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G. Israel

Publications and source records attributed to G. Israel.

At least 19 recordsLinked to original sources
arXiv2019

Detailed X-ray spectroscopy of the magnetar 1E 2259+586

Magnetic field geometry is expected to play a fundamental role in magnetar activity. The discovery of a phase-variable absorption feature in the X-ray spectrum of SGR 0418+5729, interpreted as cyclotron resonant scattering, suggests the presence of very strong non-dipolar components in the magnetic fields of magnetars. We performed a deep XMM-Newton observation of pulsar 1E 2259+586, to search for spectral features due to intense local magnetic fields. In the phase-averaged X-ray spectrum, we found evidence for a broad absorption feature at very low energy (0.7 keV). If the feature is intrinsic to the source, it might be due to resonant scattering/absorption by protons close to star surface. The line energy implies a magnetic field of ~ 10^14 G, roughly similar to the spin-down measure, ~ 6x10^13 G. Examination of the X-ray phase-energy diagram shows evidence for a further absorption feature, the energy of which strongly depends on the rotational phase (E >~ 1 keV ). Unlike similar features detected in other magnetar sources, notably SGR 0418+5729, it is too shallow and limited to a small phase interval to be modeled with a narrow phase-variable cyclotron absorption line. A detailed phase-resolved spectral analysis reveals significant phase-dependent variability in the continuum, especially above 2 keV. We conclude that all the variability with phase in 1E 2259+586 can be attributed to changes in the continuum properties which appear consistent with the predictions of the Resonant Compton Scattering model.

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arXiv2018

Peculiar Spin Frequency and Radio Profile Evolution of PSR J1119$-$6127 Following Magnetar-like X-ray Bursts

We present the spin frequency and profile evolution of the radio pulsar J1119$-$6127 following magnetar-like X-ray bursts from the system in 2016 July. Using data from the Parkes radio telescope, we observe a smooth and fast spin-down process subsequent to the X-ray bursts resulting in a net change in the pulsar rotational frequency of $\Delta\nu\approx-4\times10^{-4}$\,Hz. During the transition, a net spin-down rate increase of $\Delta\dot\nu\approx-1\times10^{-10}$\,Hz\,s$^{-1}$ is observed, followed by a return of $\dot{\nu}$ to its original value. In addition, the radio pulsations disappeared after the X-ray bursts and reappeared about two weeks later with the flux density at 1.4\,GHz increased by a factor of five. The flux density then decreased and undershot the normal flux density followed by a slow recovery back to normal. The pulsar's integrated profile underwent dramatic and short-term changes in total intensity, polarization and position angle. Despite the complex evolution, we observe correlations between the spin-down rate, pulse profile shape and radio flux density. Strong single pulses have been detected after the X-ray bursts with their energy distributions evolving with time. The peculiar but smooth spin frequency evolution of PSR~J1119$-$6127 accompanied by systematic pulse profile and flux density changes are most likely to be a result of either reconfiguration of the surface magnetic fields or particle winds triggered by the X-ray bursts. The recovery of spin-down rate and pulse profile to normal provides us the best case to study the connection between high magnetic-field pulsars and magnetars.

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arXiv2018

A new Ultraluminous X-ray source in the galaxy NGC 5907

We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4x10^{39} erg/s. The source was undetected in previous 2012 Chandra observations with a 3 sigma upper limit on the luminosity of 1.5x10^{38} erg/s, implying a flux increase of a factor of >35. We analyzed three recent 60ks/50ks Chandra and 50ks XMM-Newton observations, as well as all the available Swift observations performed between August 2017/March 2018. Until the first half of October 2017, Swift observations do not show any emission from the source. The transient entered the ULX regime in less than two weeks and its outburst was still on-going at the end of February 2018. The 0.3-10 keV spectrum is consistent with a single multicolour blackbody disc (kT~1.5 keV). The source might be a ~30 solar mass black hole accreting at the Eddington limit. However, although we did not find evidence of pulsations, we cannot rule-out the possibility that this ULX hosts an accreting neutron star.

