arXiv subjects

A. N. Parmar

Publications and source records attributed to A. N. Parmar.

At least 73 records · Page 4Linked to original sources

arXiv2000

A study of the dipping low mass X-ray binary Xþ1624-490 from the broadband BeppoSAX observation

A study of the luminous dipping LMXB X1624-490 was made using BeppoSAX. From modelling the MECS radial profile in several energy bands, the optical depth to dust scattering at 1 keV was found to be 2.4+/-0.4. The non-dip and dip spectra of X1624-490 in the energy band 1-100 keV are well-described by emission consisting of a point-like blackbody from the neutron star plus extended Comptonized emission from an ADC. The blackbody temperature was 1.31+/-0.07 keV and the Comptonized emission had photon power law index 2.0+0.5/-0.7 and cut-off energy ~12 keV. Good fits to the dip spectra were obtained by allowing the Comptonized emission to be progressively covered by an extended absorber while the blackbody was rapidly absorbed and a constant halo component accounted for dust scattering into the line-of-sight. From the dip ingress time, a diameter of the extended Comptonized emission region of 5.3+/-0.8x10^10 cm was derived, consistent with a hot, X-ray emitting corona extending to ~50% of the accretion disk radius. The source luminosity for a distance of 15 kpc is 7.3x10^37 erg/s, an appreciable fraction of the Eddington limit making X1624-490 the most luminous dipping LMXB. The half-height of the blackbody emitting region on the neutron star of 6.8+/-1.8 km agrees well with the half-height of the radiatively supported inner accretion disk of 6.3+/-2.9 km. Together with similar agreement recently obtained for 13 other LMXB in an ASCA survey (Church & Balucinska-Church 2000), this strongly supports the identification of the neutron star as the origin of the blackbody emission in LMXB and suggests a mechanism in which the disk height is the main factor determining the level of blackbody emission. Finally, from RXTE ASM data, we derive an improved orbital period of 20.87+/-0.01 hr.

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arXiv2000

BeppoSAX spectroscopy of the NGC7078 (M15) globular cluster X-ray source X2127+119

Results of a 1999 November 16--17 BeppoSAX observation of the low-mass X-ray binary X2127+119 located in the globular cluster NGC7078 are presented. The system is believed to be one where the central neutron star is normally obscured by the accretion disk, and only X-rays scattered into our line of sight by an extended accretion disk corona (ADC) are observed. The 0.1--10 keV lightcurve is energy dependent and shows two partial eclipses separated by the 17.1 hr orbital period. The 0.1--100 keV spectrum is unusually complex, but can be successfully modeled using a partially covered power-law and disk-blackbody model. Together with a column consistent with the interstellar value to NGC7078, about 60% of the source is covered by an additional column of 1E22 atom/cm2. The absorbed component may be X-rays that pass through the outer layers of the accretion disk. The energy dependent intensity variations by a factor of ~2 may be modeled as due to a changing normalization of the disk-blackbody. None of the other spectral parameters appear to clearly depend on luminosity. The same spectral model is also able to fit an archival ASCA spectrum.

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arXiv2000

Unusual quiescent X-ray activity from XTE J0421+560 (CI Cam)

We report on BeppoSAX observations of the X-ray transient XTE J0421+560 in quiescence 156, 541, and ~690 days after the maximum of the 1998 April outburst. In the first observation the source was soft with a power-law photon index of 4.0 (+1.9 -0.9) and absorption, NH, of (1.1 +4.9 -1.1) X 10^21 atom/cm2. In the second observation, the source brightened by a factor ~15 in the 1-10 keV energy range, became significantly harder with a photon index of 1.86 (+0.27 -0.32) and was strongly absorbed with NH = (4.0 +/- 0.8) X 10^23 atom/cm2. There is evidence for a narrow emission line in both spectra at \~7 keV. In the third observation, the source had faded by a factor ~$8 from the previous observation to below the BeppoSAX detection level. It is possible that these variations result from orbital motion of a compact object around the B[e] star companion with the intense, absorbed, spectrum arising during passage through dense circumstellar material. If this is the case, the system may be continuing to exhibit periodic activity.

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arXiv2000

BeppoSAX observation of the transient X-ray pulsar GS 1843+00

We present the results from both the timing and spectroscopic analysis of the transient X-ray pulsar GS 1843+00 observed by the BeppoSAX satellite on 1997 April 4, when the source was at a luminosity of ~10^{37} erg s^{-1}. GS 1843+00 shows a very hard spectrum that is well fitted by an absorbed power law (N_H \~2.3 10^{22} cm^{-2}) modified by a high energy cut-off above 6 keV. The source shows a small pulse amplitude in the whole energy band. The pulse profile evolves with energy from a double-peaked to single-peaked shape. The barycentric pulse period is 29.477+/-0.001 s.

