The first months in the lifetime of the newly born jet associated to Swift J1644+57

We describe the evolution of of Swift J1644+57, whose unique X-ray properties have led several groups to interpret its behavior as corresponding to an extraordinary event of tidal disruption of a star by a supermassive black hole in the nucleus of a (z = 0.3545) galaxy, as derived by GTC. Multiwavelength observations (optical, millimetre, centimitre) are proving to be essential to reveal the long term nature of the emission in this source. In particular, we identify for the first time the properties of a forming relativistic jet. In our interpretation of the phenomenon, we leave the still open possibility that it may correspond to the onset of a dormant AGN, but this may only be tested with longer term X-ray, millimetre and centimetre monitoring.


INTRODUCTION
On 2011 March 28 Swift triggered on a newly discovered transient, which was initially thought to be a new gammaray burst (dubbed GRB 110328A; Cummings et al. 2011). However, a second Swift trigger on this source (Barthelmy et al. 2011) started to cast doubt on the GRB nature of this source, suggesting that instead it may correspond to a new class of hard X-ray transient. This was confirmed by the pre-outburst optical detection in Palomar (Cenko et al. 2011), which strongly disfavored a cosmological longduration GRB. Furthermore, spectroscopic observations (Levan et  All this observational evidence points towards the possibility that the source is a very peculiar new type of X-ray transient (Levan et al. 2011), which may be interpreted (Bloom et al. 2011) by a tidal disruption of a main sequence star by a black hole of 10 6−7 solar masses at the center of the observed host galaxy, leading to the formation of a "mini-blazar" with the jet seen face-on. This is also supported by energetic and variability arguments, a e-mail: ajct@iaa.es which suggest that the jet is at least mildly relativistic, presents high collimation, and a spectrum dominated by synchrotron and inverse Compton emission (Bloom et al. 2011).

Millimetre observations
Our observations with the Plateau de Bure Interferometer (PdBI) started three days after the onset of the event. The counterpart was detected with an extremely high (>100) S/N ratio, on the phase center coordinates (J2000, R.A. = RA: 16:44:49.97; Dec: +57:34:59.7). The observed dual polarisation continuum bandwidth was 1 GHz. They showed that, after an initial decay, the flux at mm wavelengths displayed a significant increase by a factor of more than a factor of 2 during May 2011.

Centrimetre observations
Effelsberg observations were attempted on 2011 June 12 (no detection due to poor weather conditions) and on 2011 June 19, with a marginal detection of ∼8 mJy at 4.8 GHz. After the initial decay, the mm lightcurve displays a second bursting episode, reaching a maximum flux density even higher than the one observed after the initial Swift detection. Right: Sketch of the main features of the proposed scenario to explain the light curves of Sw 1644+57. The length scale is given in terms of Schwarzschild radii. After the forward shock passes the location where the external pressure becomes nearly constant (rather than increasing toward the black hole), a conical recollimation shock forms and collimates the jet further. Electrons are accelerated to highly relativistic energies at all three shock fronts, creating the three spectral components sketched on the bottom.

Tidal Disruption Events and AGN Outbursts
Additional observations were carried with Korean facilites, also imposing upper limits.
On 2012 February 28 we performed phase-reference observations of Sw1644+57 using the European VLBI Network (EVN) at a observing wavelength of 1.3 cm. These observations were aimed at detecting any possible proper motion, as well as to resolve its radio structure. Unfortunately, by the time of our EVN observations -337 days after the initial burst -the source had already decreased in flux well below the sensibility of our observations, preventing its detection (see Fig. 2). We have however been able to provide an upper limit (3-sigma) for its flux density at 1.3 cm of 0.97 mJy/beam.

DISCUSSION
These (and further) observations have led us to suggest that the peculiar X-ray transient Sw 1644+57 may correspond to the onset from a dormant AGN (Castro-Tirado et al. 2012). See Fig. 3. In the case of the AGN onset, we expect that the jet originated by the flaring event that triggered the Swift satellite should conserve its mm and radio emission on relatively long time scales (of several months more).
In conjunction with other multi-wavelength observations (like Swift which are being performed routinelly), ongoing (and near future) observations of Swift 1644+57 will allow: -to determine whether the nature of Swift J1644+57 is related to a short time range tidal disruption flare of a star by a supermassive black hole, or to a longer term phenomenon as the onset of a dormant AGN as proposed in Castro-Tirado et al. (2012). -to perform a detailed study of the long term evolution of the multi-wavelength afterglow emission over the first ∼24 months after the event. This will enable us to trace the evolution of the characteristic synchrotron self-absorption frequency, and the radio-mm spectral index, that will help help us to constrain among different jet models.