arXiv subjects

A. Tiengo

Publications and source records attributed to A. Tiengo.

At least 109 records · Page 6Linked to original sources

arXiv2011

The progenitor of a type Ia supernova with a short delay time?

HD49798/RXJ0648.0-4418 is the only known X-ray binary composed of a hot subdwarf and a massive white dwarf (M=1.28+/-0.05 Msun). This system, with an orbital period of 1.55 days, is the outcome of a common envelope evolution, most likely of a pair of stars with initial masses of about 8-10 Msun. When the hot subdwarf, currently in a He-burning phase, will expand again and fill its Roche-lobe, the enhanced mass transfer can rapidly bring the already massive white dwarf above the Chandrasekhar limit. The possible final fate, either a Type Ia supernova explosion or an accretion induced collapse, is particularly interesting in view of the high rotational velocity of this star, which has the shortest spin period (13 s) observed in a white dwarf.

astro-ph.SR↗

arXiv2011

Highly absorbed X-ray binaries in the Small Magellanic Cloud

Many of the high mass X-ray binaries (HMXRBs) discovered in recent years in our Galaxy are characterized by a high absorption, most likely intrinsic to the system, which hampers their detection at the softest X-ray energies. We have undertaken a search for highly-absorbed X-ray sources in the Small Magellanic Cloud (SMC) with a systematic analysis of 62 XMM-Newton SMC observations. We obtained a sample of 30 sources showing evidence for an equivalent hydrogen column density larger than 3x10^23 cm^-2. Five of these sources are clearly identified as HMXRBs: four were already known (including three X-ray pulsars) and one, XMM J005605.8-720012, reported here for the first time. For the latter, we present optical spectroscopy confirming the association with a Be star in the SMC. The other sources in our sample have optical counterparts fainter than magnitude ~16 in the V band, and many of them have possible NIR counterparts consistent with highly reddened early type stars in the SMC. While their number is broadly consistent with the expected population of background highly-absorbed active galactic nuclei, a few of them could be HMXRBs in which an early type companion is severely reddened by local material.

astro-ph.HE↗

arXiv2011

X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418

We report the results of XMM-Newton observations of HD49798/RXJ0648.0-4418, the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf. The white dwarf rotates very rapidly (P=13.2 s) and has a dynamically measured mass of 1.28+/-0.05 M_sun. Its X-ray emission consists of a strongly pulsed, soft component, well fit by a blackbody with kT~40 eV, accounting for most of the luminosity, and a fainter hard power-law component (photon index ~1.6). A luminosity of ~10^{32} erg/s is produced by accretion onto the white dwarf of the helium-rich matter from the wind of the companion, which is one of the few hot sub-dwarfs showing evidence of mass-loss. A search for optical pulsations at the South African Astronomical Observatory 1.9-m telescope gave negative results. X-rays were detected also during the white dwarf eclipse. This emission, with luminosity 2x10^{30} erg/s, can be attributed to HD 49798 and represents the first detection of a hot sub-dwarf star in the X-ray band. HD49798/RXJ0648.0-4418 is a post-common envelope binary which most likely originated from a pair of stars with masses ~8-10 M_sun. After the current He-burning phase, HD 49798 will expand and reach the Roche-lobe, causing a higher accretion rate onto the white dwarf which can reach the Chandrasekhar limit. Considering the fast spin of the white dwarf, this could lead to the formation of a millisecond pulsar. Alternatively, this system could be a Type Ia supernova progenitor with the appealing characteristic of a short time delay, being the descendent of relatively massive stars.

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arXiv2011

Long-term spectral and timing properties of the soft gamma-ray repeater SGR 1833-0832 and detection of extended X-ray emission around the radio pulsar PSR B1830-08