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arXiv2000

Ambiguities in fits to the complex X-ray spectra of starburst galaxies

Spectral fits to X-ray data from both NGC 253 and M 82 provide ambiguous results. The so-called ``best fit'' results depend on the instrument with which the data were obtained and obviously on the choice of spectral model composition. We show that different spectral models can be fit equally well to BeppoSAX data of both galaxies. Metallicities are unreliable in general, with a strong dependence on the choice of model. Preference to one particular spectral model can only be given by combining spectroscopic and imaging X-ray data from all available satellites (ROSAT, ASCA, and BeppoSAX). Based on spectra of NGC 253, we demonstrate that a model consisting of two or more thermal plasma components plus a hard power law continuum and Fe K alpha line emission can explain all observations. These model components represent the integral spectrum of thermal gas and compact sources in starburst galaxies that are most likely supernova remnants and X-ray binaries. The same model can fit the X-ray data of M 82, but there the evidence, from ROSAT imaging, of the existence of compact sources which might represent high-mass X-ray binaries is weaker. This implies that its hard X-ray emission, which is extended in ROSAT images, might - if truly diffuse - be dominated by a very hot (several keV energy) thermal gas component.

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arXiv2000

X-ray/optical observations of XTE J0421+560/CI Cam in quiescence

We report on a BeppoSAX observation of the transient X-ray source XTE J0421+560 during quiescence performed ~150 days after the 1998 April outburst. The source had an unabsorbed 0.5-10 keV flux of 6.7x10^{-12} erg/cm2/s and was still remarkably soft with most of the emission below 2 keV. The X-ray spectrum can be fit with the same two-temperature model as the outburst observations. There is evidence for the presence of an Iron emission feature at ~7 keV. We report also on a series of optical observations performed using the 72 cm Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano Telescope of the Bologna Observatory. The optical spectrum includes very strong Balmer emission lines, He I, [Fe II], and [O I] features. From the observed L_x/Lopt~10^{-3} the quiescent optical emission cannot be due to re-processing of the X-rays, but has to be generated in the optical companion or the circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due to the optical star if it is of spectral type OB. Although the nature of the compact object present in the XTE J0421+560/CI Cam system cannot be firmly established, we speculate that it is most probably a white dwarf.

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arXiv2000

BeppoSAX and Chandra Observations of SAXJ0103.2-7209=2E0101.5-7225: a new Persistent 345s X-ray Pulsar in the SMC

We report the results of a 1998 July BeppoSAX observation of a field in the SMC which led to the discovery of 345s pulsations in the X-ray flux of SAXJ0103.2-7209. The BeppoSAX X-ray spectrum is well fit by an absorbed power-law with photon index 1.0 plus a black body component with kT=0.1keV. The unabsorbed luminosity in the 2-10 keV energy range is 1.2x10^{36} erg/s. In a very recent Chandra observation the 345s pulsations are also detected. The available period measurements provide a constant period derivative of -1.7s/yr over the last three years making SAXJ0103.2-7209 one of the most rapidly spinning-up X-ray pulsars known. The BeppoSAX position is consistent with that of the Einstein source 2E0101.5-7225 and the ROSAT source RXJ0103.2-7209. This source was detected at a luminosity level of few 10^{35}-10^{36} erg/s in all datasets of past X-ray missions since 1979. The ROSAT HRI and Chandra positions are consistent with that of a m_V=14.8 Be spectral type star already proposed as the likely optical counterpart of 2E0101.5-7225. We conclude that SAXJ0103.2-7209 and 2E0101.5-7225 are the same source, a relatively young and persistent X-ray pulsar in the SMC.

