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Research

I am broadly interested in high-energy astrophysics, Galactic center science and cosmic-ray physics. My research has focused on X-ray observation of the Galactic center/Galactic plane and X-ray instrumentation. In the following is a summary of major results obtained by my research.

X-ray Flares from Sgr A*

The Galactic center supermassive black hole Sagittarius A* (Sgr A*) is the closest such object and is thus an ideal target for investigation of galactic nuclei and their activity cycles. Sgr A* is an underluminous black hole with a bolometric luminosity about 10^-9 times its Eddington luminosity. Its current X-ray quiescent state is punctuated by flares. These flares are believed to take place at only a few Schwarzschild radius from the supermassive black hole.Therefore, they hold the key to derive the physical properties (e.g. magnetic field strength, electron energy and density) under strong gravity. The necessary first step to fulfill this goal is to determine the flare radiation mechanism through spectral analysis. I analyzed the ~1 Ms 2012-2015 NuSTAR Sgr A* observations, and found a total of nine flares using Bayesian block analysis. My current flare data show that the flares can have a range of photon index values. There is a trend that brighter flares process harder spectra , though with huge error bars. Such phenomena can be explained by the viable radiation model magnetic reconnection.

In the 2016-2017 NuSTAR Galactic center legacy program, we have allocated ~470 ks exposure time to Sgr A* multi-wavelength observation campaign, aiming at detecting at least one very bright flare and simultaneously determining its spectra in different wavelengths. This is crucial to pin down the flare radiation mechanism.


My Related Publications

4. Zhang, S., Baganoff, F. K., Barriere, N. M., Tomsick, J. A., Ponti, G., Neilsen, J., Dexter, J., Mori, K., Hailey, C. J., et al., 2016, in prep.
Hard X-ray Flaring Properties of Sagittarius A* During NuSTAR Galactic Center Monitoring

3. Li, Y. P., Yuan, F., Yuan, Q., Wang, Q. D., Chen, P. F., Neilsen, J., Fang, T. T., Zhang, S., & Dexter, J., 2015, ApJ, 810, 1
Statistics of X-ray Flares of Sagittarius A*: Evidence for Solar like Self-organized Criticality Phenomena.

2. Feng, H., Ho, L. C., Kaaret, P., Tao, L., Yamaoka, K., Zhang, S., & Grise, F., 2015, ApJ, 807, 185
A Luminous X-ray Flare from The Nucleus of The Dormant Bulgeless Spiral Galaxy NGC 247

1. Barriere, N. M., Tomsick, J. A., Baganoff, F. K., Boggs, S. E., Christensen, F. E., Craig, W. W, Dexter, J., Grefenstette, B., Hailey, C. J., Harrison, F. A., Madsen, K. K., Mori, K., Stern, D., Zhang, W. W., Zhang, S., & Zoglauer, A., 2014, ApJ, 786, 46
NuSTAR Detection of High-energy X-ray Emission and Rapid Variability from Sagittarius A* Flares.



Past Giant X-ray Sgr A* Outburst reflected by Molecular Clouds

Whether Sgr A* has ever experienced more substantial increases in activity as observed in low-luminosity Active Galactic Nuclei is under discussion. Indication of more substantial increases of Sgr A* activity has come from Galactic center molecular clouds.

In the populous Central Molecular Zone, Sgr B2 is the densest and most massive molecular cloud. Both its iron line and the continuum emission have decayed since 2001, which has been explained as due to reflection of a giant Sgr A* X-ray outburst about 100 years ago. I studied the remaining hard X-ray emission from Sgr B2 in 2013 and investigated its origin. Two prominent substructres of Sgr B2 are resolved above 10 keV: a newly emerging cloud G0.66-0.13 and the central Sgr B2 90" radius region, which contains compact cores and surrounding diffuse emission. Due to the lack of statistics, I found it inconclusive whether the remaining level of Sgr B2 emission is still decreasing or has reached a constant background level in 2013. A decreasing X-ray emission can be best explained by the X-ray reflection model. In this scenario, I for the first time test the reflection model in a self-consistent way using both the iron line and the continuum emission. The past Sgr A* outburst luminosity is constrained to L~5x10^38 erg/s. Interestingly, the newly discovered cloud feature G0.66-0.13 shows different timing variability. I interpreted it as a molecular clump located in the Sgr Bw envelope reflecting the same Sgr A* outburst. In contrast, if the Sgr B2 X-ray emission has reached a constant background level, it would reveal a contribution from low-energy cosmic-ray proton (LECRp) bombardment. In this scenario, I calculated the required cosmic-ray ion power as dW/dt=(1-4)x10^39 erg/s. Future observations will distinguish between a continually decreasing and a constant X-ray emission from Sgr B2.