SGR 1833-0832 was discovered on 2010 March 19 thanks to the Swift detection of a short hard X-ray burst and follow-up X-ray observations. Since then, it was repeatedly observed with Swift, Rossi X-ray Timing Explorer, and XMM-Newton. Using these data, which span about 225 days, we studied the long-term spectral and timing characteristics of SGR 1833-0832. We found evidence for diffuse emission surrounding SGR 1833-0832, which is most likely a halo produced by the scattering of the point source X-ray radiation by dust along the line of sight, and we show that the source X-ray spectrum is well described by an absorbed blackbody, with temperature kT=1.2 keV and absorbing column nH=(10.4+/-0.2)E22 cm^-2, while different or more complex models are disfavoured. The source persistent X-ray emission remained fairly constant at about 3.7E-12 erg/cm^2/s for the first 20 days after the onset of the bursting episode, then it faded by a factor 40 in the subsequent 140 days, following a power-law trend with index alpha=-0.5. We obtained a phase-coherent timing solution with the longest baseline (225 days) to date for this source which, besides period P=7.5654084(4) s and period derivative dP/dt=3.5(3)E-12 s/s, includes higher order period derivatives. We also report on our search of the counterpart to the SGR at radio frequencies using the Australia Telescope Compact Array and the Parkes radio telescope. No evidence for radio emission was found, down to flux densities of 0.9 mJy (at 1.5 GHz) and 0.09 mJy (at 1.4 GHz) for the continuum and pulsed emissions, respectively, consistently with other observations at different epochs.

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arXiv2011

The first deep X-ray and optical observations of the closest isolated radio pulsar

With a distance of 170 pc, PSR J2144-3933 is the closest isolated radio pulsar currently known. It is also the slowest and least energetic radio pulsar; indeed, its radio emission is difficult to account for with standard pulsar models, since its position in the P-Pdot diagram is far beyond typical "death lines". Here we present the first deep X-ray and optical observations of PSR J2144-3933, performed in 2009 with XMM-Newton and the VLT, from which we can set one of the most robust upper limits on the surface temperature of a neutron star. We have also explored the possibility of measuring the neutron star mass from the gravitational lensing effect on a background optical source.

astro-ph.HE↗

arXiv2011

IKT 16: A Composite SNR in the SMC

Aims: IKT 16 is an X-ray and radio-faint supernova remnant (SNR) in the Small Magellanic Cloud (SMC). A previous X-ray study of this SNR found a hard X-ray source near its centre. Using all available archival and proprietary XMM-Newton data, alongside new multi-frequency radio-continuum surveys and optical observations at H-alpha and forbidden [SII] and [OIII] lines, we aim to constrain the properties of the SNR and discover the nature of the hard source within. Methods: We combine XMM-Newton datasets to produce the highest quality X-ray image of IKT 16 to date. We use this, in combination with radio and optical images, to conduct a multi-wavelength morphological analysis of the remnant. We extract separate spectra from the SNR and the bright source near its centre, and conduct spectral fitting of both regions. Results: We find IKT 16 to have a radius of 37+-3 pc, with the bright source located 8+-2 pc from the centre. This is the largest known SNR in the SMC. The large size of the remnant suggests it is likely in the Sedov-adiabatic phase of evolution. Using a Sedov model to fit the SNR spectrum, we find an electron temperature kT of 1.03+-0.12 keV and an age of 14700 yr. The absorption found requires the remnant to be located deep within the SMC. The bright source is fit with a power law with index 1.58+-0.07, and is associated with diffuse radio emission extending towards the centre of the SNR. We argue that this source is likely to be the neutron star remnant of the supernova explosion, and infer its transverse kick velocity to be 580+-100 km/s. The X-ray and radio properties of this source strongly favour a pulsar wind nebula (PWN) origin.

astro-ph.HE↗

arXiv2011

The XMM-Newton survey of the Small Magellanic Cloud: XMMUJ005011.2-730026 = SXP214, a Be/X-ray binary pulsar

In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC), a region to the east of the emission nebula N19 was observed in November 2009. To search for new candidates for high mass X-ray binaries the EPIC PN and MOS data of the detected point sources were investigated and their spectral and temporal characteristics identified. A new transient (XMMUJ005011.2-730026= SXP214) with a pulse period of 214.05 s was discovered; the source had a hard X-ray spectrum with power-law index of ~0.65. The accurate X-ray source location permits the identification of the X-ray source with a ~15th magnitude Be star, thereby confirming this system as a new Be/X-ray binary.

astro-ph.HE↗

arXiv2011

Multi-instrument X-ray monitoring of the January 2009 outburst from the recurrent magnetar candidate 1E 1547.0-5408

