Galactic Center Minispiral: Interaction Modes of Neutron Stars

Streams of gas and dust in the inner parsec of the Galactic center form a distinct feature known as the Minispiral, which has been studied in radio waveband as well as in the infrared wavebands. A large fraction of the Minispiral gas is ionized by radiation of OB stars present in the Nuclear Star Cluster (NSC). Based on the inferred mass in the innermost parsec ($\sim 10^6$ solar masses), over $\sim 10^3$ -- $10^4$ neutron stars should move in the sphere of gravitational influence of the SMBH. We estimate that a fraction of them propagate through the denser, ionized medium concentrated mainly along the three arms of the Minispiral. Based on the properties of the gaseous medium, we discuss different interaction regimes of magnetised neutron stars passing through this region. Moreover, we sketch expected observational effects of these regimes. The simulation results may be applied to other galactic nuclei hosting NSC, where the expected distribution of the interaction regimes is different across different galaxy types.Comment: 12 pages, 17 figures, published in Acta Polytechnic

Coordinated NIR/mm observations of flare emission from Sagittarius A*

Context. We report on a successful, simultaneous observation and modelling of the millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart associated with the supermassive (4 × 10^6 M_☉) black hole at the Galactic centre (GC). We present a mm/sub-mm light curve of Sgr A* with one of the highest quality continuous time coverages. Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*. Methods. Our non-relativistic modelling is based on simultaneous observations carried out in May 2007 and 2008, using the NACO adaptive optics (AO) instrument at the ESO's VLT and the mm telescope arrays CARMA in California, ATCA in Australia, and the 30 m IRAM telescope in Spain. We emphasize the importance of multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modelling. We present a new method for obtaining concatenated light curves of the compact mm-source Sgr A* from single dish telescopes and interferometers in the presence of significant flux density contributions from an extended and only partially resolved source. Results. The observations detect flaring activity in both the mm domain and the NIR. Inspection and modelling of the light curves show that in the case of the flare event on 17 May 2007, the mm emission follows the NIR flare emission with a delay of 1.5±0.5 h. On 15 May 2007, the NIR flare emission is also followed by elevated mm-emission. We explain the flare emission delay by an adiabatic expansion of source components. For two other NIR flares, we can only provide an upper limit to any accompanying mm-emission of about 0.2 Jy. The derived physical quantities that describe the flare emission give a source component expansion speed of ν_(exp) ~ 0.005c–0.017c, source sizes of about one Schwarzschild radius, flux densities of a few Janskys, and spectral indices of α = 0.6 to 1.3. These source components peak in the THz regime. Conclusions. These parameters suggest that either the adiabatically expanding source components have a bulk motion greater than ν_(exp) or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*. Applying the flux density values or limits in the mm- and X-ray domain to the observed flare events constrains the turnover frequency of the synchrotron components that are on average not lower than about 1 THz, such that the optically thick peak flux densities at or below these turnover frequencies do not exceed, on average, about ~1 Jy

Multiple accretion events as a trigger for Sgr A* activity

Gas clouds are present in the Galactic centre, where they orbit around the supermassive black hole. Collisions between these clumps reduce their angular momentum, and as a result some of the clumps are set on a plunging trajectory. Constraints can be imposed on the nature of past accretion events based on the currently observed X-ray reflection from the molecular clouds. We discuss accretion of clouds in the context of enhanced activity of Sgr A* during the past few hundred years. We put forward a scenario according to which gas clouds bring material close to the horizon of the black hole on <~0.1 parsec scale. We have modelled the source intrinsic luminosity assuming that multiple events occur at various moments in time. These events are characterized by the amount of accreted material and the distribution of angular momentum. We parameterized the activity in the form of a sequence of discrete events, followed the viscous evolution, and calculated the luminosity of the system from the time-dependent accretion rate across the inner boundary. Accreting clumps settle near a circularization radius, spread there during the viscous time, and subsequently feed the black hole over a certain period. A significant enhancement (by factor of ten) of the luminosity is only expected if the viscous timescale of the inflow is very short. On the other hand, the increase in source activity is expected to be much less prominent if the latter timescale is longer and a considerable fraction of the material does not reach the centre. A solution is obtained under two additional assumptions: (i) the radiative efficiency is a decreasing function of the Eddington ratio; (ii) the viscous decay of the luminosity proceeds somewhat faster than the canonical L(t)~t^{-5/3} profile. We applied our scheme to the case of G2 cloud in the Galactic centre to obtain constraints on the core-less gaseous cloud model.Comment: Astronomy and Astrophysics accepte