My Related Publications

2. Zhang, S., Hailey, C. J., Mori, K., Clavel, M., Terrier, R., Goldwurm, A., Bauer, F. E., Boggs, S. E., Craig, W. W., Christensen, F. E., Harrison, F. A., Hong, J., Nynka, M., Stern, D., Soldi, S., Tomsick, J. A., & Zhang, W. W., 2015, APJ, 815, 132
Hard X-ray Morphological and Spectral Studies of the Galactic Center Molecular Cloud Sgr B2: Constraining Past Sgr A* Flaring Activity.

1. Mori, K., Hailey, C. J., Krivonos, R., Hong, J., Ponti G., Bauer, F., Perez, K., Nynka, M., Zhang, S., Tomsick, J. A., Alexander, D. M, Baganoff, F. K., Barret, D., Barriere, N. M., Boggs, S. E., Canipe, A. M., Christensen, F. E., Craig, W. W., Forster, K., Giommi, P., Grefenstette, B. W., Grindlay, J. E., Harrison, F. A., Hornstrup, A., Kitaguchi, T., Koglin, J. E., Luu, V., Madsen, K. K., Mao, P. H., Miyasaka, H., Perri, M., Pivovaroff, M. J., Puccetti, S., Rana, V., Stern, D., Westergaad, N. J., Zhang, W. W., Zoglauer, A. 2015, ApJ, 814, 94
NuSTAR Hard X-ray Survey of the Galactic Center Region I: Hard X-ray Morphology and Spectroscopy of the Diffuse Emission.



G2's Effect on Future Sgr A* Activities

The gas cloud G2 orbits the Galactic center on a highly eccentric trajectory with a pericenter passage of only a few thousand Schwarzschild radii. This rare event is supposed to provide valuable information on the physics of accretion flows and the environment of the black hole. The sudden brightening-up of the Galactic center in early 2013 turned out not related to the G2 event but instead led us to discover a Galactic center transient magnetar SGR J1745-29. The NuSTAR monitoring of Sgr A* in 2012-2015, during the G2 pericenter passage, showed no evidence of significantly increased quiescent luminosity of Sgr A*. However, a recent study showed that the flare rate and luminosity a few months after the G2 pericenter passage are higher than before. It is uncertain whether this is caused by the G2 infall, or simply due to more frequent monitoring of Sgr A* and flare clusters. Our future Sgr A* observations will reveal whether the G2 infall can affect the Sgr A* flare behaviors.


My Related Publications

2. Ponti, G., De Marco, B., Morris, M. R., Merloni, A., Munoz-Darias, T., Clavel, M., Haggard, D., Zhang, S., Nandra, K., Gillessen, S., Mori, K., Neilsen, J., Rea, N., Degenaar, N., Terrier, R., & Goldwurm, A., 2015, MNRAS, 454, 1525
Fifteen Years of XMM-Newton and Chandra Monitoring of Sgr A*: Evidence for a Recent Increase in the Bright Flaring Rate.

1. Mori, K., Gotthelf, E. V., Zhang, S., An, H. J., Baganoff, F. K., Barriere, N. M., Beloborodov, A. A., Boggs, S. E., Christensen, F. E., Craig, W. W., Dufour, F., Grefenstette, B. W., Hailey, C. J., Harrison, F. A., Hong, J., Kaspi, V. M., Kennea, J. A., Madsen, K. K., Markwardt, C. B., Nynka, M., Stern, D., Tomsick, J. A., & Zhang, W. W., 2013, ApJL, 770, 23
NuSTAR Discovery of a 3.76-second Transient Magnetar Near Sagittarius A*.



Galactic Center Magnetic Filamentary Structures

Non-thermal filamentary structures are unique in the Galactic center. They serve as power tools to probe the magnetic field and relativistic particles in the Galactic center. Sgr A-E is the brightest X-ray filament detected up to ~50 keV by NuSTAR. I ruled out the PWN and SNR-cloud interaction scenarios and interpreted it as a magnetic flux tube, where locally enhanced magnetic field traps relativistic CR electrons. To produce synchrotron emission up to 50 keV, the required cosmic-ray electron energy has to be up to ~100-200 TeV. Therefore, the X-ray filaments provide evidence of existence of TeV cosmic-ray electrons in the Galactic center. I proposed that the origin of these TeV electrons could be PeV cosmic-ray proton and molecular cloud interaction.


My Related Publications

3. Zhang, S., Hailey, C. J., Baganoff, F. K., Bauer, F. E., Boggs, S. E., Craig, W. W, Christensen, F. E., Gotthelf, E. V., Harrison, F. A., Mori, K., Nynka, M., Stern, D., Tomsick, J. A., & Zhang, W. W. 2014, ApJ, 784, 6
High Energy X-Ray Detection of G359.89-0.08 (Sgr A-E): Magnetic Flux Tube Emission Powered by Cosmic Rays?

