arXiv subjects

A. Tiengo

Publications and source records attributed to A. Tiengo.

At least 163 records · Page 9Linked to original sources

arXiv2005

A first look with Chandra at SGR 1806-20 after the Giant Flare: significant spectral softening and rapid flux decay

We report on the results of a ~30 ks Chandra pointing of the soft gamma-ray repeater SGR 1806-20, the first X-ray observation with high spectral resolution performed after the 2004 December 27 Giant Flare. The source was found in a bursting active phase and with a significantly softer spectrum than that of the latest observations before the Giant Flare. The observed flux in the 2-10 keV range was ~2.2 x10^-11 erg/s/cm^2, about 20% lower than that measured three months before the event. This indicates that, although its giant flare was ~100 times more intense than those previously observed in two other soft gamma-ray repeaters, the post flare X-ray flux decay of SGR 1806-20 has been much faster. The pulsed fraction was about 3%, a smaller value than that observed before the flare. We discuss the different properties of the post-flare evolution of SGR 1806-20 in comparison to those of SGR 1900+14 and interpret the results as a strong evidence that a magnetospheric untwisting occurred (or is occurring) after the Giant Flare.

astro-ph↗

arXiv2005

XMM-Newton and VLT observations of the afterglow of GRB040827

The field of the Gamma-Ray Burst GRB 040827 was observed with XMM-Newton and with the ESO/VLT starting ~6 and ~12 hours after the burst, respectively. A fading X-ray afterglow is clearly detected with the XMM-Newton/EPIC instrument, with a time decay t^(-delta), with delta=1.41+/-0.10. Its spectrum is well described by a power law (photon index Gamma=2.3+/-0.1) affected by an absorption largely exceeding (by a factor ~5) the expected Galactic one, requiring the contribution of an intrinsic, redshifted absorber. In the optical/NIR range, the afterglow emission was observed in the Ks band, as a weak source superimposed to the host galaxy, with magnitude Ks=19.44+/-0.13 (12 hours after the GRB, contribution from the host subtracted); in other bands the flux is dominated by the host galaxy. Coupling constraints derived from X-ray spectral fitting and from photometry of the host, we estimated a gas column density in the range (0.4-2.6)x10^22 cm^-2 in the GRB host galaxy, likely located at a redshift 0.5<z<1.7. GRB 040827 stands out as the best example of an X-ray afterglow with intrinsic absorption.

astro-ph↗

arXiv2005

A XMM-Newton View of the Soft Gamma-ray Repeater SGR 1806--20: Long Term Variability in the pre-Super Giant Flare Epoch

The low energy (<10 keV) X-ray emission of the Soft Gamma-ray Repeater SGR1806-20 has been studied by means of four XMM-Newton observations carried out in the last two years, the latter performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5. The source was caught in different states of activity: over the 2003-2004 period the 2-10 keV flux doubled with respect to the historical level observed previously. The long term raise in luminosity was accompanied by a gradual hardening of the spectrum, with the power law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations are consistent with an average spin-down rate of 5.5x10e-10 s/s, higher than the values observed in the previous years. The long-term behavior of SGR1806-20 exhibits the correlation between spectral hardness and spin-down rate previously found only by comparing the properties of different sources (both SGRs and Anomalous X-ray Pulsars). The best quality spectrum (obtained on 6 September 2004) cannot be fitted by a single power law, but it requires an additional blackbody component (kT=0.79 keV, R_BB = 1.9 (d/15 kpc)^2 km), similar to the spectra observed in other SGRs and in Anomalous X-ray Pulsars. No spectral lines were found in the persistent emission, with equivalent width upper limits in the range 30-110 eV. Marginal evidence for an absorption feature at 4.2 keV is present in the cumulative spectrum of 69 bursts detected in September-October 2004.

astro-ph↗

arXiv2005

Three XMM-Newton observations of the Anomalous X-ray Pulsar 1E 1048.1-5937: long term variations in spectrum and pulsed fraction

