Shadow, spectrum, and radiative efficiency of a thin accretion disk around the Janis–Newman–Winicour naked singularity
Dilshodbek Bardiev
1,*
1 Ulugh Beg Astronomical Institute , Astronomy St. 33, Tashkent 100052, Uzbekistan
Abstract
A central question raised by the weak cosmic censorship conjecture is whether gravitational collapse can leave behind a naked singularity that is, even in principle, observationally distinguishable from a black hole of the same mass. We address this question for the Janis–Newman–Winicour (JNW) spacetime, the unique static, spherically symmetric, asymptotically flat solution of Einstein gravity minimally coupled to a massless scalar field, which describes a naked curvature singularity for hair parameter 0 < γ < 1 and reduces to the Schwarzschild black hole at γ = 1. Modeling the disk matter as a scalar-coupled test particle with effective inertial mass \(m_*=m(1+g_s\Phi)\), a prescription previously applied to the Bocharova–Bronnikov–Melnikov–Bekenstein spacetime, we build a Novikov–Thorne thin accretion disk around the JNW geometry and compute three largely independent horizon-scale observables: the radiative efficiency, the disk-integrated spectrum, and the ray-traced image. The two structural thresholds that organize our analysis – the existence of a photon sphere (γ > 1/2) and the existence of a stable disk inner edge \(\gamma_c\equiv1/\sqrt{5}\approx0.447\) – are individually known, but we stress that they are numerically distinct, so that a narrow window \(\gamma_c<\gamma<1/2\) exists in which an ordinary accretion disk forms around a naked singularity that possesses no shadow boundary at all. Outside this window, the shadow size, radiative efficiency, and disk spectrum all vary only smoothly and quantitatively with γ and \(g_s\), so that cleanly separating a JNW naked singularity from a same-mass Schwarzschild black hole is considerably harder. We cross-validate the ray-traced shadow area against an independent analytic calculation of the critical impact parameter, finding agreement to within 0.2%, and discuss the prospects and limitations of using each of the three observables to discriminate a naked singularity from a black hole.
References
[1] Penrose, R., Riv. Nuovo Cimento 1, 252 (1969), reprinted in Gen. Relativ. Gravit. 34, 1141 (2002)
[2] Event Horizon Telescope Collaboration, Astrophys. J. Lett. 875, L1 (2019)
[3] Event Horizon Telescope Collaboration, Astrophys. J. Lett. 875, L6 (2019)
[4] Event Horizon Telescope Collaboration, Astrophys. J. Lett. 930, L12 (2022)
[5] Shaikh, R. and Joshi, P. S., J. Cosmol. Astropart. Phys. 2019 (10), 064
[6] Novikov, I. D. and Thorne, K. S., in Black Holes, edited by DeWitt, C. and DeWitt, B. S. (Gordon and Breach, New York, 1973), pp. 343–450
[7] Page, D. N. and Thorne, K. S., Astrophys. J. 191, 499 (1974)
[8] Thorne, K. S., Astrophys. J. 191, 507 (1974)
[9] Bardeen, J. M., Press, W. H. and Teukolsky, S. A., Astrophys. J. 178, 347 (1972)
[10] Shakura, N. I. and Sunyaev, R. A., Astron. Astrophys. 24, 337 (1973)
[11] Shakura, N. I. and Sunyaev, R. A., Mon. Not. R. Astron. Soc. 175, 613 (1976)