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arXiv2017

Discovery of periodic dips in the brightest hard X-ray source of M31 with EXTraS

We performed a search for eclipsing and dipping sources in the archive of the EXTraS project - a systematic characterization of the temporal behaviour of XMM-Newton point sources. We discovered dips in the X-ray light curve of 3XMM J004232.1+411314, which has been recently associated with the hard X-ray source dominating the emission of M31. A systematic analysis of XMM-Newton observations revealed 13 dips in 40 observations (total exposure time $\sim$0.8 Ms). Among them, four observations show two dips, separated by $\sim$4.01 hr. Dip depths and durations are variable. The dips occur only during low-luminosity states (L$_{0.2-12}<1\times10^{38}$ erg s$^{-1}$), while the source reaches L$_{0.2-12}\sim2.8\times10^{38}$ erg s$^{-1}$. We propose this system to be a new dipping Low-Mass X-ray Binary in M31 seen at high inclination (60$^{\circ}$-80$^{\circ}$), the observed dipping periodicity is the orbital period of the system. A blue HST source within the Chandra error circle is the most likely optical counterpart of the accretion disk. The high luminosity of the system makes it the most luminous dipper known to date.

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arXiv2017

Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of gamma-rays, a gravitational wave signal, and a transient optical/near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named "macronovae" or "kilonovae", are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short gamma-ray burst at z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational wave source GW 170817 and gamma-ray burst GRB 170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03-0.05 solar masses of material, including high-opacity lanthanides.

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arXiv2017

GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226

We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational-wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of $90$ deg$^2$ and $72$ deg$^2$ for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the $r-$band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational-wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB150827A detected by {\it Fermi}-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational-wave events are evaluated for the present and future follow-up surveys.

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arXiv2017

Magnetar-like X-ray bursts suppress pulsar radio emission

Rotation-powered pulsars and magnetars are two different observational manifestations of neutron stars: rotation powered pulsars are rapidly spinning objects that are mostly observed as pulsating radio sources, while magnetars, neutron stars with the highest known magnetic fields, often emit short-duration X-ray bursts. Here we report simultaneous observations of the high-magnetic-field radio pulsar PSR J1119-6127 at X-ray, with XMM-Newton & NuSTAR, and at radio energies with Parkes radio telescope, during a period of magnetar-like bursts. The rotationally powered radio emission shuts off coincident with the occurrence of multiple X-ray bursts, and recovers on a time scale of ~70 seconds. These observations of related radio and X-ray phenomena further solidify the connection between radio pulsars and magnetars, and suggest that the pair plasma produced in bursts can disrupt the acceleration mechanism of radio emitting particles.

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arXiv2017

Paving the way to simultaneous multi-wavelength astronomy

Whilst astronomy as a science is historically founded on observations at optical wavelengths, studying the Universe in other bands has yielded remarkable discoveries, from pulsars in the radio, signatures of the Big Bang at submm wavelengths, through to high energy emission from accreting, gravitationally-compact objects and the discovery of gamma-ray bursts. Unsurprisingly, the result of combining multiple wavebands leads to an enormous increase in diagnostic power, but powerful insights can be lost when the sources studied vary on timescales shorter than the temporal separation between observations in different bands. In July 2015, the workshop "Paving the way to simultaneous multi-wavelength astronomy" was held as a concerted effort to address this at the Lorentz Center, Leiden. It was attended by 50 astronomers from diverse fields as well as the directors and staff of observatories and spaced-based missions. This community white paper has been written with the goal of disseminating the findings of that workshop by providing a concise review of the field of multi-wavelength astronomy covering a wide range of important source classes, the problems associated with their study and the solutions we believe need to be implemented for the future of observational astronomy. We hope that this paper will both stimulate further discussion and raise overall awareness within the community of the issues faced in a developing, important field.