With two consecutive outbursts recorded in four months (October 2008 and January 2009), and a possible third outburst in 2007, 1E 1547.0-5408 is one of the most active transient anomalous X-ray pulsars known so far. Thanks to extensive X-ray observations, obtained both in the quiescent and active states, 1E 1547.0-5408 represents a very promising laboratory to get insights into the outburst properties and magnetar emission mechanisms. We performed a detailed timing and spectral analysis of four Chandra, three INTEGRAL, and one XMM observations collected over a two week interval after the outburst onset in January 2009. Several Swift pointings, covering a 1.5 year interval, were also analyzed in order to monitor the decay of the X-ray flux. We compare the characteristics of the two outbursts, as well as those of the active and quiescent states. We also discuss the long-term X-ray flux history of 1E 1547.0-5408 since its first detection in 1980, and show that the source displays three flux levels: low, intermediate and high.

astro-ph.HE↗

arXiv2011

X-ray and optical observations of the closest isolated radio pulsar

With a parallactic distance of 170 pc, PSR J2144-3933 is the closest isolated radio pulsar currently known. It is also the slowest (P = 8.51 s) and least energetic (Edot = 2.6E28 erg/s) radio pulsar; its radio emission is difficult to account for with standard pulsar models, since the position of PSR J2144-3933 in the period-period derivative diagram is far beyond the typical radio `death lines'. Here we present the first deep X-ray and optical observations of PSR J2144-3933, performed in 2009 with XMM-Newton and European Southern Observatory (ESO)/Very Large Telescope (VLT), from which we derive, assuming a blackbody emission spectrum, a surface temperature upper limit of 2.3E5 K for a 13 km radius neutron star, 4.4E5 K for a 500 m radius hot spot and 1.9E6 K for a 10 m radius polar cap. In addition, our non-detection of PSR J2144-3933 constrains its non-thermal luminosity to be <30 per cent and <2 per cent of the pulsar rotational energy loss in the 0.5-2 keV X-ray band and in the B optical band, respectively.

astro-ph.HE↗

arXiv2011

SGR 0418+5729: a low-magnetic-field magnetar

Soft gamma-ray repeaters and anomalous X-ray pulsars are a small (but growing) group of X-ray sources characterized by the emission of short bursts and by a large variability in their persistent flux. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields 1E14-1E15 G). We found evidence for a magnetar with a low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of soft gamma-ray repeaters. New X-ray observations show that its dipolar magnetic field cannot be greater than 8E12 G, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of magnetic-field strengths, ages and evolutionary stages than observed so far.

astro-ph.HE↗

arXiv2010

The XMM-Newton survey of the Small Magellanic Cloud: Discovery of the 11.866 s Be/X-ray binary pulsar XMMUJ004814.0-732204 (SXP11.87)

One of the goals of the XMM-Newton survey of the Small Magellanic Cloud is the study of the Be/X-ray binary population. During one of our first survey observations a bright new transient - XMMUJ004814.0-732204 - was discovered. We present the analysis of the EPIC X-ray data together with optical observations, to investigate the spectral and temporal characteristics of XMMUJ004814.0-732204. We found coherent X-ray pulsations in the EPIC data with a period of (11.86642 +/- 0.00017) s. The X-ray spectrum can be modelled by an absorbed power-law with indication for a soft excess. Depending on the modelling of the soft X-ray spectrum, the photon index ranges between 0.53 and 0.66. We identify the optical counterpart as a B = 14.9mag star which was monitored during the MACHO and OGLE-III projects. The optical light curves show regular outbursts by ~0.5 mag in B and R and up to 0.9 mag in I which repeat with a time scale of about 1000 days. The OGLE-III optical colours of the star are consistent with an early B spectral type. An optical spectrum obtained at the 1.9 m telescope of the South African Astronomical Observatory in December 2009 shows H_alpha emission with an equivalent width of 3.5 +/- 0.6 A. The X-ray spectrum and the detection of pulsations suggest that XMMUJ004814.0-732204 is a new high mass X-ray binary pulsar in the SMC. The long term variability and the H_alpha emission line in the spectrum of the optical counterpart identify it as a Be/X-ray binary system.