[12] Luminet, J. P., Astron. Astrophys. 75, 228 (1979)
[13] Hollywood, J. M. and Melia, F., Astrophys. J. 443, L17 (1995)
[14] Hollywood, J. M., Melia, F., Close, L. M. et al., Astrophys. J. Lett. 448, L21 (1995)
[15] Hollywood, J. M. and Melia, F., Astrophys. J. Suppl. 112, 423 (1997)
[16] Kulkarni, A. K., Penna, R. F., Shcherbakov, R. V. et al., Mon. Not. R. Astron. Soc. 414, 1183 (2011)
[17] James, O., von Tunzelmann, E., Franklin, P. and Thorne, K. S., Class. Quantum Grav. 32, 065001 (2015)
[18] Rahaman, F., Manna, T., Shaikh, R. et al., Nucl. Phys. B 972, 115548 (2021)
[19] Zheng, H.-B., Wu, M.-Q., Li, G.-P. and Jiang, Q.-Q., Eur. Phys. J. C 85, 46 (2025)
[20] Wang, P., Guo, S., Li, L.-F. et al., Eur. Phys. J. C 85, 747 (2025)
[21] Schee, J. and Stuchlík, Z., J. Cosmol. Astropart. Phys. 2022 (1), 054
[22] Schee, J. and Stuchlík, Z., Eur. Phys. J. C 79, 988 (2019)
[23] Li, S., Mirzaev, T., Abdujabbarov, A. A. et al., Phys. Rev. D 106, 084041 (2022)
[24] Boshkayev, K., Konysbayev, T., Kurmanov, Y. et al., Astrophys. J. 936, 96 (2022)
[25] Boshkayev, K., Konysbayev, T., Kurmanov, E. et al., Phys. Rev. D 104, 084009 (2021)
[26] Boshkayev, K., Idrissov, A., Luongo, O. and Malafarina, D., Mon. Not. R. Astron. Soc. 496, 1115 (2020)
[27] Heydari-Fard, M., Honarvar, S. G. and Heydari-Fard, M., Mon. Not. R. Astron. Soc. 521, 708 (2023)
[28] Banerjee, I., Mandal, B. and SenGupta, S., Mon. Not. R. Astron. Soc. 500, 481 (2021)
[29] Chowdhury, A. N., Patil, M., Malafarina, D. and Joshi, P. S., Phys. Rev. D 85, 104031 (2012)
[30] Gyulchev, G., Nedkova, P., Vetsov, T. and Yazadjiev, S., Phys. Rev. D 100, 024055 (2019)
[31] Gyulchev, G., Kunz, J., Nedkova, P. et al., Eur. Phys. J. C 80, 1017 (2020)
[32] Turimov, B., Rahimov, O., Davlataliev, A. et al., Phys. Dark Univ. 48, 101855 (2025)
[33] Turimov, B., Usanov, S. and Khamroev, Y., Phys. Dark Univ. 48, 101876 (2025)
[34] Turimov, B., Davlataliev, A., Usmanov, Y. et al., Eur. Phys. J. C 84, 1098 (2024)
[35] Turimov, B., Urinov, S., Mardiev, I. et al., Arab. J. Math. 15, 421 (2026)
[36] Brans, C. and Dicke, R. H., Phys. Rev. 124, 925 (1961)
[37] Damour, T. and Esposito-Farese, G., Class. Quantum Grav. 9, 2093 (1992)
[38] Damour, T. and Esposito-Farese, G., Phys. Rev. Lett. 70, 2220 (1993)
[39] Turimov, B., Davlataliev, A., Abdujabbarov, A. and Ahmedov, B., Phys. Rev. D 110, 084053 (2024)
[40] Janis, A. I., Newman, E. T. and Winicour, J., Phys. Rev. Lett. 20, 878 (1968)
[41] Fisher, I. Z., Zh. Eksp. Teor. Fiz. 18, 636 (1948), English translation available on arXiv (1999), arXiv:gr-qc/9911008
[42] Bergmann, O. and Leipnik, R., Phys. Rev. 107, 1157 (1957)
[43] Buchdahl, H. A., Phys. Rev. 115, 1325 (1959)
[44] Wyman, M., Phys. Rev. D 24, 839 (1981)
[45] Virbhadra, K. S., Int. J. Mod. Phys. A 12, 4831 (1997)
[46] Joshi, A. B., Dey, D., Joshi, P. S. and Bambhaniya, P., Phys. Rev. D 102, 024022 (2020)
Bardiev, D., Shadow, spectrum, and radiative efficiency of a thin accretion disk around the Janis–Newman–Winicour naked singularity, Turanian J. Vol. 2, No. 3 (020303), 2026.