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arXiv2017

Broad-band characteristics of seven new hard X-ray selected cataclysmic variables

We present timing and spectral analysis of a sample of seven hard X-ray selected Cataclysmic Variable candidates based on simultaneous X-ray and optical observations collected with XMM-Newton , complemented with Swift/BAT and INTEGRAL/IBIS hard X-ray data and ground-based optical photometry. For six sources, X-ray pulsations are detected for the first time in the range $\rm \sim296-6098\,s$, identifying them as members of the magnetic class. Swift J0927.7-6945, Swift J0958.0-4208, Swift J1701.3-4304, Swift J2113.5+5422, and possibly PBC J0801.2-4625, are Intermediate Polars (IPs), while Swift J0706.8+0325 is a short (1.7 h) orbital period Polar, the 11$^{\rm th}$ hard X-ray selected identified so far. X-ray orbital modulation is also observed in Swift J0927.7-6945 (5.2 h) and Swift J2113.5+5422 (4.1 h). Swift J1701.3-4304 is discovered as the longest orbital period (12.8 h) deep eclipsing IP. The spectra of the magnetic systems reveal optically thin multi-temperature emission between 0.2 and 60 keV. Energy dependent spin pulses and the orbital modulation in Swift J0927.7-6945 and Swift J2113.5+5422 are due to intervening local high density absorbing material ($\rm N_H\sim10^{22-23}\,cm^{-2}$). In Swift J0958.0-4208 and Swift J1701.3-4304, a soft X-ray blackbody (kT$\sim$50 and $\sim$80 eV) is detected, adding them to the growing group of "soft" IPs. White dwarf masses are determined in the range $\rm \sim0.58-1.18\,M_{\odot}$, indicating massive accreting primaries in five of them. Most sources accrete at rates lower than the expected secular value for their orbital period. Formerly proposed as a long-period (9.4 h) novalike CV, Swift J0746.3-1608 shows peculiar spectrum and light curves suggesting either an atypical low-luminosity CV or a low mass X-ray binary.

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arXiv2015

Swift J0525.6+2416 and IGR J04571+4527: two new hard X-ray selected magnetic cataclysmic variables identified with XMM-Newton

IGR J04571+4527 and Swift J0525.6+2416 are two hard X-ray sources detected in the Swift/BAT and INTEGRAL/IBIS surveys. They were proposed to be magnetic cataclysmic variables of the Intermediate Polar (IP) type, based on optical spectroscopy. IGR J04571+4527 also showed a 1218 s optical periodicity, suggestive of the rotational period of a white dwarf, further pointing towards an IP classification. We here present detailed X-ray (0.3-10 keV) timing and spectral analysis performed with XMM-Newton, complemented with hard X-ray coverage (15-70 keV) from Swift/BAT. These are the first high signal to noise observations in the soft X-ray domain for both sources, allowing us to identify the white dwarf X-ray spin period of Swift J0525.6+2416 (226.28 s), and IGR J04571+4527 (1222.6 s). A model consisting of multi-temperature optically thin emission with complex absorption adequately fits the broad-band spectrum of both sources. We estimate a white dwarf mass of about 1.1 and 1.0 solar masses for IGR J04571+4527 and Swift J0525.6+2416, respectively. The above characteristics allow us to unambiguously classify both sources as IPs, confirming the high incidence of this subclass among hard X-ray emitting Cataclysmic Variables.

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arXiv2015

Giant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?

Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3\times10^{38}$ erg s$^{-1}$. This extends the total source dynamic range to $\gtrsim$10$^6$, the largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence for pulsations at a period of 11.6 s is also reported. We show that these observations challenge, for the first time, the maximum theoretical luminosity achievable by an SFXT and propose that this giant outburst was due to the formation of a transient accretion disc around the compact object.

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arXiv2013

XMM-Newton and Swift observations of XTE J1743-363

XTEJ1743-363 is a poorly known hard X-ray transient, that displays short and intense flares similar to those observed from Supergiant Fast X-ray Transients. The probable optical counterpart shows spectral properties similar to those of an M8 III giant, thus suggesting that XTEJ1743-363 belongs to the class of the Symbiotic X-ray Binaries. In this paper we report on the first dedicated monitoring campaign of the source in the soft X-ray range with XMM-Newton and Swift/XRT. T hese observations confirmed the association of XTEJ1743-363 with the previously suggested M8 III giant and the classification of the source as a member of the Symbiotic X-ray binaries. In the soft X-ray domain, XTEJ1743-363 displays a high absorption (~6x10^22 cm^-2 ) and variability on time scales of hundreds to few thousand seconds, typical of wind accreting systems. A relatively faint flare (peak X-ray flux 3x10^-11 erg/cm^2/s) lasting ~4 ks is recorded during the XMM-Newton observation and interpreted in terms of the wind accretion scenario.

