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Post Info TOPIC: SGR J1745-2900
Blobrana


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Title: A possible origin of the Galactic Center magnetar SGR 1745-2900
Author: Quan Cheng, Shuang-Nan Zhang, Xiao-Ping Zheng

Since a large population of massive O/B stars and putative neutron stars (NSs) located in the vicinity of the Galactic center (GC), intermediate-mass X-ray binaries (IMXBs) constituted by a NS and a B-type star probably exist here. We investigate the evolutions of accreting NSs in IMXBs (similar to M82 X-2) with a ~5.2 solar mass companion, and orbit period \simeq 2.53 day. By adopting an mildly super-Eddington rate \dot{M}=6 x 10^-8 solar masses yr^-1 for the early Case B Roche-lobe overflow (RLOF) accretion, we find only in accreting NSs with quite elastic crusts (slippage factor s=0.05), the toroidal magnetic fields can be amplified within 1 Myrs, which is assumed to be the longest duration of the RLOF. These IMXBs will evolve into NS+white dwarf (WD) binaries if they are dynamical stable. However, before the formation of NS+WD binaries, the high stellar density in the GC will probably lead to frequent encounters between the NS+evolved star binaries (in post-early Case B mass transfer phase) and NSs or exchange encounters with other stars, which may produce single NSs. These NSs will evolve into magnetars when the amplified poloidal magnetic fields diffuse out to the NS surfaces. Consequently, our results provide a possible explanation for the origin of the GC magnetar SGR 1745-2900. Moreover, the accreting NSs with s>0.05 will evolve into millisecond pulsars (MSPs). Therefore, our model reveals the GC magnetars and MSPs could both originate from the special kind of IMXBs.

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Blobrana


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RE: SGR J1745-2900
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Title: Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
Author: F. Yusef-Zadeh, R. Diesing, M. Wardle, L. O. Sjouwerman, M. Royster, W. D. Cotton, D. Roberts, C. Heinke

We present radio continuum light curves of the magnetar SGR J1745-2900 and Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations between 2012 and 2014. During this period, a powerful X-ray outburst from SGR J1745-2900 occurred on 2013-04-24. Enhanced radio emission is delayed with respect to the X-ray peak by about seven months. In addition, the flux density of the emission from the magnetar fluctuates by a factor of 2 to 4 at frequencies between 21 and 41 GHz and its spectral index varies erratically. Here we argue that the excess fluctuating emission from the magnetar arises from the interaction of a shock generated from the X-ray outburst with the orbiting ionised gas at the Galactic center. In this picture, variable synchrotron emission is produced by ram pressure variations due to inhomogeneities in the dense ionised medium of the Sgr A West bar. The pulsar with its high transverse velocity is moving through a highly blue-shifted ionised medium. This implies that the magnetar is at a projected distance of ~0.1 pc from Sgr A* and that the orbiting ionised gas is partially or largely responsible for a large rotation measure detected toward the magnetar. Despite the variability of Sgr A* expected to be induced by the passage of the G2 cloud, monitoring data shows a constant flux density and spectral index during this period

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Blobrana


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SGR 1745-2900: Magnetar Near Supermassive Black Hole Delivers Surprises

Since its discovery two years ago when it gave off a burst of X-rays, astronomers have been actively monitoring the magnetar, dubbed SGR 1745-2900, with Chandra and the European Space Agency's XMM-Newton. The main image of the graphic shows the region around the Milky Way's black hole in X-rays from Chandra (red, green, and blue are the low, medium, and high-energy X-rays respectively). The inset contains Chandra's close-up look at the area right around the black hole, showing a combined image obtained between 2005 and 2008 (left) when the magnetar was not detected, during a quiescent period, and an observation in 2013 (right) when it was caught as a bright point source during the X-ray outburst that led to its discovery.
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Blobrana


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ATel 5922: Continued Chandra monitoring observations of the Galactic Center magnetar

 



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Blobrana


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A strong magnetic field around the supermassive black hole at the centre of the Galaxy

Earths nearest candidate supermassive black hole lies at the centre of the Milky Way1. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment



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Blobrana


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RE: SGR J1745-2900
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Title: Radio properties of the magnetar near Sagittarius A* from observations with the Australia Telescope Compact Array
Authors: Ryan M. Shannon, Simon Johnston (CSIRO Astronomy and Space Science)

