https://doi.org/10.1140/epjc/s10052-024-13721-6
Regular Article - Theoretical Physics
X-ray pulsed light curves of highly compact neutron stars as probes of scalar–tensor theories of gravity
1
PPGCosmo, Universidade Federal do Espírito Santo, Av. Fernando Ferrari, 29075-910, Vitória, Espirito Santo, Brazil
2
ICRANet-Ferrara, Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, 44122, Ferrara, FE, Italy
3
Núcleo de Astrofísica e Cosmologia (Cosmo-Ufes) and Departamento de Física, Universidade Federal do Espírito Santo, Av. Fernando Ferrari, 29075-910, Vitória, ES, Brazil
4
Departamento de Física, Universidade do Estado de Santa Catarina, Rua Paulo Malschitzki, 89219-710, Joinville, SC, Brazil
5
Departamento de Astronomia, Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG), Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-090, São Paulo, Brazil
6
Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716, Warsaw, Poland
7
ICRANet, Piazza della Repubblica 10, 65122, Pescara, Italy
8
ICRA, Dipartamento di Fisica, Sapienza Universitá di Roma, Piazzale Aldo Moro 5, 00185, Rome, Italy
9
Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122, Ferrara, Italy
10
INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133, Rome, Italy
Received:
10
September
2024
Accepted:
15
December
2024
Published online:
30
December
2024
The strong gravitational potential of neutron stars (NSs) makes them ideal astrophysical objects for testing extreme gravity phenomena. We explore the potential of NS X-ray pulsed light curve observations to probe deviations from general relativity (GR) within the scalar–tensor theory (STT) of gravity framework. We compute the flux from a single, circular, finite-size hot spot, accounting for light bending, Shapiro time delay, and Doppler effect. We focus on the high-compactness regime, i.e., close to the critical GR value 
, over which multiple images of the spot appear and impact crucially the light curves. Our investigation is motivated by the increased sensitivity of the pulse to the scalar charge of the spacetime in such high compactness regimes, making these systems exceptionally suitable for scrutinizing deviations from GR, notably phenomena such as spontaneous scalarization, as predicted by STT. We find significant differences in NS observables, e.g., the flux of a single spot can differ up to 80% with respect to GR. Additionally, reasonable choices for the STT parameters that satisfy astrophysical constraints lead to changes in the NS radius relative to GR of up to approximately 10%. Consequently, scalar parameters might be better constrained when uncertainties in NS radii decrease, where this could occur with the advent of next-generation gravitational wave detectors, such as the Einstein Telescope and LISA, as well as future electromagnetic missions like eXTP and ATHENA. Thus, our findings suggest that accurate X-ray data of the NS surface emission, jointly with refined theoretical models, could constrain STTs.
© The Author(s) 2024
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Funded by SCOAP3.
