Multiwavelength study of the flaring activity of Sagittarius A in 2014 February−April

Context. The supermassive black hole named Sgr A* is located at the dynamical center of the Milky Way. This closest supermassive black hole is known to have a luminosity several orders of magnitude lower than the Eddington luminosity. Flares coming from the Sgr A* environment can be observed in infrared, X-ray, and submillimeter wavelengths, but their origins are still debated. Interestingly, the close passage of the Dusty S-cluster Object (DSO)/G2 near Sgr A* may increase the black hole flaring activity and could therefore help us to better constrain the radiation mechanisms from Sgr A*. Aims. Our aim is to study the X-ray, infrared, and radio flaring activity of Sgr A* close to the time of the DSO/G2 pericenter passage in order to constrain the physical properties and origin of the flares. Methods. Simultaneous observations were made with XMM-Newton and WFC3 onboard HST during the period Feb.–Apr. 2014, in addition to coordinated observations with SINFONI at ESO’s VLT, VLA in its A-configuration, and CARMA. Results. We detected two X-ray flares on 2014 Mar. 10 and Apr. 2 with XMM-Newton , three near-infrared (NIR) flares with HST on 2014 Mar. 10 and Apr. 2, and two NIR flares on 2014 Apr. 3 and 4 with VLT. The X-ray flare on 2014 Mar. 10 is characterized by a long rise (~7700 s) and a rapid decay (~844 s). Its total duration is one of the longest detected so far in X-rays. Its NIR counterpart peaked well before (4320 s) the X-ray maximum, implying a dramatic change in the X-ray-to-NIR flux ratio during this event. This NIR/X-ray flare is interpreted as either a single flare where variation in the X-ray-to-NIR flux ratio is explained by the adiabatic compression of a plasmon, or two distinct flaring components separated by 1.2 h with simultaneous peaks in X-rays and NIR. We identified an increase in the rising radio flux density at 13.37 GHz on 2014 Mar. 10 with the VLA that could be the delayed radio emission from a NIR/X-ray flare that occurred before the start of our observation. The X-ray flare on 2014 Apr. 2 occurred for HST during the occultation of Sgr A* by the Earth, therefore we only observed the start of its NIR counterpart. With NIR synchrotron emission from accelerated electrons and assuming X-rays from synchrotron self-Compton emission, the region of this NIR/X-ray flare has a size of 0.03−7 times the Schwarzschild radius and an electron density of 10 8.5 –10 10.2 cm -3 , assuming a synchrotron spectral index of 0.3−1.5. When Sgr A* reappeared to the HST view, we observed the decay phase of a distinct bright NIR flare with no detectable counterpart in X-rays. On 2014 Apr. 3, two 3.2-mm flares were observed with CARMA, where the first may be the delayed (4.4 h) emission of a NIR flare observed with VLT. Conclusions. We observed a total of seven NIR flares, with three having a detected X-ray counterpart. The physical parameters of the flaring region are less constrained for the NIR flare without a detected X-ray counterpart, but none of the possible radiative processes (synchrotron, synchrotron self-Compton, or inverse Compton) can be ruled out for the production of the X-ray flares. The three X-ray flares were observed during the XMM-Newton total effective exposure of ~256 ks. This flaring rate is statistically consistent with those observed during the 2012 Chandra XVP campaign, implying that no increase in the flaring activity was triggered close to the pericenter passage of the DSO/G2. Moreover, higher flaring rates had already been observed with Chandra and XMM-Newton without any increase in the quiescent level, showing that there is no direct link between an increase in the flaring rate in X-rays and the change in the accretion rate.