arXiv subjects

P. Esposito

Publications and source records attributed to P. Esposito.

At least 37 records · Page 2Linked to original sources

arXiv2024

The restless population of bright X-ray sources of NCG 3621

We report on the multi-year evolution of the population of X-ray sources in the nuclear region of NGC 3621 based on Chandra, XMM-Newton and Swift observations. Among these, two sources, X1 and X5, after their first detection in 2008, seem to have faded below the detectability threshold, a most interesting fact as X1 is associated with the AGN of the galaxy. Two other sources, X3 and X6 are presented for the first time, the former showing a peculiar short-term variability in the latest available dataset, suggesting an egress from eclipse, hence belonging to the handful of known eclipsing ultra-luminous X-ray sources. One source, X4, previously known for its "heart-beat", i.e. a characteristic modulation in its signal with a period of $\approx1$ h, shows a steady behaviour in the latest observation. Finally, the brightest X-ray source in NGC 3621, here labelled X2, shows steady levels of flux across all the available datasets but a change in its spectral shape, reminiscent of the behaviours of Galactic disk-fed X-ray binaries.

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arXiv2024

An X-ray and radio view of the 2022 reactivation of the magnetar SGRJ1935+2154

Recently, the Galactic magnetar SGR J1935+2154 has garnered attention due to its emission of an extremely luminous radio burst, reminiscent of Fast Radio Bursts (FRBs). SGR J1935+2154 is one of the most active magnetars, displaying flaring events nearly every year, including outbursts as well as short and intermediate bursts. Here, we present our results on the properties of the persistent and bursting X-ray emission from SGR J1935+2154, during the initial weeks following its outburst on October 10, 2022. The source was observed with XMM-Newton and NuSTAR (quasi-)simultaneously during two epochs, separated by $\sim$5 days. The persistent emission spectrum is well described by an absorbed blackbody plus power-law model up to an energy of $\sim$25 keV. No significant changes were observed in the blackbody temperature ($kT_{\rm BB}\sim$ 0.4 keV) and emitting radius ($R_{\rm BB}\sim$ 1.9 km) between the two epochs. However, we observed a slight variation in the power-law parameters. Moreover, we detected X-ray pulsations in all the datasets and derived a spin period derivative of $\dot{P} = 5.52(5) \times 10^{-11}$ ss. This is 3.8 times larger than the value measured after the first recorded outburst in 2014. Additionally, we performed quasi-simultaneous radio observations using three 25--32-m class radio telescopes for a total of 92.5 hr to search for FRB-like radio bursts and pulsed emission. However, our analysis did not reveal any radio bursts or periodic emission.

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arXiv2023

A soft and transient ultraluminous X-ray source with 6-h modulation in the NGC 300 galaxy

We investigate the nature of CXOU J005440.5-374320 (J0054), a peculiar bright ($\sim$$4\times10^{39}$ erg/s) and soft X-ray transient in the spiral galaxy NGC 300 with a 6-hour periodic flux modulation that was detected in a 2014 Chandra observation. Subsequent observations with Chandra and XMM-Newton, as well as a large observational campaign of NGC 300 and its sources performed with the Swift Neil Gehrels Observatory, showed that this source exhibits recurrent flaring activity: four other outbursts were detected across $\sim$8 years of monitoring. Using data from the Swift/UVOT archive and from the XMM-Newton/OM and Gaia catalogues, we noted the source is likely associated with a bright blue optical/ultraviolet counterpart. This prompted us to perform follow-up observations with the Southern African Large Telescope in December 2019. With the multi-wavelength information at hand, we discuss several possibilities for the nature of J0054. Although none is able to account for the full range of the observed peculiar features, we found that the two most promising scenarios are a stellar-mass compact object in a binary system with a Wolf$-$Rayet star companion, or the recurrent tidal stripping of a stellar object trapped in a system with an intermediate-mass ($\sim1000$ $M_\odot$) black hole.

