption of a stray star, may therefore offer a unique opportunity to observe and study the birth of a relativistic jet. On 2011 March 25, the Swift {\gamma}-ray satellite discovered an unusual transient source (Swift J164449.3+573451) potentially representing such an event. Here we present the discovery of a luminous radio transient associated with Swift J164449.3+573451, and an extensive set of observations spanning centimeter to millimeter wavelengths and covering the first month of evolution. These observations lead to a positional coincidence with the nucleus of an inactive galaxy, and provide direct evidence for a newly-formed relativistic outflow, launched by transient accretion onto a million solar mass black hole. While a relativistic outflow was not predicted in this scenario, we show that the tidal disruption of a star naturally explains the high-energy properties, radio luminosity, and the inferred rate of such events. The weaker beaming in the radio compared to {\gamma}-rays/X-rays, suggests that radio searches may uncover similar events out to redshifts of z ~ 6.

• The paper reports the unique observation of a nascent relativistic outflow, with EVLA data indicating a Lorentz factor between 1.2 and 2.
• It employs multiwavelength radio monitoring and spectral energy distribution analysis to confirm self-absorbed synchrotron emission peaking at millimeter wavelengths.
• The findings imply that tidal disruption events can trigger relativistic jets, offering new insights into high-energy accretion processes in dormant galaxies.

Analysis of Relativistic Outflow Formation in Gamma-Ray Transient SwiftJ164449.3+573451

The study presented in this paper reports on the exceptional observation of a relativistic outflow from the transient gamma-ray source SwiftJ164449.3+573451. Unlike continuous relativistic jets in active galactic nuclei (AGN), which are typically unobservable at their nascent stage due to their prolonged life spans (>10 years), this event offers a unique opportunity to study the onset of a relativistic jet, potentially resulting from the transient accretion onto a supermassive black hole (SMBH).

The discovery was initiated by the detection of a transient source by the Swift γ-ray satellite on March 25, 2011. Follow-up radio observations using the Expanded Very Large Array (EVLA) identified a brightening radio source coincident with an inactive galaxy's nucleus, suggesting the formation of a relativistic jet. This was evidenced by a newly formed relativistic outflow associated with transient accretion onto a 10^6 solar mass (M⊙) black hole, an occurrence not conventionally predicted in such scenarios. The apparent beaming effect, detected in radio frequencies and weaker than in γ-rays or X-rays, indicates the potential for further discovery of similar events at redshifts of z ∼ 6.2.

Key observational data were collected spanning centimeter to millimeter wavelengths over one month. The temporal evolution of the observed spectral energy distribution (SED) was analyzed, yielding a power law attributed to self-absorbed synchrotron emission, peaking in the millimeter range. An inferred mildly relativistic expansion with a Lorentz factor (Γ) of approximately 1.2 to 2 indicates ongoing but stable relativistic expansion corresponding with the initial γ-ray detection.

The study hypothesizes that the outflow’s energy exceeds the Eddington luminosity of the black hole, suggesting a highly collimated outflow possibly induced by tidal disruption of a star. Supporting this claim, the host galaxy’s observed properties imply the absence of an active AGN, reinforcing the likelihood of an external disruption as the cause.

Several robust numerical findings and theoretical implications arise from this investigation:

• Equipartition Analysis: The study reports an equipartition radius of approximately 1 µas and supports the existence of a relativistic outflow with a Lorentz factor of Γ ≈ 1.2, implying a mass outflow enhanced by relativistic effects.
• Scintillation Evidence: Observations of interstellar scintillation at multiple radio frequencies provide an empirical cross-verification of the outflow’s relativistic nature, projecting angular sizes consistent with the expected Fresnel scale modulations.
• Gamma-Ray to Radio Correlation: Although initial γ-ray emissions coincide with the emergence of a radio transient, the lack of synchronous variability in X-ray beams prompts the inference of a separate emission origin, possibly close to the outflow base.

The implications of these findings suggest the following avenues for further exploration in high-energy astrophysics:

• Radio Monitoring: Continued observations will enable precise calorimetry of the jet, independent of beaming effects, and provide insights into the ambient medium interactions in the vicinity of dormant SMBHs.
• Resolving Radio Sources: Long-term monitoring with VLBA is predicted to resolutely confirm relativistic expansion on a timescale of approximately two years.
• Comparative Analysis of Event Rates: The rarity of such events, inferred to occur at a rate of ~0.1 Gpc^-3 yr^-1, suggests that events like SwiftJ164449.3+573451 could be distinctively insightful, offering a diagnostic tool for judging beaming effects and event energy budgets.

Ultimately, while the paper does provide striking support for theories involving tidal disruption events as progenitors of transient relativistic outflows, it also raises new questions about the nature of initial accretion and jet collimation mechanisms. Future research to locate and study similar events may benefit from focusing on radio signatures and the challenges inherent in high optical extinction scenarios, enriching understanding relevant to both galactic core transient phenomena and broader relativistic jet studies.