We report the results of a recent (July 2004) XMM-Newton Target of Opportunity observation of the Anomalous X-ray pulsar 1E 1048.1-5937, together with a detailed re-analysis of previous observations carried out in 2000 and 2003. In July 2004 the source had a 2-10 keV flux of 6.2$\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ and a pulsed fraction P$_F$=0.68. The comparison of the three data sets shows the presence of an anti-correlation between flux and pulsed fraction, implying that previous estimates of the source energetics based on the assumption of a large and constant pulsed fraction might be significantly underestimated. The source spectrum is well described by a power law plus blackbody model (kT~0.63 keV, photon index $Γ$~2.7-3.5) or, alternatively, by the sum of two blackbodies of which the hotter is Comptonized by relativistic electrons. In this case the temperatures are kT${_1}$~0.2-0.3 keV and kT${_2}$~0.4-0.5 keV and the emitting area of the cooler component is consistent with the whole neutron star surface. The long term luminosity variation of a factor >~2 is accompanied by relatively small variations in the spectral shape. Phase resolved spectroscopy indicates a harder spectrum in correspondence of the pulse maximum. No spectral features have been detected with 4$σ$ limits on the equivalent width in the range ~10-220 eV, depending on line energy and width.

astro-ph↗

arXiv2004

INTEGRAL and XMM-Newton Observations of GRB040106

On January 6th 2004, the IBAS burst alert system triggered the 8th gamma-ray burst (GRB) to be located by the INTEGRAL satellite. The position was determined and publicly distributed within 12s, prompting ESA's XMM-Newton to execute a ToO observation just 5 hours later, during which an X-ray afterglow was detected. The GRB had a duration ~52s with two distinct pulses separated by \~42s. Here we present the results of imaging and spectral analyses of the prompt emission from INTEGRAL data and the X-ray afterglow from XMM-Newton data. The gamma-ray spectrum is consistent with a single power-law of photon index -1.72 +/- 0.15. The fluence (20-200 keV) was 8.2 x 10^-7 erg cm^-2. The X-ray afterglow (F_nu (t) propto nu^-beta_X t^-delta) was extremely hard with beta_X = 0.47 +/- 0.01 and delta = 1.46 +/- 0.04. The 2-10 keV flux 11 hours after the burst was 1.1 x 10^-12 erg cm^-2 s^-1. The time profile of the GRB is consistent with the observed trends from previous analysis of BATSE GRBs. We find that the X-ray data are not well-fit by either a simple spherical fireball or by a speading jet, expanding into a homogeneous medium or a wind environment. Based on previously determined correlations between GRB spectra and redshift, we estimate a redshift of ~0.9^+0.5_-0.4 (1 sigma) and a lower limit on the isotropic radiated energy of ~5 x 10^51 erg in this burst.

astro-ph↗

arXiv2004

Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar XTE J1810-197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004) was fainter by Delta H=0.7+/-0.2 and Delta Ks=0.5+/-0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810-197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5-10 keV absorbed flux of the source was 2.2x10^-11 erg/s/cm^2, which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of Anomalous X-ray Pulsars.

astro-ph↗

arXiv2004

A first XMM-Newton look at the relativistic double pulsar PSR J0737-3039

We present the results of a 50 ks long X-ray observation of the relativistic double pulsar system PSR J0737-3039 obtained with the XMM-Newton satellite in March 2004. The source has a soft spectrum (power law photon index = 3.5(+0.5,-0.3)) and a 0.2-10 keV luminosity of ~3x10^30 erg/s (assuming a distance of 500 pc), consistent with the values derived from a previous Chandra observation. No flux variations have been detected, implying the absence of large orbital modulations. The high time resolution of the EPIC instrument has allowed us to perform the first search for X-ray pulsations from this system. The result was negative, with an upper limit of 60% on the pulsed fraction of the 22 ms pulsar.

astro-ph↗

arXiv2004

Late evolution of the X-ray afterglow of GRB 030329

The X-ray afterglow of the Gamma-Ray Burst GRB 030329, associated to SN2003dh at z=0.1685, has been observed with XMM-Newton 258 days after the burst explosion. A source with flux of (6.2 +/- 2.3) 10^{-16} erg cm^{-2} s^{-1} (0.5-2 keV) has been detected at the GRB position. This measurement, together with a re-analysis of the previous X-ray observations, indicates a flattening of the X-ray light curve ~40 days after the burst. This is in remarkable agreement with the scenario invoking the presence of two jets with different opening angles. The wider jet should be responsible for the observed flattening due to its transition into the non-relativistic Sedov-Taylor phase.

astro-ph↗

arXiv2004

Pronounced long term flux variability of the Anomalous X-ray Pulsar 1E1048.1-5937