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arXiv2023

Probing the nature of the X-ray source IGR J16327-4940 with Chandra

We report on the results of a Chandra observation of the source IGRJ16327-4940, suggested to be a high mass X-ray binary hosting a luminous blue variable star (LBV). The source field was imaged by ACIS-I in 2023 to search for X-ray emission from the LBV star and eventually confirm this association. No X-ray emission is detected from the LBV star, with an upper limit on the X-ray luminosity of L$_{\rm 0.5-10 keV}<2.9(^{+1.6} _{-1.1})\times10^{32}$ erg/s (at the LBV distance d=12.7$^{+3.2} _{-2.7}$ kpc). We detected 21 faint X-ray sources, 8 of which inside the INTEGRAL error circle. The brightest one is the best candidate soft X-ray counterpart of IGRJ16327-4940, showing a hard power law spectrum and a flux corrected for the absorption UF$_{\rm 0.5-10 keV}$=$2.5\times10^{-13}$ erg/cm2/s, mplying a luminosity of $3.0\times10^{33}$ d$_{10~kpc}^2$ erg/s. No optical/near-infrared counterparts have been found. Previous X--ray observations of the source field with Swift/XRT and ART-XC did not detect any source consistent with the INTEGRAL position. These findings exclude the proposed LBV star as the optical association, and pinpoint the most likely soft X-ray counterpart. In this case, the source properties suggest a low mass X-ray binary, possibly a new member of the very faint X-ray transient class.

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arXiv2023

Discovery of a magnetar candidate X-ray pulsar in the Large Magellanic Cloud

During a systematic search for new X-ray pulsators in the XMM-Newton archive, we discovered a high amplitude ($PF\simeq86\%$) periodic ($P\simeq7.25\,\mathrm{s}$) modulation in the X-ray flux of 4XMM J045626.3-694723 (J0456 hereafter), a previously unclassified source in the Large Magellanic Cloud (LMC). The period of the modulation is strongly suggestive of a spinning neutron star (NS). The source was detected only during one out of six observations in 2018-2022. Based on an absorbed power-law spectral model with photon slope of $Γ\simeq 1.9$, we derive a 0.3-10 keV luminosity of $L_\mathrm{X}\simeq2.7\times10^{34}$ erg cm$^{-2}$ s$^{-1}$ for a distance of 50 kpc. The X-ray properties of J0456 are at variance with those of variable LMC X-ray pulsars hosted in high-mass X-ray binary systems with a Be-star companion. Based on SALT spectroscopic observations of the only optical object that matches the X-ray uncertainty region, we cannot completely rule out that J0456 is a NS accreting from a late-type (G8-K3) star, an as-yet-unobserved binary evolutionary outcome in the MCs. We show that the source properties are in better agreement with those of magnetars. J0456 may thus be second known magnetar in the LMC after SGR 0526-66.

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arXiv2023

X-ray nondetection of PSR J0250+5854

We conducted a deep XMM$\unicode{x2013}$Newton observing campaign on the 23.5-s radio pulsar PSR J0250+5854 in order to better understand the connection between long-period, radio-emitting neutron stars and their high-energy-emitting counterparts. No X-ray emission was detected resulting in an upper limit in the bolometric luminosity of PSR J0250+5854 of $<$10$^{31}$ erg s$^{-1}$ for an assumed blackbody with a temperature of 85 eV, typical of an X-ray Dim Isolated Neutron Star (XDINS). We compared the upper limit in the bolometric luminosity of PSR J0250+5854 with the known population of XDINSs and found that the upper limit is lower than the bolometric luminosity of all but one XDINS. We also compared PSR J0250+5854 with SGR 0418+5729, the magnetar with low dipole magnetic field strength, where the upper limit suggests that if PSR J0250+5854 has a thermal hot spot like SGR 0418+5729, it would have a blackbody temperature of $<$200 eV, compared to 320 eV of the magnetar.

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arXiv2022

Deep X-ray and radio observations of the first outburst of the young magnetar Swift J1818.0-1607

Swift J1818.0-1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of B~3E14 G, which is very young compared to the Galactic pulsar population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in March 2020 until October 2021, as well as INTEGRAL upper limits on its hard X-ray emission. The 1-10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of kT_BB~1.1 keV, and apparent reduction in the radius of the emitting region from ~0.6 to ~0.2 km. We also confirm the bright diffuse X-ray emission around the source extending between ~50'' and ~110''. A timing analysis revealed large torque variability, with an average spin-down rate nudot~-2.3E-11 Hz^2 that appears to decrease in magnitude over time. We also observed Swift J1818.0-1607 with the Karl G. Jansky Very Large Array (VLA) on 2021 March 22. We detected the radio counterpart to Swift J1818.0-1607 measuring a flux density of S_v = 4.38+/-0.05 mJy at 3 GHz, and a half ring-like structure of bright diffuse radio emission located at ~90'' to the west of the magnetar. We tentatively suggest that the diffuse X-ray emission is due to a dust scattering halo and that the radio structure may be associated with the supernova remnant of this young pulsar, based on its morphology.