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arXiv2000

Broad-band BeppoSAX observation of the low-mass X-ray binary X1822-371

Results of a 1997 September 9-10 BeppoSAX observation of the 5.57 hr low-mass X-ray binary (LMXRB) X1822-371 are presented. The 0.3-40 keV spectrum is unusually complex and cannot be fit by any of the standard models applied to other LMXRB. At least two components are required. One component has a shape consistent with that expected from the Comptonization of an input soft (Wein) spectrum while the other, contributing ~40% of the 1-10 keV flux, is consistent with being a blackbody. In addition, there is a ``dip'' in the spectrum which can be modeled by a 1.33 +0.05 -0.11 keV absorption edge with an optical depth, tau, of 0.28 +/- 0.06. If the same model is fit to ASCA Solid-State Imaging Spectrometer spectra obtained in 1993 and 1996, then reasonable fits are also obtained, with a similar absorption feature required. The nature of this feature is highly uncertain; its energy corresponds to the K-edges of highly ionized Ne x and neutral Mg, or to an L-edge of moderately ionized Fe. Surprisingly, no strong (tau > 0.05) Fe-K or (tau > 0.18) O-K edges are visible. The folded lightcurve of X1822-371 is similar to previous observations, except that no strong softening is seen near the eclipse. An updated orbital ephemeris is provided.

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arXiv1999

XEUS - The X-ray Evolving Universe Spectroscopy Mission

XEUS is under study by ESA as part of the Horizon 2000+ program to utilize the International Space Station (ISS) for astronomical applications. XEUS will be a long-term X-ray observatory with an initial mirror area of 6m2 at 1 keV that will be grown to 30m2 following a visit to the ISS. The 1 keV spatial resolution is expected to be 2-5'' HEW. XEUS will consist of separate detector and mirror spacecraft aligned by active control to provide a focal length of 50m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting sensitivity will then be ~4 10-18 erg/cm2/s - around 250 times better than XMM, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.

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arXiv1999

Discovery of a faint 437 s X-ray pulsar 1SAX J1452.8-5949

A new pulsar, 1SAX J1452.8-5949, was discovered during a BeppoSAX galactic plane survey on 1999 July 20 at R.A.=14h 52m 49s, Dec=-59 49' 18'' (J2000) with a 90% confidence uncertainty radius of 50''. Coherent pulsations were detected with a barycentric period of a 437.4 +/- 1.4 s. The X-ray spectrum can be modeled by a power-law with a photon index of 1.4 +/- 0.6 and absorption consistent with the galactic value in the direction of the source ($1.9 \times 10^{22}$ atom cm$^{-2}$). An Fe K line with a equivalent width of $\approxgt$1.3 keV may be present in the spectrum. The unabsorbed 2-10 keV flux is $9 \times 10^{-13}$ erg cm$^{-2}$s$^{-1}$. The X-ray properties and lack of an obvious optical counterpart are consistent with a Be star companion at a distance of between approximately 6 and 12 kpc which implies a luminosity of (4-15)$\times 10^{33}$ erg s$^{-1}$.

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arXiv1999

BeppoSAX spectroscopy of the Hercules X-1 short-on state

We present results of a 5.7 day duration BeppoSAX observation of the short-on state of Her X-1 and a short observation during the decline of the preceding main-on state. The 0.1-10 keV spectra can be fit with a power-law and blackbody model together with Fe emission features at 1.0 keV and 6.5 keV. During the later stages of the short-on state there are long intervals when the absorption is $\approxgt$$5 \times 10^{22}$ atom cm$^{-2}$. These intervals become longer and occur ~0.3 day earlier in each orbital cycle as the short-on state progresses. During the intervals of high absorption the 0.1 keV blackbody is still clearly detected. This may indicate the presence of separate scattered and absorbed spectral components, although other explanations such as partial covering or a partially ionized absorber cannot be excluded. During the rest of the short-on state the ratio of flux in the blackbody compared to the power-law is consistent with that in the main-on state. This supports the view that much of the 35 day modulation is caused by an energy independent process, such as electron scattering. The discovery of strong absorption late in the short-on state is consistent with the predictions of the warped disk model (Petterson 1977, see also Schandl & Meyer 1994) where the end of the short-on state is caused by the accretion disk moving into the line of sight to the neutron star. The pulse phase difference between the blackbody and the power-law maxima is 250 +/- 20 degrees in both observations (separated by 0.43 of a 35 day cycle). This constant phase difference is consistent with the blackbody originating at the inner edge of a precessing accretion disk.