Reflection nebulae in the Galactic Center: the case for soft X-ray imaging polarimetry

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas.Comment: 7 pages, 3 figures, 2 tables, accepted for publication in A&

Galactic Centre observations with local mm-telescope arrays

Radio interferometry observations are a powerful tool to study the central regions of the Galactic centre (GC). High angular resolution observations at millimetre wavelengths can disentangle the emission of Sgr A* from the thermal emission of the circum nuclear disk (CND) and the mini-spiral surrounding it. In this thesis I present the results of radio interferometric observations of the GC region with local millimetre-telescope arrays such as CARMA and ATCA, supplemented by observations in the infrared (IR) with ESO's VLT telescopes. My goal is to analyse the emission mechanisms present in the central few parsecs of the GC region, in particular the extended thermal emission of the ionized gas and dust of the mini-spiral region and the non-thermal emission of Sagittarius A* (Sgr A*), the radio source associated with the supermassive black hole (SMBH) at the GC. Observations were carried out in March and April 2009 at 1.3 and 3 mm with the mm telescope array CARMA in California, in June 2006 with the mid-infrared (MIR) instrument VISIR at ESO's VLT, and in the NIR Br_gamma in August 2009 with VLT NACO. I present high angular resolution continuum maps of the GC at 3 and 1.3 mm and the highest resolution spectral index map obtained at these wavelengths. I obtain a spectral index of 0.5 for Sgr A*, indicating an inverted synchrotron spectrum and a mixture of negative and positive values in the extended emission of the minispiral, including the thermal free-free emission from the ionized gas, and a possible contribution of dust at 1.3 mm. I infer the physical properties of the dust and gas in the mini-spiral by comparing the radio continuum maps to the MIR continuum map, and the Br_gamma line emission map in the NIR. An extinction map at NIR wavelengths for the mini-spiral region, showing extinctions ranging from 1.8-3 magnitudes, was also produced. To study the flaring activity of Sgr A*, global coordinated multiwavelength campaigns were carried out in 2007 and 2008 using ESO's VLT and the mm telescope arrays CARMA, ATCA, and the 30 m IRAM telescope in Spain, which detected four flares in the NIR, of which three were covered later by the mm data. I develop a new method for obtaining concatenated light curves of the compact mm-source Sgr A* from single dish telescopes and interferometers in the presence of significant flux density contributions from an extended and only partially resolved source, and model the observed flares in the NIR and mm using an adiabatic expansion model involving synchrotron source components. I derive physical quantities such as expansion velocities ranging from 0.005c-0.017c, source sizes of about one Schwarzschild radius, turnover frequencies of a few THz, flux densities of a few Janskys, and spectral indices of 0.6 to 1.3. These parameters suggest either a bulk motion of the adiabatically expanding source components greater than the expansion velocity or a confinement of expanding material within a corona or disk in the immediate surroundings of Sgr A*

Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region

We explore the conditions for the thermal instability to operate in the mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold media are known to coexist. The photoionisation Cloudy calculations are performed for different physical states of plasma. We neglect the dynamics of the material and concentrate on the study of the parameter ranges where the thermal instability may operate, taking into account the past history of Sgr A* bolometric luminosity. We show that the thermal instability does not operate at the present very low level of the Sgr A* activity. However, Sgr A* was much more luminous in the past. For the highest luminosity states the two-phase medium can be created up to 1.4 pc from the centre. The presence of dust grains tends to suppress the instability, but the dust is destroyed in the presence of strong radiation field and hot plasma. The clumpiness is thus induced in the high activity period, and the cooling/heating timescales are long enough to preserve later the past multi-phase structure. The instability enhances the clumpiness of the mini-spiral medium and creates a possibility of episodes of enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the range of masses and sizes of clouds; under the conditions of Sgr A*, the likely values come out $1$ - $10^2M_{\oplus}$ for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure