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Professor Tim Roberts' Outputs (139)

Skipping a beat: discovery of persistent quasi-periodic oscillations associated with pulsed fraction drop of the spin signal in M51 ULX-7 (2024)
Journal Article
Imbrogno, M., Motta, S. E., Amato, R., Israel, G. L., Rodríguez-Castillo, G. A., Brightman, M., Casella, P., Bachetti, M., Fürst, F., Stella, L., Pinto, C., Pintore, F., Tombesi, F., Gúrpide, A., Middleton, M. J., Salvaggio, C., Tiengo, A., Belfiore, A., De Luca, A., Esposito, P., …Salvaterra, R. (2024). Skipping a beat: discovery of persistent quasi-periodic oscillations associated with pulsed fraction drop of the spin signal in M51 ULX-7. Astronomy & Astrophysics, 689, Article A284. https://doi.org/10.1051/0004-6361/202450432

Examining the nature of the ultraluminous X-ray source Holmberg II X-1 (2024)
Journal Article
Barra, F., Pinto, C., Middleton, M., Di Salvo, T., Walton, D. J., Gúrpide, A., & Roberts, T. P. (2024). Examining the nature of the ultraluminous X-ray source Holmberg II X-1. Astronomy & Astrophysics, 682, Article A94. https://doi.org/10.1051/0004-6361/202348471

We present a comprehensive spectral analysis of the ultraluminous X-ray source Holmberg II X-1 using broadband and high-resolution X-ray spectra taken with the XMM-Newton satellite over a period of 19 yr, taking advantage of data from a recent campai... Read More about Examining the nature of the ultraluminous X-ray source Holmberg II X-1.

NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation (2024)
Journal Article
Luangtip, W., & Roberts, T. P. (2024). NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation. Monthly Notices of the Royal Astronomical Society, 528(1), 418-431. https://doi.org/10.1093/mnras/stae023

We present a study of the X-ray source NGC 2403 XMM4 (4XMM J073702.2+653934) based on 20 yr of archival observations
with XMM–Newton, Chandra, Swift, and NuSTAR. Although it has previously been classified as an ultraluminous X-ray source
(ULX), we... Read More about NGC 2403 XMM4: evidence for a super-Eddington neutron star with a possible transient pulsation.

The high energy X-ray probe ( HEX-P ): studying extreme accretion with ultraluminous X-ray sources (2023)
Journal Article
Bachetti, M., Middleton, M. J., Pinto, C., Gúrpide, A., Walton, D. J., Brightman, M., …Wolter, A. (2023). The high energy X-ray probe ( HEX-P ): studying extreme accretion with ultraluminous X-ray sources. Frontiers in Astronomy and Space Sciences, 10, Article 1289432. https://doi.org/10.3389/fspas.2023.1289432

Introduction: Ultraluminous X-ray sources (ULXs) represent an extreme class of accreting compact objects: from the identification of some of the accretors as neutron stars to the detection of powerful winds travelling at 0.1–0.2 c, the increasing evi... Read More about The high energy X-ray probe ( HEX-P ): studying extreme accretion with ultraluminous X-ray sources.

Digging a little deeper: characterizing three new extreme ULX candidates (2023)
Journal Article
Roberts, T. P., Walton, D. J., Mackenzie, A. D. A., Heida, M., & Scaringi, S. (2023). Digging a little deeper: characterizing three new extreme ULX candidates. Monthly Notices of the Royal Astronomical Society, 525(3), 3330-3343. https://doi.org/10.1093/mnras/stad2367

A prime motivation for compiling catalogues of any celestial X-ray source is to increase our numbers of rare subclasses. In this work, we take a recent multimission catalogue of ultraluminous X-ray sources (ULXs) and look for hitherto poorly-studied... Read More about Digging a little deeper: characterizing three new extreme ULX candidates.

The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: The Case of the Circumnuclear Starburst Ring in NGC 7552 (2023)
Journal Article
West, L., Garofali, K., Lehmer, B. D., Prestwich, A., Eufrasio, R., Luangtip, W., …Zezas, A. (2023). The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: The Case of the Circumnuclear Starburst Ring in NGC 7552. Astrophysical Journal, 952(1), Article 22. https://doi.org/10.3847/1538-4357/acd9aa

Luminous infrared galaxies (LIRGs), the most extreme star-forming galaxies in the nearby (D < 30 Mpc) universe, show a notable X-ray emission deficiency (up to a factor of ∼10) compared with predictions from scaling relations of galaxy-wide high-mass... Read More about The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: The Case of the Circumnuclear Starburst Ring in NGC 7552.

Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1 (2023)
Journal Article
Fürst, F., Walton, D. J., Israel, G. L., Bachetti, M., Barret, D., Brightman, M., …Webb, N. (2023). Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1. Astronomy & Astrophysics, 672, Article A140. https://doi.org/10.1051/0004-6361/202245048

NGC 5907 ULX1 is the most luminous ultra-luminous X-ray pulsar (ULXP) known to date, reaching luminosities in excess of 1041 erg s−1. The pulsar is known for its fast spin-up during the on-state. Here, we present a long-term monitoring of the X-ray f... Read More about Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1.

The hyperluminous X‐ray source population (2023)
Journal Article
MacKenzie, A., Roberts, T., & Walton, D. (2023). The hyperluminous X‐ray source population. Astronomische Nachrichten, 344(4), Article e230028. https://doi.org/10.1002/asna.20230028

We have recently published a catalog of 1843 candidate ultraluminous X-ray sources (ULXs). This is the largest catalog of ULXs to date and was built by cross-correlating recent serendipitous source catalogs from the XMM-Newton, Swift, and Chandra obs... Read More about The hyperluminous X‐ray source population.

Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1 (2022)
Journal Article
Barra, F., Pinto, C., Walton, D., Kosec, P., D’Aì, A., Di Salvo, T., Del Santo, M., Earnshaw, H., Fabian, A., Fuerst, F., Marino, A., Pintore, F., Robba, A., & Roberts, T. (2022). Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1. Monthly Notices of the Royal Astronomical Society, 516(3), 3972-3983. https://doi.org/10.1093/mnras/stac2453

Ultraluminous X-ray sources (ULXs) are the most extreme among X-ray binaries in which the compact object, a neutron star or a black hole, accretes matter from the companion star, and exceeds a luminosity of 1039 ergs−1 in the X-ray energy band alone.... Read More about Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1.

The variability behavior of NGC 925 ULX-3 (2022)
Journal Article
Earnshaw, H. P., Brightman, M., Harrison, F. A., Heida, M., Jaodand, A., Middleton, M. J., Roberts, T. P., & Walton, D. J. (2022). The variability behavior of NGC 925 ULX-3. Astrophysical Journal, 934(1), Article 42. https://doi.org/10.3847/1538-4357/ac79b0

We report the results of a 2019-2021 monitoring campaign with Swift and associated target-of-opportunity observations with XMM-Newton and NuSTAR, examining the spectral and timing behavior of the highly variable ultraluminous X-ray source (ULX) NGC 9... Read More about The variability behavior of NGC 925 ULX-3.

A transient ultraluminous X-ray source in NGC 55 (2022)
Journal Article
Robba, A., Pinto, C., Pintore, F., Rodriguez, G., Ambrosi, E., Barra, F., Cusumano, G., D’Aì, A., Del Santo, M., Kosec, P., Marino, A., Middleton, M., Roberts, T., Salvaggio, C., Soria, R., Wolter, A., & Walton, D. (2022). A transient ultraluminous X-ray source in NGC 55. Monthly Notices of the Royal Astronomical Society, 515(4), 4669-4674. https://doi.org/10.1093/mnras/stac2018

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1039 erg s−1. The average number of ULXs per galaxy is still not well constrained, especially given the uncertainty on the fraction of ULX trans... Read More about A transient ultraluminous X-ray source in NGC 55.

A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2 (2022)
Journal Article
Sathyaprakash, R., Roberts, T., Grisé, F., Kaaret, P., Ambrosi, E., Done, C., Gladstone, J., Kajava, J., Soria, R., & Zampieri, L. (2022). A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2. Monthly Notices of the Royal Astronomical Society, 511(4), 5346-5362. https://doi.org/10.1093/mnras/stac402

NGC 1313 X-2 is one of the few known pulsating ultraluminous X-ray sources (PULXs), and so is thought to contain a neutron star that accretes at highly super-Eddington rates. However, the physics of this accretion remains to be determined. Here, we r... Read More about A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2.

The impact of precession on the observed population of ULXs (2021)
Journal Article
Khan, N., Middleton, M., Wiktorowicz, G., Dauser, T., Roberts, T., & Wilms, J. (2022). The impact of precession on the observed population of ULXs. Monthly Notices of the Royal Astronomical Society, 509(2), 2493-2506. https://doi.org/10.1093/mnras/stab3049

The discovery of neutron stars (NSs) powering several ultraluminous X-ray sources (ULXs) raises important questions about the nature of the underlying population. In this paper, we build on previous work studying simulated populations by incorporatin... Read More about The impact of precession on the observed population of ULXs.