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arXiv2022

X-ray observation of the Roche-lobe filling white dwarf plus hot subdwarf system ZTF J213056.71+442046.5

ZTF J213056.71+442046.5 is the prototype of a small class of recently discovered compact binaries composed of a white dwarf and a hot subdwarf that fills its Roche-lobe. Its orbital period of only 39 min is the shortest known for the objects in this class. Evidence for a high orbital inclination (i=86 deg) and for the presence of an accretion disk has been inferred from a detailed modeling of its optical photometric and spectroscopic data. We report the results of an XMM-Newton observation carried out on 2021 January 7. ZTF J213056.71+442046.5 was clearly detected by the Optical Monitor, which showed a periodic variability in the UV band (200-400 nm), with a light curve similar to that seen at longer wavelengths. Despite accretion on the white dwarf at an estimated rate of the order of 10^{-9} M_sun/yr, no X-rays were detected with the EPIC instrument, with a limit of ~10^{30} erg/s on the 0.2-12 keV luminosity. We discuss possible explanations for the lack of a strong X-ray emission from this system.

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arXiv2022

XMM-Newton discovery of very high obscuration in the candidate Supergiant Fast X-ray Transient AX J1714.1-3912

We have analysed an archival XMM-Newton EPIC observation that serendipitously covered the sky position of a variable X-ray source AX J1714.1-3912, previously suggested to be a Supergiant Fast X-ray Transient (SFXT). During the XMM-Newton observation the source is variable on a timescale of hundred seconds and shows two luminosity states, with a flaring activity followed by unflared emission, with a variability amplitude of a factor of about 50. We have discovered an intense iron emission line with a centroid energy of 6.4 keV in the power law-like spectrum, modified by a large absorption (NH around 1e24 cm-2), never observed before from this source. This X-ray spectrum is unusual for an SFXT, but resembles the so-called "highly obscured sources", high mass X-ray binaries (HMXBs) hosting an evolved B[e] supergiant companion (sgB[e]). This might suggest that AX J1714.1-3912 is a new member of this rare type of HMXBs, which includes IGR J16318-4848 and CI Camelopardalis. Increasing this small population of sources would be remarkable, as they represent an interesting short transition evolutionary stage in the evolution of massive binaries. Nevertheless, AX J1714.1-3912 appears to share X-ray properties of both kinds of HMXBs (SFXT vs sgB[e] HMXB). Therefore, further investigations of the companion star are needed to disentangle the two hypothesis.

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arXiv2022

Recurrent X-ray flares of the black hole candidate in the globular cluster RZ 2109 in NGC 4472

We report on the systematic analysis of the X-ray observations of the ultra-luminous X-ray source XMMU J122939.7+075333 located in the globular cluster RZ 2109 in the Virgo galaxy NGC 4472. The inclusion of observations and time intervals ignored in previous works and the careful selection of extraction regions and energy bands have allowed us to identify new flaring episodes, in addition to the ones that made it one of the best black hole candidates in globular clusters. Although most observations are too short and sparse to recognize a regular pattern, the spacing of the three most recent X-ray flares is compatible with a ~34 hours recurrence time. If confirmed by future observations, such behavior, together with the soft spectrum of the X-ray flares, would be strikingly similar to the quasi-periodic eruptions recently discovered in galactic nuclei. Following one of the possible interpretations of these systems and of a peculiar class of extra-galactic X-ray transients, we explore the possibility that XMMU J122939.7+075333 might be powered by the partial disruption of a white dwarf by an intermediate mass (M~700 Msun) black hole.

