We derive the extinction curve towards the Galactic Center from 1 to 19 µm. We use hydrogen emission lines of the minispiral observed by ISO-SWS and SINFONI. The extinction free flux reference is the 2 cm continuum emission observed by the VLA. Towards the inner 14 ′′ ×20 ′′ we find an extinction of A 2.166 µm = 2.62 ± 0.11, with a power-law slope of α = −2.11 ± 0.06 shortward of 2.8 µm, consistent with the average near infrared slope from the recent literature. At longer wavelengths, however, we find that the extinction is grayer than shortward of 2.8 µm. We find it is not possible to fit the observed extinction curve with a dust model consisting of pure carbonaceous and silicate grains only, and the addition of composite particles, including ices, is needed to explain the observations. Combining a distance dependent extinction with our distance independent extinction we derive the distance to the GC to be R 0 = 7.94±0.65 kpc. Towards Sgr A* (r < 0.5 ′′ ) we obtain A H = 4.21±0.10, A Ks = 2.42 ± 0.10 and A L ′ = 1.09 ± 0.13.

This paper reports measurements of Sgr A* made with NACO in L ′ -band (3.80 µm), Ks-band (2.12 µm) and H-band (1.66 µm) and with VISIR in N-band (11.88 µm) at the ESO VLT 1 , as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 4 April, 2007, a very bright flare was observed from Sgr A* simultaneously at L ′ -band and X-ray wavelengths. No emission was detected 1 The Very Large Telescope (VLT) at the European Southern Observatory (ESO) on Paranal, Chile: Program IDs 179.B-0261(A) and Program ID: 079.B-0929(A).using VISIR. The resulting SED has a blue slope (β > 0 for νL ν ∝ ν β , consistent with νL ν ∝ ν 0.4 ) between 12 micron and 3.8 micron.For the first time our high quality data allow a detailed comparison of infrared and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak) and prominent substructures in the 3.8 micron light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L ′ -band lightcurve we find that the flaring region must be no more than 1.2 R S in size.The high X-ray to infrared flux ratio, blue νL ν slope MIR to L ′ -band, and the soft νL ν spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 M black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2−8 keV luminosity of about 33 × 10 33 erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered submillimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2-455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of Pdot<2e-10 s/s, which implies that the spin-down luminosity is Edot<3e33 erg/s. The X-ray luminosity of the pulsar is L_X = 3(+10,-2)e33 (D/5 kpc)^2 erg/s, and the spectrum can be described by a kT = 0.61+/-0.02 keV blackbody with a radius of R_bb = 0.27+/-0.03 (D/5 kpc}) km. Deep infrared observations reveal no counterpart with K<18.5, which rules out binary companions with M>1 Msun. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2-455216 only 1.6' from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses M_i=35 Msun and >50 post-main-sequence stars that indicate the cluster is 4+/-1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i>40 Msun. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars.Comment: 4 pages, 5 figures. Final version to match ApJL (added one figure since v2

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with midinfrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 μm excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 μm excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from ∼10 to ∼20 M . Within the central 400 × 50 pc (|l| < 1.• 3 and |b| < 10 ) the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M yr −1 . Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10 5 yr. This suggestion is also consistent with estimates of SFRs within the last ∼10 7 yr showing a peak around 10 5 yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.

We present an observation with the Chandra X-ray Observatory of the unusual radio source G359.23-0.82 ("the Mouse"), along with updated radio timing data from the Parkes radio telescope on the coincident young pulsar J1747-2958. We find that G359.23-0.82 is a very luminous X-ray source (L X [0.5 − 8.0 keV] = 5 × 10 34 ergs s −1 for a distance of 5 kpc), whose morphology consists of a bright head coincident with PSR J1747-2958, plus a 45 ′′ -long narrow tail whose power-law spectrum steepens with distance from the pulsar. We thus confirm that G359.23-0.82 is a bow-shock pulsar wind nebula powered by PSR J1747-2958; the nebular stand-off distance implies that the pulsar is moving with a Mach number of ∼ 60, suggesting a space velocity ≈ 600 km s −1 through gas of density ≈ 0.3 cm −3 . We combine the theory of ion-dominated pulsar winds with hydrodynamic simulations of pulsar bow shocks to show that a bright elongated X-ray and radio feature extending 10 ′′ behind the pulsar represents the surface of the wind termination shock. The X-ray and radio "trails" seen in other pulsar bow shocks may similarly represent the surface of the termination shock, rather than particles in the postshock flow as is usually argued. The tail of the Mouse contains two components: a relatively broad region seen only at radio wavelengths, and a narrow region seen in both radio and X-rays. We propose that the former represents material flowing from the wind shock ahead of the pulsar's motion, while the latter corresponds to more weakly magnetized material streaming from the backward termination shock. This study represents the first consistent attempt to apply our understanding of "Crab-like" nebulae to the growing group of bow shocks around high-velocity pulsars.

Context. Our Galaxy hosts at its dynamical center Sgr A*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. However, the recent observations of occasional rapid X-ray flares from Sgr A* provide constraints on the accretion and radiation mechanisms at work close to its event horizon. Aims. Our aim is to investigate the flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares. Methods. In Spring 2007, we observed Sgr A* with XMM-Newton with a total exposure of ∼230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of Sgr A* for comparison. Results. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (∼3 ks) and spectral characteristics to the very bright flare observed in ), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward Sgr A* (A V ∼ 25 mag), i.e., 4.5 × 10 22 cm −2 . However, our fitting of the Sgr A* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase. Conclusions. The two brightest X-ray flares observed so far from Sgr A* exhibited similar soft spectra.

We have carried out Very Large Array (VLA) continuum observations to study the variability of Sgr A Ã at 43 GHz (k ¼ 7 mm) and 22 GHz (k ¼ 13 mm). A low level of flare activity has been detected with a duration of $2 hr at these frequencies, showing the peak flare emission at 43 GHz leading the 22 GHz peak flare by $20-40 minutes. The overall characteristics of the flare emission are interpreted in terms of the plasmon model of van der Laan by considering the ejection and adiabatic expansion of a uniform, spherical plasma blob due to flare activity. The observed peak of the flare emission with a spectral index, À , of ¼ 1:6 is consistent with the prediction that the peak emission shifts toward lower frequencies in an adiabatically expanding self-absorbed source. We present the expected synchrotron light curves for an expanding blob, as well as the peak frequency emission, as a function of the energy spectral index constrained by the available flaring measurements in near-IR, submillimeter, millimeter, and radio wavelengths. We note that the blob model is consistent with the available measurements; however, we cannot rule out the jet of Sgr A Ã . If expanding material leaves the gravitational potential of Sgr A Ã , the total mass-loss rate of nonthermal and thermal particles is estimated to be 2 ; 10 À8 M yr À1 . We discuss the implication of the mass-loss rate, since this value matches closely the estimated accretion rate based on polarization measurements.