astro-ph.HE↗

arXiv2010

A low-magnetic-field Soft Gamma Repeater

Soft gamma repeaters and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields, B~10^{14}-10^{15} Gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5x10^{12} Gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages and evolutionary stages than observed so far.

astro-ph.HE↗

arXiv2010

The 2008 October Swift detection of X-ray bursts/outburst from the transient SGR-like AXP 1E 1547.0-5408

We report on the detailed study of the 2008 October outburst from the anomalous X-ray pulsar (AXP) 1E 1547.0-5408 discovered through the Swift/Burst Alert Telescope (BAT) detection of SGR-like short X-ray bursts on 2008 October 3. The Swift/X-ray Telescope (XRT) started observing the source after less than 100 s since the BAT trigger, when the flux (about 6E-11 erg/cm^2/s in the 2-10 keV range) was >50 times higher than its quiescent level. Swift monitored the outbursting activity of 1E 1547.0-5408 on a daily basis for approximately three weeks. This strategy allowed us to find a phase-coherent solution for the source pulsations after the burst, which, besides period and period derivative, requires a positive Period second derivative term (spin-down increase). The time evolution of the pulse shape is complex and variable, with the pulsed fraction increasing from 20% to 50% within the Swift observational window. The XRT spectra can be fitted well by means of a single component, either a power-law (PL) or a blackbody (BB). During the very initial phases of the outburst the spectrum is hard, with a PL photon index about 2 (or kT about 1.4 keV) which steepens to about 4 (or kT about 0.8 keV) within one day from the BAT trigger, though the two components are likely present simultaneously during the first day spectra. An INTEGRAL observation carried out five days after the trigger provided an upper limit of about 2E-11 erg/cm^2/s to the emission of 1E 1547.0-5408 in the 18-60 keV band.

astro-ph.HE↗

arXiv2010

The December 2009 gamma-ray flare of 3C 454.3: the multifrequency campaign

During the month of December, 2009 the blazar 3C 454.3 became the brightest gamma-ray source in the sky, reaching a peak flux F ~2000E-8 ph/cm2/s for E > 100 MeV. Starting in November, 2009 intensive multifrequency campaigns monitored the 3C 454 gamma-ray outburst. Here we report the results of a 2-month campaign involving AGILE, INTEGRAL, Swift/XRT, Swift/BAT, RossiXTE for the high-energy observations, and Swift/UVOT, KANATA, GRT, REM for the near-IR/optical/UV data. The GASP/WEBT provided radio and additional optical data. We detected a long-term active emission phase lasting ~1 month at all wavelengths: in the gamma-ray band, peak emission was reached on December 2-3, 2009. Remarkably, this gamma-ray super-flare was not accompanied by correspondingly intense emission in the optical/UV band that reached a level substantially lower than the previous observations in 2007-2008. The lack of strong simultaneous optical brightening during the super-flare and the determination of the broad-band spectral evolution severely constrain the theoretical modelling. We find that the pre- and post-flare broad-band behavior can be explained by a one-zone model involving SSC plus external Compton emission from an accretion disk and a broad-line region. However, the spectra of the Dec. 2-3, 2009 super-flare and of the secondary peak emission on Dec. 9, 2009 cannot be satisfactorily modelled by a simple one-zone model. An additional particle component is most likely active during these states.

astro-ph.CO↗

arXiv2010

An XMM-Newton view of Planetary Nebulae in the Small Magellanic Cloud. The X-ray luminous central star of SMP SMC 22

During an X-ray survey of the Small Magellanic Cloud, carried out with the XMM-Newton satellite, we detected significant soft X-ray emission from the central star of the high-excitation planetary nebula SMP SMC 22. Its very soft spectrum is well fit with a non local thermodynamical equilibrium model atmosphere composed of H, He, C, N, and O, with abundances equal to those inferred from studies of its nebular lines. The derived effective temperature of 1.5x10^5 K is in good agreement with that found from the optical/UV data. The unabsorbed flux in the 0.1-0.5 keV range is about 3x10^{-11} erg cm^-2 s^-1, corresponding to a luminosity of 1.2x10^37 erg/s at the distance of 60 kpc. We also searched for X-ray emission from a large number of SMC planetary nebulae, confirming the previous detection of SMP SMC 25 with a luminosity of (0.2-6)x10^35 erg/s (0.1-1 keV). For the remaining objects that were not detected, we derived flux upper limits corresponding to luminosity values from several tens to hundreds times smaller than that of SMP SMC 22. The exceptionally high X-ray luminosity of SMP SMC 22 is probably due to the high mass of its central star, quickly evolving toward the white dwarf's cooling branch, and to a small intrinsic absorption in the nebula itself.