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arXiv2021

Detecting the intrinsic X-ray emission from the O-type donor star and the residual accretion in a Supergiant Fast X-ray Transient during its faintest state

We report on the results of an XMM-Newton observation of the Supergiant Fast X-ray Transient (SFXT) IGR J08408-4503 performed in June 2020. The source is composed by a compact object (likely a neutron star) orbiting around an O8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. Analysis of spectra measured during the flare and during quiescence is performed. The quiescent state shows a continuum spectrum well deconvolved to three spectral models: two components are from a collisionally-ionized plasma (with temperatures kT1=0.24 keV and kT2=0.76 keV), together with a power law model (photon index of 2.55), dominating above 2 keV. The X-ray flux emitted at this lowest level is 3.2$\times10^{-13}$ erg/cm2/s (0.5-10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85$\times10^{32}$ erg/s (at 2.2 kpc). The two temperature collisionally-ionized plasma is intrinsic to the stellar wind of the donor star, while the power law can be interpreted as emission due to residual, low level accretion onto the compact object. The X-ray luminosity contributed by the power law component only, in the lowest state, is (4.8$\pm{1.4})\times10^{31}$ erg/s, the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by XMM-Newton, X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, as well as a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical radiation-driven settling accretion stage.

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arXiv2021

A deep $XMM-Newton$ observation of the X-Persei-like binary system CXOU J225355.1+624336

We report on the follow-up $XMM-Newton$ observation of the persistent X-ray pulsar CXOU J225355.1+624336, discovered with the CATS@BAR project on archival $Chandra$ data. The source was detected at $f_{\rm X}$(0.5-10 keV) = 3.4$\times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, a flux level which is fully consistent with the previous observations performed with $ROSAT$, $Swift$, and $Chandra$. The measured pulse period $P$ = 46.753(3) s, compared with the previous measurements, implies a constant spin down at an average rate $\dot P = 5.3\times 10^{-10}$ s s$^{-1}$. The pulse profile is energy dependent, showing three peaks at low energy and a less structured profile above about 3.5 keV. The pulsed fraction slightly increases with energy. We described the time-averaged EPIC spectrum with four different emission models: a partially covered power law, a cut-off power law, and a power law with an additional thermal component (either a black body or a collisionally ionized gas). In all cases we obtained equally good fits, so it was not possible to prefer or reject any emission model on the statistical basis. However, we disfavour the presence of the thermal components, since their modeled X-ray flux, resulting from a region larger than the neutron star surface, would largely dominate the X-ray emission from the pulsar. The phase-resolved spectral analysis showed that a simple flux variation cannot explain the source variability and proved that it is characterized by a spectral variability along the pulse phase. The results of the $XMM-Newton$ observation confirmed that CXOU J225355.1+624336 is a BeXB with a low-luminosity ($L_{\rm X} \sim 10^{34-35}$ erg s$^{-1}$), a limited variability, and a constant spin down. Therefore, they reinforce the source classification as a persistent BeXB.

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arXiv2021

IKT16: Discovery of a 22 ms energetic rotation-powered pulsar in the Small Magellanic Cloud

We report here on the discovery with XMM-Newton of pulsations at 22 ms from the central compact source associated with IKT16, a supernova remnant in the Small Magellanic Cloud (SMC). The measured spin period and spin period derivative correspond to 21.7661076(2) ms and $2.9(3)\times10^{-14}$ s,s$^{-1}$, respectively. Assuming standard spin-down by magnetic dipole radiation, the spin-down power corresponds to $1.1\times10^{38}$,erg,s$^{-1}$ implying a Crab-like pulsar. This makes it the most energetic pulsar discovered in the SMC so far and a close analogue of PSR J0537--6910, a Crab-like pulsar in the Large Magellanic Cloud. The characteristic age of the pulsar is 12 kyr. Having for the first time a period measure for this source, we also searched for the signal in archival data collected in radio with the Parkes telescope and in Gamma-rays with the Fermi/LAT, but no evidence for pulsation was found in these energy bands.

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arXiv2021

The EXTraS Project: Exploring the X-ray transient and variable sky

Temporal variability in flux and spectral shape is ubiquitous in the X-ray sky and carries crucial information about the nature and emission physics of the sources. The EPIC instrument on board the XMM-Newton observatory is the most powerful tool for studying variability even in faint sources. Each day, it collects a large amount of information about hundreds of new serendipitous sources, but the resulting huge (and growing) dataset is largely unexplored in the time domain. The project called Exploring the X-ray transient and variable sky (EXTraS) systematically extracted all temporal domain information in the XMM-Newton archive. This included a search and characterisation of variability, both periodic and aperiodic, in hundreds of thousands of sources spanning more than eight orders of magnitude in timescale and six orders of magnitude in flux, and a search for fast transients that were missed by standard image analysis. All results, products, and software tools have been released to the community in a public archive. A science gateway has also been implemented to allow users to run the EXTraS analysis remotely on recent XMM datasets. We give details on the new algorithms that were designed and implemented to perform all steps of EPIC data analysis, including data preparation, source and background modelling, generation of time series and power spectra, and search for and characterisation of different types of variabilities. We describe our results and products and give information about their basic statistical properties and advice on their usage. We also describe available online resources. The EXTraS database of results and its ancillary products is a rich resource for any kind of investigation in almost all fields of astrophysics. Algorithms and lessons learnt from our project are also a very useful reference for any current and future experiment in the time domain.