We present XMM-Newton and Chandra observations of 1E1048.1-5937, being the first to show evidence for a significant variation in the X-ray luminosity of this Anomalous X-ray Pulsar (AXP). While during the first XMM-Newton (2000 December) and Chandra (2001 July) observations the source had a flux consistent with that measured on previous occasions (~5x10e-12 ergs/cmq/s), two more recent observations found it at a considerably higher flux level of 2x10e-11 ergs/cmq/s (2002 August; Chandra) and 10e-11 ergs/cmq/s (2003 June; XMM-Newton). All the spectra are fit by the sum of a blackbody with kT~0.6 keV and a power law with photon index ~3. No significant changes were seen in the spectral parameters, while the pulsed fraction in the 0.6-10 keV energy range decreased from ~90% in 2000 to ~53% in 2003. The spectral invariance does not support the presence of two physically distinct components in the AXP emission. The sparse coverage of the data does not permit us to unambiguously relate the observed variations to the two bursts seen from this source in the fall of 2001.

astro-ph↗

arXiv2004

The X-ray afterglow of GRB030329 at early and late times

Thanks to its extraordinary brightness, the X-ray afterglow of GRB030329 could be studied by XMM-Newton up to two months after the prompt Gamma-ray emission. We present the results of two XMM-Newton observations performed on May 5 and 29, as well as an analysis of the Rossi-XTE data of the early part of the afterglow, discussing in particular the stability of the X-ray spectrum and presenting upper limits on the presence of X-ray emission lines.

astro-ph↗

arXiv2003

GRB 030227: the first multiwavelength afterglow of an INTEGRAL GRB

We present multiwavelength observations of a gamma-ray burst detected by INTEGRAL (GRB 030227) between 5.3 hours and ~1.7 days after the event. Here we report the discovery of a dim optical afterglow (OA) that would not have been detected by many previous searches due to its faintess (R~23). This OA was seen to decline following a power law decay with index Alpha_R= -0.95 +/- 0.16. The spectral index Beta_opt/NIR yielded -1.25 +/- 0.14. These values may be explained by a relativistic expansion of a fireball (with p = 2.0) in the cooling regime. We also find evidence for inverse Compton scattering in X-rays.

astro-ph↗

arXiv2003

The X-ray afterglow of GRB030329

We report on XMM-Newton and Rossi-XTE observations of the bright (fluence $\sim$ 10$^{-4}$ erg cm$^{-2}$) and nearby (z=0.1685) Gamma-Ray Burst GRB030329 associated to SN2003dh. The first Rossi-XTE observation, 5 hours after the burst, shows a flux decreasing with time as a power law with index 0.9$\pm$0.3. Such a decay law is only marginally consistent with a further Rossi-XTE measurement (at t-t$_{GRB}\sim$30 hr). Late time observations of this bright afterglow at X-ray wavelengths have the advantage, compared to optical observations, of not being affected by contributions from the supernova and host galaxy. A first XMM-Newton observation, at t-t$_{GRB}\sim$37 days, shows a flux of 4$\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (0.2-10 keV). The spectrum is a power law with photon index $Γ$=1.9 and absorption $<2.5\times10^{20}$ cm$^{-2}$, consistent with the Galactic value. A further XMM-Newton pointing at t-t$_{GRB}\sim$61 days shows a flux fainter by a factor $\sim$2. The combined Rossi-XTE and XMM-Newton measurements require a break at t$\sim$0.5 days in the afterglow decay, with a power law index increasing from 0.9 to 1.9, similar to what is observed in the early part of the optical afterglow. The extrapolation of the XMM-Newton spectra to optical frequencies lies a factor of $\sim10$ below simultaneous measurements. This is likely due to the presence of SN2003dh.

astro-ph↗

arXiv2003

INTEGRAL and XMM-Newton observations of the weak GRB 030227

We present INTEGRAL and XMM-Newton observations of the prompt gamma-ray emission and the X-ray afterglow of GRB030227, the first GRB for which the quick localization obtained with the INTEGRAL Burst Alert System (IBAS) has led to the discovery of X-ray and optical afterglows. GRB030227 had a duration of about 20 s and a peak flux of 1.1 photons cm^-2 s^-1 in the 20-200 keV energy range. The time averaged spectrum can be fit by a single power law with photon index about 2 and we find some evidence for a hard to soft spectral evolution. The X-ray afterglow has been detected starting only 8 hours after the prompt emission, with a 0.2-10 keV flux decreasing as t^-1 from 1.3x10e-12 to 5x10e-13 erg cm^-2 s^-1. The afterglow spectrum is well described by a power law with photon index 1.94+/-0.05 modified by a redshifted neutral absorber with column density of several 10e22 cm^-2. A possible emission line at 1.67 keV could be due to Fe for a redshift z=3, consistent with the value inferred from the absorption.

astro-ph↗

arXiv2003

XMM-Newton Spectroscopy of the Accretion-Driven Millisecond X-ray Pulsar XTE J1751-305 in Outburst