We have carried out observations of the newly-discovered magnetar in the direction of Sagittarius A* using the Australia Telescope Compact Array in four frequency bands from 4.5 to 20 GHz. Radio pulsations are clearly detected at all frequencies. We measure the pulsar's dispersion measure to be 1650 ± 50 cm^-3 pc, the highest of any of the known pulsars. Once Faraday rotation has been taken into account, the pulse profile is almost completely linearly polarized at all frequencies and has a small degree of circular polarization. The rotation measure of -67000 ± 500 rad m^-2 is the largest ever measured in an astronomical object. The combination of the dispersion and rotation measures implies an integrated magnetic field strength of -50 uG along the line of sight. This object therefore joins the small class of radio emitting magnetars. Follow-up observations using single dishes are underway and will no doubt characterise this object further.

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Title: Swift Discovery of a New Soft Gamma Repeater, SGR J1745-29, near Sagittarius A*
Authors: J. A. Kennea, D. N. Burrows, C. Kouveliotou, D. M. Palmer, E. Gogus, Y. Kaneko, P. A. Evans, N. Degenaar, M. Reynolds, J. M. Miller, R. Wijnands, K. Mori, N. Gehrels

Starting in February 2013, Swift has been performing short daily monitoring observations of the G2 gas cloud near Sgr A* with the X-ray Telescope to determine whether the cloud interaction leads to an increase in the flux from the Galactic center. On 2013 April 24 Swift detected an order of magnitude rise in the X-ray flux from the region near Sgr A*. Initially thought to be a flare from Sgr A*, detection of a short hard X-ray burst from the same region by the Burst Alert Telescope suggested that the flare was from an unresolved new Soft Gamma Repeater, SGR J1745-29. Here we present the discovery of SGR J1745-29 by Swift, including analysis of data before, during, and after the burst. We find that the spectrum in the 0.3-10 keV range is well fit by an absorbed blackbody model with kTBB ~ 1.1 keV and absorption consistent with previously measured values from the quiescent emission from Sgr A*, strongly suggesting that this source is at a similar distance. Only one SGR burst has been detected so far from the new source, and the persistent light curve shows little evidence of decay in approximately 2 weeks of monitoring after outburst. We discuss this light curve trend and compare it with those of other well covered SGR outbursts.

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Title: NuSTAR discovery of a 3.76-second transient magnetar near Sagittarius A*
Authors: Kaya Mori, Eric V. Gotthelf, Shuo Zhang, Hongjun An, Frederick K. Baganoff, Nicolas M. Barriere, Andrei Beloborodov, Steven E. Boggs, Finn E. Christensen, William W. Craig, Francois Dufour, Brian W. Grefenstette, Charles J. Hailey, Fiona Anne Harrison, Jaesub Hong, Victoria M. Kaspi, Jamie A. Kennea, Kristin K. Madsen, Craig B. Markwardt, Melania Nynka, Daniel Stern, John Tomsick, William Zhang

We report the discovery of 3.76-s pulsations from the new burst source in the direction of Sgr A* using data obtained with the NuSTAR Observatory. We detect a strong signal from SGR J1745-2900 presenting a complex pulse profile modulated with pulsed fraction 27±3 % in the 3-10 keV band. Two observations spaced 9 days apart allow us to determine a preliminary spin-down rate of Pdot = (6.8±1.5)x10^{-12}. This implies a magnetic field B = 1.6x10^14 G, spin-down power Edot = 5.0x10^33 erg/s, and characteristic age P/2Pdot = 8.8x10^3 yr, for the rotating dipole model. However, we note that the current Pdot may be erratic, especially during outburst. The flux and modulation remained steady during the observations and the 3-79 keV spectrum is well fitted by a combined blackbody plus power-law model with temperature kT_BB = 0.96±0.02 keV and photon index = 1.5±0.4, respectively. The neutral hydrogen column density (nH ~ 1.4x10^23 cm^{-2}) measured by NuSTAR and Swift suggests that SGR J1745-2900 is located at or near the Galactic Center. The lack of an X-ray counterpart to SGR J1745-2900 in the published Chandra survey catalogue sets a quiescent 2-8 keV luminosity limit of Lx ~< 10^32 erg/s. The bursting, timing, and spectral properties of SGR J1745-2900 indicate the source is a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5x10^35 erg/s for a distance of 8 kpc.

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