A multimission catalogue of ultraluminous X-ray source candidates (2021)
Journal Article
Walton, D., Mackenzie, A., Gully, H., Patel, N., Roberts, T., Earnshaw, H., & Mateos, S. (2022). A multimission catalogue of ultraluminous X-ray source candidates. Monthly Notices of the Royal Astronomical Society, 509(2), 1587-1604. https://doi.org/10.1093/mnras/stab3001

We present a new, multimission catalogue of ultraluminous X-ray source (ULX) candidates, based on recent data releases from each of the XMM–Newton, Swift, and Chandra observatories (the 4XMM-DR10, 2SXPS, and CSC2 catalogues, respectively). This has b... Read More about A multimission catalogue of ultraluminous X-ray source candidates.

The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1 (2021)
Journal Article
D'a, ì. A., Pinto, C., Del Santo, M., Pintore, F., Soria, R., Robba, A., Ambrosi, E., Alston, W., Barret, D., Fabian, A., Fürst, F., Kara, E., Kosec, P., Middleton, M., Roberts, T., Rodriguez-Castillo, G., & Walton, D. (2021). The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1. Monthly Notices of the Royal Astronomical Society, 507(4), 5567-5579. https://doi.org/10.1093/mnras/stab2427

Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems... Read More about The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1.

Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2 (2021)
Journal Article
Robba, A., Pinto, C., Walton, D., Soria, R., Kosec, P., Pintore, F., Roberts, T., Alston, W., Middleton, M., Cusumano, G., Earnshaw, H., Fuerst, F., Sathyaprakash, R., Kyritsis, E., & Fabian, A. (2021). Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2. Astronomy & Astrophysics, 652, Article A118. https://doi.org/10.1051/0004-6361/202140884

Context. It is thought that ultraluminous X-ray sources (ULXs) are mainly powered by super-Eddington accreting neutron stars or black holes as shown by the recent discovery of X-ray pulsations and relativistic winds. Aims. This work presents a follow... Read More about Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2.

Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13 (2021)
Journal Article
Fuerst, F., Walton, D., Heida, M., Bachetti, M., Pinto, C., Middleton, M., Brightman, M., Earnshaw, H., Barret, D., Fabian, A., Kretschmar, P., Pottschmidt, K., Ptak, A., Roberts, T., Stern, D., Webb, N., & Wilms, J. (2021). Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13. Astronomy & Astrophysics, 651, Article A75. https://doi.org/10.1051/0004-6361/202140625

Ultra-luminous X-ray pulsars (ULXPs) provide a unique opportunity to study persistent super-Eddington accretion. Here we present the results of a long-term monitoring campaign of ULXP NGC 7793 P13, focusing on the pulse period evolution and the deter... Read More about Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13.

XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows (2021)
Journal Article
Pinto, C., Soria, R., Walton, D., D'Ai, A., Pintore, F., Kosec, P., Alston, W., Fuerst, F., Middleton, M., Roberts, T., Del Santo, M., Barret, D., Ambrosi, E., Robba, A., Earnshaw, H., & Fabian, A. (2021). XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows. Monthly Notices of the Royal Astronomical Society, 505(4), 5058-5074. https://doi.org/10.1093/mnras/stab1648

Most ultraluminous X-ray sources (ULXs) are believed to be powered by super-Eddington accreting neutron stars and, perhaps, black holes. Above the Eddington rate the disc is expected to thicken and to launch powerful winds through radiation pressure.... Read More about XMM-Newton campaign on the ultraluminous X-ray source NGC 247 ULX-1: outflows.

Quasi-periodic dipping in the ultraluminous X-ray source, NGC 247 ULX-1 (2021)
Journal Article
Alston, W., Pinto, C., Barret, D., D'Ai, A., Del Santo, M., Earnshaw, H., Fabian, A., Fuerst, F., Kara, E., Kosec, P., Middleton, M., Parker, M., Pintore, F., Robba, A., Roberts, T., Sathyaprakash, R., Walton, D., & Ambrosi, E. (2021). Quasi-periodic dipping in the ultraluminous X-ray source, NGC 247 ULX-1. Monthly Notices of the Royal Astronomical Society, 505(3), 3722-3729. https://doi.org/10.1093/mnras/stab1473

Most ultraluminous X-ray sources (ULXs) are believed to be stellar mass black holes or neutron stars accreting beyond the Eddington limit. Determining the nature of the compact object and the accretion mode from broad-band spectroscopy is currently a... Read More about Quasi-periodic dipping in the ultraluminous X-ray source, NGC 247 ULX-1.