2. Nynka, M., Hailey, C. J., Zhang, S., Morris, M. M., Zhao, J. H., Goss, M., Bauer, F. E., Boggs, S. E., Craig, W. W., Christensen, F. E., Gotthelf, E. V., Harrison, F. A., Mori, K., Perez, K. M., Stern, D., & Zhang, W. W. 2015, ApJ, 800, 119
G359.97-0.038: A Hard X-Ray Filament Associated with a Supernova Shell- Molecular Cloud Interaction.

1. Mori, K., Hailey, C. J., Krivonos, R., Hong, J., Ponti G., Bauer, F., Perez, K., Nynka, M., Zhang, S., Tomsick, J. A., Alexander, D. M, Baganoff, F. K., Barret, D., Barriere, N. M., Boggs, S. E., Canipe, A. M., Christensen, F. E., Craig, W. W., Forster, K., Giommi, P., Grefenstette, B. W., Grindlay, J. E., Harrison, F. A., Hornstrup, A., Kitaguchi, T., Koglin, J. E., Luu, V., Madsen, K. K., Mao, P. H., Miyasaka, H., Perri, M., Pivovaroff, M. J., Puccetti, S., Rana, V., Stern, D., Westergaad, N. J., Zhang, W. W., Zoglauer, A. 2015, ApJ, 814, 94
NuSTAR Hard X-ray Survey of the Galactic Center Region I: Hard X-ray Morphology and Spectroscopy of the Diffuse Emission



Galactic Plane Survey

I simulated the NuSTAR Galactic plane survey using a ray-trace tool NuSim. Based on my simulation, my colleague and I made observation strategies of the whole survey. Some key results of the survey not mentioned above are: discovery of a new hard X-ray diffuse emission component (Perez et al. 2015); intermediate Polars which can explain the newly discovered hard X-ray diffuse emission(Hailey et al. 2016); hard X-ray diffuse features and point sources detected in the survey (Mori et al. 2015, Hong et al. 2016); neutron star low-mass X-ray binary (LMHB) 1E 1743.1-2843 (Lotti et al. 2016).


My Related Publications

5. Hong, J., Mori, K., Hailey, C. J., Nynka, M., Zhang, S., Gotthelf, E., Fornasini, F., Krivonos, R., Bauer, F., Perez, K., Tomsick, J. A., Bodaghee, A., Chiu, J. L., Stern, D., Grindlay, J. E. et al. 2016, submitted to ApJ
NuSTAR Hard X-ray Survey of the Galactic Center Region II: X-ray Point Sources.

4. Hailey, C. J., Mori, K., Perez, K., Canipe, A. M., Hong, J., Tomsick, J. A., Boggs, S. E., Christensen, F. E., Craig, W. W., Fornasini, F., Grindlay, J. E., Harrison, F. A., Nynka, M., Rahoui, F., Stern, D., Zhang, S., & Zhang, W. W. 2016, submitted to ApJ
Evidence for Intermediate Polars as the Origin of the Galactic Center Hard X-ray Emission.

3. Lotti, S., Natalucci, L., Mori, K., Baganoff, F. K., Boggs, S. E., Christensen, F. E., Craig, W. W., Hailey, C. J., Harrison, F. A., Hong, J., Krivonos, R. A., Rahoui, F., Stern, D., Tomsick, J. A., Zhang, S., Zhang, W. W., 2016, submitted to ApJ
NuSTAR and XMM-Newton Observations of 1E1743.1-2843: Indications of a Neutron Star LMXB Nature of the Compact Object.

2. Perez, K., Hailey, C., Bauer, F. E., Krinovos, R. A., Mori, K., Baganoff, F. K., Barriere, N. M., Boggs, S. E., Christensen, F. E., Craig, W. W., Grefenstette, B. W., Grindlay, J. E., Harrison, F. A., Hong, J., Madsen, K. K., Nynka, M., Stern, D., Tomsick, J. A., Wik, D., R., Zhang, S., Zhang, W. W., & Zoglauer, A., 2015, Nature, 520, 646
Extended Hard X-ray Emission in the Inner Few Parsecs of the Galaxy.

1. Nynka, M., Hailey, C. J., Mori, K., Baganoff, F. K., Bauer, F. E., Boggs, S. E., Craig, W. W., Christensen, F. E., Gotthelf, E. V., Harrison, F. A., Hong, J., Perez, K. M., Stern, D., Zhang, S., & Zhang, W. W, 2013, ApJL, 778, 31
High-energy X-rays from J174545.5-285829, the Cannonball: A Candidate Pulsar.

Webmaster: Shuo Zhang, last updates: Jan. 2016