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Register a new userThe Proper Motion of the Galactic Center Pulsar Relative to Sagittarius A*
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We measure the proper motion of the pulsar PSR J1745-2900 relative to the Galactic Center massive black hole, Sgr A*, using the Very Long Baseline Array (VLBA). The pulsar has a transverse velocity of 236 +/- 11 km s^-1 at position angle 22 +/- 2 deg East of North at a projected separation of 0.097 pc from Sgr A*. Given the unknown radial velocity, this transverse velocity measurement does not conclusively prove that the pulsar is bound to Sgr A*; however, the probability of chance alignment is very small. We do show that the velocity and position is consistent with a bound orbit originating in the clockwise disk of massive stars orbiting Sgr A* and a natal velocity kick of <~ 500 km s^-1. An origin among the isotropic stellar cluster is possible but less probable. If the pulsar remains radio-bright, multi-year astrometry of PSR J1745-2900 can detect its acceleration and determine the full three-dimensional orbit. We also demonstrate that PSR J1745-2900 exhibits the same angular broadening as Sgr A* over a wavelength range of 3.6 cm to 0.7 cm, further confirming that the two sources share the same interstellar scattering properties. Finally, we place the first limits on the presence of a wavelength-dependent shift in the position of Sgr A*, i.e., the core shift, one of the expected properties of optically-thick jet emission. Our results for PSR J1745-2900 support the hypothesis that Galactic Center pulsars will originate from the stellar disk and deepens the mystery regarding the small number of detected Galactic Center pulsars.
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We report measurements with the Very Long Baseline Array of the proper motion of Sgr A* relative to two extragalactic radio sources spanning 18 years. The apparent motion of Sgr A* is -6.411 +/- 0.008 mas/yr along the Galactic plane and -0.219 +/- 0.007 mas/yr toward the North Galactic Pole. This apparent motion can almost entirely be attributed to the effects of the Suns orbit about the Galactic center. Removing these effects yields residuals of -0.58 +/- 2.23 km/s in the direction of Galactic rotation and -0.85 +/- 0.75 km/s toward the North Galactic Pole. A maximum-likelihood analysis of the motion, both in the Galactic plane and perpendicular to it, expected for a massive object within the Galactic center stellar cluster indicates that the radiative source, Sgr A*, contains more than about 25% of the gravitational mass of 4 x 10^6 Msun deduced from stellar orbits. The intrinsic size of Sgr A* is comparable to its Schwarzschild radius, and the implied mass density of >4 x 10^23 Msun/pc^-3 very close to that expected for a black hole, providing overwhelming evidence that it is indeed a super-massive black hole. Finally, the existence of intermediate-mass black holes more massive than 3 x 10^4 Msun between approximately 0.003 and 0.1 pc from Sgr A*are excluded.
We present a measurement of the proper motion of the presumed pulsar in the evolved composite supernova remnant (SNR) MSH 15-56 whose pulsar wind nebula (PWN) has been disrupted by the supernova (SN) reverse shock. Using Chandra X-ray observations acquired over a baseline of 15 years, we measure a pulsar velocity of 720 (+290/-215) km/s and a direction of motion of 14 +/- 22 degrees west of south. We use this measurement to constrain a hydrodynamical model for the evolution of this system and find that its morphology is well-described by an SNR expanding in an ambient density gradient that increases from east to west. The effect of the density gradient and the pulsars motion is an asymmetric interaction between the SN reverse shock and the PWN that displaces the bulk of the PWN material away from the pulsar, towards the northeast. The simulation is consistent with an SNR age of 11,000 years, an SN ejecta mass of 10 solar masses, and an average surrounding density of 0.4 cm^-3. However, a combination of a higher SN ejecta mass and ambient density can produce a similar SNR morphology at a later age.