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arXiv2012

Extended X-ray emission around RRAT J1819-1458

We present new imaging and spectral analysis of the recently discovered extended X-ray emission around the high-magnetic-field rotating radio transient RRAT J1819-1458. We used two Chandra observations, taken on 2008 May 31 and 2011 May 28. The diffuse X-ray emission was detected with a significance of ~19sigma in the image obtained by combining the two observations. Long-term spectral variability has not been observed. Possible scenarios for the origin of this diffuse X-ray emission, further detailed in Camero-Arranz et al. (2012), are here discussed.

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arXiv2011

The nature of the X-ray transient MAXI J0556-332

Phase-resolved spectroscopy of the newly discovered X-ray transient MAXI J0556-332 has revealed the presence of narrow emission lines in the Bowen region that most likely arise on the surface of the mass donor star in this low mass X-ray binary. A period search of the radial velocities of these lines provides two candidate orbital periods (16.43+/-0.12 and 9.754+/-0.048 hrs), which differ from any potential X-ray periods reported. Assuming that MAXI J0556-332 is a relatively high inclination system that harbors a precessing accretion disk in order to explain its X-ray properties, it is only possible to obtain a consistent set of system parameters for the longer period. These assumptions imply a mass ratio of q~0.45, a radial velocity semi-amplitude of the secondary of K_2~190 km/s and a compact object mass of the order of the canonical neutron star mass, making a black hole nature for MAXI J0556-332 unlikely. We also report the presence of strong N III emission lines in the spectrum, thereby inferring a high N/O abundance. Finally we note that the strength of all emission lines shows a continuing decay over the ~1 month of our observations.

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arXiv2011

XMM-Newton observations of the supergiant fast X-ray transients XTEJ1739-302, IGRJ08408-4503 and IGRJ18410-0535

We report here on the XMM-Newton observations of the three supergiant fast X-ray transients (SFXT) XTE J1739-302, IGRJ08408-4503, and IGRJ18410-0535. For the latter source we only discuss some preliminary results of our data analysis. Some interpretation is provided for the timing and spectral behavior of the three sources in terms of the different theoretical models proposed so far to interpret the behavior of the SFXTs.

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arXiv2011

XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient

IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been ascribed to the combined effect of the intense magnetic field and rotation of the compact object hosted in them and/or the presence of dense structures ("clumps") in the wind of their supergiant companion. IGR J18410-0535 was observed for 45 ks by XMM-Newton as part of a program designed to study the quiescent emission of supergiant fast X-ray transients and clarify the origin of their peculiar X-ray variability. We carried out an in-depth spectral and timing analysis of these XMM-Newton data. IGR J18410-0535 underwent a bright X-ray flare that started about 5 ks after the beginning of the observation and lasted for \sim15 ks. Thanks to the capabilities of the instruments on-board XMM-Newton, the whole event could be followed in great detail. The results of our analysis provide strong convincing evidence that the flare was produced by the accretion of matter from a massive clump onto the compact object hosted in this system. By assuming that the clump is spherical and moves at the same velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl \simeq1.4\times10^(22) g and Rcl \simeq8\times10^(11) cm. These are in qualitative agreement with values expected from theoretical calculations. We found no evidence of pulsations at \sim4.7 s after investigating coherent modulations in the range 3.5 ms-100 s. A reanalysis of the archival ASCA and Swift data of IGR J18410-0535, for which these pulsations were previously detected, revealed that they were likely to be due to a statistical fluctuation and an instrumental effect, respectively.

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arXiv2011

Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole

Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in the massive black hole at the centre of our Milky Way galaxy could strongly ionize the upper atmosphere of the Earth, if beamed towards us.

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