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arXiv2021

The X-ray evolution and geometry of the 2018 outburst of XTE J1810-197

After 15 years, in late 2018, the magnetar XTE J1810-197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM-Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from $\sim7.2\times10^{-12}$ s s$^{-1}$ to $\sim1.5\times10^{-11}$ s s$^{-1}$ after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810-197 is well-modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810-197 has likely evolved relative to that found for the 2003 event.

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arXiv2021

Multi-band observations of Swift J0840.7-3516: a new transient ultra-compact X-ray binary candidate

We report on multi-band observations of the transient source Swift J0840.7-3516, which was detected in outburst in 2020 February by the Neil Gehrels Swift Observatory. The outburst episode lasted just ~5 days, during which the X-ray luminosity quickly decreased from ~3E37 erg/s at peak down to ~5E33 erg/s in quiescence (0.3-10 keV; at 10 kpc). Such a marked and rapid decrease in the flux was also registered at UV and optical wavelengths. In outburst, the source showed considerable aperiodic variability in the X-rays on timescales as short as a few seconds. The spectrum of the source in the energy range 0.3-20 keV was well described by a thermal, blackbody-like, component plus a non-thermal, power law-like, component and it softened considerably as the source returned to quiescence. The spectrum of the optical counterpart in quiescence showed broad emission features associated mainly with ionised carbon and oxygen, superposed on a blue continuum. No evidence for bright continuum radio emission was found in quiescence. We discuss possible scenarios for the nature of this source, and show that the observed phenomenology points to a transient ultra-compact X-ray binary system.

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arXiv2021

Time Domain Astronomy with the THESEUS Satellite

THESEUS is a medium size space mission of the European Space Agency, currently under evaluation for a possible launch in 2032. Its main objectives are to investigate the early Universe through the observation of gamma-ray bursts and to study the gravitational waves electromagnetic counterparts and neutrino events. On the other hand, its instruments, which include a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focusing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal for carrying out unprecedented studies in time domain astrophysics. In addition, the presence onboard of a 70 cm near infrared telescope will allow simultaneous multi-wavelegth studies. Here we present the THESEUS capabilities for studying the time variability of different classes of sources in parallel to, and without affecting, the gamma-ray bursts hunt.

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arXiv2021

New X-ray observations of the hot subdwarf binary HD49798 / RXJ0648.0-4418

HD49798 / RXJ0648.0-4418 is the only confirmed X-ray binary in which the mass donor is a hot subdwarf star of O spectral type and, most likely, it contains a massive white dwarf (1.28$\pm$0.05 M$_{\rm SUN}$) with a very fast spin period of 13.2 s. Here we report the results of new XMM-Newton pointings of this peculiar binary, carried out in 2018 and in 2020, together with a reanalysis of all the previous observations. The new data indicate that the compact object is still spinning-up at a steady rate of $(-2.17\pm0.01)\times10^{-15}$ s s$^{-1}$, consistent with its interpretation in terms of a young contracting white dwarf. Comparison of observations obtained at similar orbital phases, far from the ecplise, shows evidence for long term variability of the hard ($>$0.5 keV) spectral component at a level of $\sim$(70$\pm$20)\%, suggesting the presence of time-dependent inhomogeneities in the weak stellar wind of the HD49798 subdwarf. To investigate better the soft spectral component that dominates the X-ray flux from this system, we computed a theoretical model for the thermal emission expected from an atmosphere with element abundances and surface gravity appropriate for this massive white dwarf. This model gives a best fit with effective temperature of T$_{\rm eff}$=2.25$\times$10$^5$ K and an emitting area with radius of $\sim$1600 km, larger than that found with blackbody fits. This model also predicts a contribution of the pulsed emission from the white dwarf in the optical band significantly larger than previously thought and possibly relevant for optical variability studies of this system.

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