Understanding the origin of the flaring activity from the Galactic center supermassive black hole, Sagittarius A*, is a major scientific goal of the NuSTAR Galactic plane survey campaign. We report on the data obtained between July 2012 and April 2015, including 27 observations on Sgr A* with a total exposure of ~ 1 Ms. We found a total of ten X-ray flares detected in the NuSTAR observation window, with luminosities in the range of $L_{3-79~keV}$~$(0.2-4.0) times 10^{35}~erg~s^{-1}$. With this largest hard X-ray Sgr A* flare dataset to date, we studied the flare spectral properties. Seven flares are detected above 5{sigma} significance, showing a range of photon indices ({Gamma} ~ 2.0-2.8) with typical uncertainties of +/-0.5 (90% confidence level). We found no significant spectral hardening for brighter flares as indicated by a smaller sample. The accumulation of all the flare spectra in 1-79 keV can be well fit with an absorbed power-law model with {Gamma}=2.2+/-0.1, and does not require the existence of a spectral break. The lack of variation in X-ray spectral index with luminosity would point to a single mechanism for the flares and is consistent with the synchrotron scenario. Lastly, we present the quiescent state spectrum of Sgr A*, and derived an upper limit on the quiescent luminosity of Sgr A* above 10 keV to be $L_{Xq, 10-79 keV}$ < $(2.9{pm}0.2) times 10^{34}~erg~s^{-1}$.
The leading explanation of the $textit{Fermi}$ Galactic center $gamma$-ray excess is the extended emission from a unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Such a population would, along with the prompt $gamma$ rays, also inject large quantities of electrons/positrons ($e^pm$) into the interstellar medium. These $e^pm$ could potentially inverse-Compton (IC) scatter ambient photons into $gamma$ rays that fall within the sensitivity range of the upcoming Cherenkov Telescope Array (CTA). In this article, we examine the detection potential of CTA to this signature by making a realistic estimation of the systematic uncertainties on the Galactic diffuse emission model at TeV-scale $gamma$-ray energies. We forecast that, in the event that $e^pm$ injection spectra are harder than $E^{-2}$, CTA has the potential to robustly discover the IC signature of a putative Galactic bulge MSP population sufficient to explain the GCE for $e^pm$ injection efficiencies in the range $approx 2.9-74.1%$, or higher, depending on the level of mismodeling of the Galactic diffuse emission components. On the other hand, for spectra softer than $E^{-2.5}$, a reliable CTA detection would require an unphysically large $e^pm$ injection efficiency of $gtrsim 158%$. However, even this pessimistic conclusion may be avoided in the plausible event that MSP observational and/or modeling uncertainties can be reduced. We further find that, in the event that an IC signal were detected, CTA can successfully discriminate between an MSP and a dark matter origin for the radiating $e^pm$.
We obtained six observations of PSR J1741-2054 using the $Chandra$ ACIS-S detector totaling $sim$300 ks. By registering this new epoch of observations to an archival observation taken 3.2 years earlier using X-ray point sources in the field of view, we have measured the pulsar proper motion at $mu =109 pm 10 {rm mas yr}^{-1}$ in a direction consistent with the symmetry axis of the observed H$alpha$ nebula. We investigated the inferred past trajectory of the pulsar but find no compelling association with OB associations in which the progenitor may have originated. We confirm previous measurements of the pulsar spectrum as an absorbed power law with photon index $Gamma$=2.68$pm$0.04, plus a blackbody with an emission radius of (4.5$^{+3.2}_{-2.5})d_{0.38}$ km, for a DM-estimated distance of $0.38d_{0.38}$ kpc and a temperature of $61.7pm3.0$ eV. Emission from the compact nebula is well described by an absorbed power law model with a photon index of $Gamma$ = 1.67$pm$0.06, while the diffuse emission seen as a trail extending northeast of the pulsar shows no evidence of synchrotron cooling. We also applied image deconvolution techniques to search for small-scale structures in the immediate vicinity of the pulsar, but found no conclusive evidence for such structures.
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