astro-ph.SR↗

arXiv2010

An ultra-massive fast-spinning white dwarf in a peculiar binary system

White dwarfs typically have masses in a narrow range centered at about 0.6 solar masses (Msun). Only a few ultra-massive white dwarfs (M>1.2 Msun) are known. Those in binary systems are of particular interest because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the XMM-Newton satellite, we show that the X-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 Msun, based only on dynamical measurements. This ultra-massive white dwarf in a post-common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a Type Ia supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.

astro-ph.HE↗

arXiv2010

Early X-ray and optical observations of the soft gamma-ray repeater SGR 0418+5729

Emission of two short hard X-ray bursts on 2009 June 5 disclosed the existence of a new soft gamma-ray repeater, now catalogued as SGR 0418+5729. After a few days, X-ray pulsations at a period of 9.1 s were discovered in its persistent emission. SGR 0418+5729 was monitored almost since its discovery with the Rossi X-ray Timing Explorer (2-10 keV energy range) and observed many times with Swift (0.2-10 keV). The source persistent X-ray emission faded by a factor 10 in about 160 days, with a steepening in the decay about 19 days after the activation. The X-ray spectrum is well described by a simple absorbed blackbody, with a temperature decreasing in time. A phase-coherent timing solution over the 160 day time span yielded no evidence for any significant evolution of the spin period, implying a 3-sigma upper limit of 1.1E-13 s/s on the period derivative and of 3E+13 G on the surface dipole magnetic field. Phase-resolved spectroscopy provided evidence for a significant variation of the spectrum as a function of the stellar rotation, pointing to the presence of two emitting caps, one of which became hotter during the outburst. Finally, a deep observation of the field of SGR 0418+5729 with the new Gran Telescopio Canarias 10.4-m telescope allowed us to set an upper limit on the source optical flux of i'>25.1 mag, corresponding to an X-ray-to-optical flux ratio exceeding 10000, consistent with the characteristics of other magnetars.

astro-ph.HE↗

arXiv2010

Multiwavelength observations of 3C 454.3. III. Eighteen months of AGILE monitoring of the "Crazy Diamond"

We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) carried out in July 2007-January 2009. We show the results of the AGILE campaigns which took place on May-June 2008, July-August 2008, and October 2008-January 2009. During the May 2008-January 2009 period, the source average flux was highly variable, from an average gamma-ray flux F(E>100MeV) > 200E-8 ph/cm2/s in May-June 2008, to F(E>100MeV)~80E-8 ph/cm2/s in October 2008-January 2009. The average gamma-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law (Gamma_GRID ~ 2.0 to 2.2). Only 3-sigma upper limits can be derived in the 20-60 keV energy band with Super-AGILE. During July-August 2007 and May-June 2008, RXTE measured a flux of F(3-20 keV)= 8.4E-11 erg/cm2/s, and F(3-20 keV)=4.5E-11 erg/cm2/s, respectively and a constant photon index Gamma_PCA=1.65. Swift/XRT observations were carried out during all AGILE campaigns, obtaining a F(2-10 keV)=(0.9-7.5)E-11 erg/cm2/s and a photon index Gamma_XRT=1.33-2.04. BAT measured an average flux of ~5 mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period from the radio to the optical. A correlation analysis between the optical and the gamma-ray fluxes shows a time lag of tau=-0.4 days. An analysis of 15 GHz and 43 GHz VLBI core radio flux observations shows an increasing trend of the core radio flux, anti- correlated with the higher frequency data. The modeling SEDs, and the behavior of the long-term light curves in different energy bands, allow us to compare the jet properties during different emission states, and to study the geometrical properties of the jet on a time-span longer than one year.

astro-ph.HE↗