We present an analysis of the first high-resolution spectra measured from an accretion-driven millisecond X-ray pulsar in outburst. We observed XTE J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ksec. Using a simple absorbed blackbody plus power-law model, we measure an unabsorbed flux of (6.6 +/- 0.1) * 10^(-10) erg/cm^2/s (0.5--10.0 keV). A hard power-law component (Gamma = 1.44 +/- 0.01) contributes 83% of the unabsorbed flux in the 0.5-10.0 keV band, but a blackbody component (kT = 1.05 +/- 0.01 keV) is required. We find no clear evidence for narrow or broad emission or absorption lines in the time-averaged spectra, and the sensitivity of this observation has allowed us to set constraining upper-limits on the strength of important features. The lack of line features is at odds with spectra measured from some other X-ray binaries which share some similarities with XTE J1751-305. We discuss the implications of these findings on the accretion flow geometry in XTE J1751-305.

astro-ph↗

arXiv2002

Status of the EPIC/MOS calibration

The XMM-Newton observatory has the largest collecting area flown so far for an X-ray imaging system, resulting in a very high sensitivity over a broad spectral range. In order to exploit fully these performances, a very accurate calibration of the XMM-Newton instruments is required, and has led to an extensive ground and flight calibration program. We report here on the current status of the EPIC/MOS cameras calibrations, highlighting areas for which a reasonably good accuracy has been achieved, and noting points where further work is needed.

astro-ph↗

arXiv2001

The X-ray Source at the Center of the Cassiopeia A Supernova Remnant

We present the first results of an XMM-Newton observation of the central X-ray source in the Cas A supernova remnant. The spectrum can be fit equally well with an absorbed steep power law (alpha~3, N_H~1.5x10^22 cm^-2) or with a bremsstrahlung with temperature kT~2.4 keV and N_H~10^22 cm^-2. A blackbody model (kT~0.7 keV) gives a slightly worse fit and requires a column density N_H~5x10^21 cm^-2 and an emitting area with radius ~0.3 km (for d=3.4 kpc). A search for pulsations for periods longer than 150 ms gave negative results. The 3 sigma upper limits on the pulsed fraction are ~13% for P>0.3 s and ~7% for P>3 s. The overall properties of the central X-ray source in Cas A are difficult to explain in terms of a rapidly spinning neutron star with a canonical magnetic field of ~10^12 G, and are more similar to those of slowly rotating neutron stars such as the Anomalous X-ray Pulsars.

astro-ph↗

arXiv2001

The Anomalous X-ray Pulsar 1E 1048.1-5937: Phase resolved spectroscopy with the XMM-Newton satellite

We report on an observation of the Anomalous X-ray Pulsar 1E 1048.1-5937 performed with the XMM-Newton satellite. The phase averaged spectrum of 1E 1048.1-5937 is well described by the sum of a power law with photon index ~2.9 and a blackbody with temperature ~0.6 keV, without evidence for significant absorption or emission lines. The above spectral parameters do not vary during the phases corresponding to the broad pulse, while the off pulse emission shows a different spectrum characterized by a soft excess at energies below ~1.5 keV. The XMM-Newton observation and a re-analysis of archival BeppoSAX data, show that the spectral parameters and flux of 1E 1048.1-5937 did not change significantly during observations spanning the last four years. All the data are consistent with a 2-10 keV luminosity varying in the range ~(5-7) 10^33 erg/s (for a distance of 3 kpc).

astro-ph↗

arXiv2001

The central region of M31 observed with XMM-Newton. II. Variability of the individual sources

We present the results of a study of the variability of X-ray sources in the central 30' of the nearby Andromeda Galaxy (M31) based on XMM Performance Verification observations. Two observations of this field, with a total exposure time of about 50 ks, were performed in June and December of 2000. We found 116 sources brighter than a limiting luminosity of 6 x 10(35) erg/s (0.3--12 keV, d=760 kpc). For the ~60 brightest sources, we searched for periodic and non-periodic variability; at least 15% of these sources appear to be variable on a time scale of several months. We discovered a new bright transient source ~2.9' from the nucleus in the June observation; this source faded significantly and was no longer detected in December. The behaviour of the object is similar to a handful of Galactic LMXB transients, most of which are supposed to harbor black holes. We detected pulsations with a period of \~865 s from a source with a supersoft spectrum. The flux of this source decreased significantly between the two XMM observations. The detected period is unusually short and points to a rapidly spinning magnetized white dwarf. The high luminosity and transient nature of the source suggest its possible identification with classical or symbiotic nova, some of which were observed earlier as supersoft sources.

astro-ph↗