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arXiv1999

An X-ray study of the dipping low mass X-ray binary XB 1323-619

During a BeppoSAX observation of the low-mass X-ray binary dip source XB 1323-619 a total of 10 type I X-ray bursts and parts of 12 intensity dips were observed. During non-bursting, non-dipping intervals, the 1-150 keV BeppoSAX spectrum can be modelled by a cutoff power-law with a photon index of 1.48 +/- 0.01, a cutoff energy of 44.1 +5.1/-4.4 keV together with a blackbody with kT of 1.77 +/- 0.25 keV contributing ~15% of the 2-10 keV flux. Absorption equivalent to 3.88 +/- 0.16x10^22 H atom cm^(-2) is required. The dips repeat with a period of 2.938 +/- 0.020 hr and span 40% of the orbital cycle. During dips the maximum reduction in 2-10 keV intensity is ~65%. The spectral changes during dips are complex and cannot be modelled by a simple absorber because of the clear presence of part of the non-dip spectrum which is not absorbed. Spectral evolution in dipping can be well modelled by progressive covering of the cutoff power-law component which must be extended, plus rapid absorption of the point-source blackbody. One of the bursts is double and 4 of the bursts occurred during dipping intervals. These bursts have 2-10 keV peak count rates reduced by only 22% on average from those occurring outside the dips, and are not heavily absorbed. One explanation for this lack of absorption is that the bursts temporarily ionize the absorbing material responsible for the dips.

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arXiv1999

BeppoSAX spectroscopy of the globular cluster X-ray source X1746-371 (NGC6441)

During a BeppoSAX observation of the X-ray source X1746-371 located in the globular cluster NGC6441 a type I X-ray burst, parts of 4 intensity dips, and energy dependent flaring were detected. The dips repeat every 5.8 +0.3 -0.9 hr and show no obvious energy dependence. If the dips are due to electron scattering this energy independence implies an abundance >130 times less than solar, confirming an earlier measurement. Since the overall cluster abundance is close to solar this low abundance is unexpected. Photoionization of the absorbing material, obscuration of an extended source, and variations in multiple components that combine to produce an apparent energy independence are all excluded. Thus, the nature of the dips remains uncertain. The best-fit model to the overall 0.3-30 keV spectrum is a disk-blackbody with a temperature of 2.82 +/- 0.04 keV, together with a cutoff power-law with a photon index of -0.32 +/- 0.80 and a cutoff energy of 0.90 +/- 0.26 keV. Absorption, consistent with the optical extinction to NGC6441 of (0.28 +/- 0.04) 10^22 atom/cm2 is required. All previous best-fit spectral models for this source are excluded at high confidence. The spectrum is dominated by the blackbody-like component, with the cutoff power-law only contributing an average of 12% of the 1-10 keV flux. During flaring intervals the contribution of this component decreases to \~6% with variations in the intensity of the blackbody-like component being responsible for most of the flaring activity.

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arXiv1999

X-ray observations during a Her X-1 anomalous low-state

Results of a 1999 July 8-10 BeppoSAX observation during an anomalous low-state of Her X-1 are presented. The standard on-state power-law and blackbody continuum model is excluded at high confidence unless partial covering is included. This gives a power-law photon index of 0.63 +/- 0.02 and implies that 0.28 +/- 0.03 of the flux undergoes additional absorption of (27 +/- 7) 10^22 atom/cm2. 11% of the observed 0.1-10 keV flux is from the 0.068 +/- 0.015 keV blackbody. 1.237747(2) s pulses with a semi-amplitude of 2.1 +/- 0.8% are detected at >99.5% confidence and confirmed by RXTE measurements. This implies that Her X-1 underwent substantial spin-down close to the start of the anomalous low-state. The spectral and temporal changes are similar to those recently reported from 4U1626-67. These may result from a strongly warped disk that produces a spin-down torque. The X-ray source is then mostly viewed through the inner regions of the accretion disk. A similar mechanism could be responsible for the Her X-1 anomalous low-states. Shadowing by such an unusually warped disk could produce observable effects in the optical and UV emission from the companion star.

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arXiv1999

The BeppoSAX 1-8 keV cosmic background spectrum

The spectrum of the 1.0-8.0 keV cosmic X-ray background (CXB) at galactic latitudes > 25 degrees has been measured using the BeppoSAX LECS and MECS instruments. The spectrum is consistent with a power-law of photon index 1.40 +/- 0.04 and normalization 11.7 +/- 0.5 photon cm^{-2} s^{-1} keV^{-1} sr^{-1} at 1 keV. Our results are in good agreement with previous ROSAT PSPC (Georgantopuolos et al. 1996), ASCA GIS (Myaji et al. 1998) and rocket (McCammon & Sanders 1990) measurements. On the contrary, previous measurements with the HEAO1 A2 (Marshall et al. 1980) and the ASCA SIS (Gendreau et al. 1995) instruments, are characterized by normalizations which are respectively 35% and 25% smaller than ours.

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arXiv1999

The broad-band spectrum of OAO1657-415 with BeppoSAX: In search of cyclotron lines

We report on a 30 ks observation of the high-mass X-ray binary pulsar OAO1657-415 performed by BeppoSAX on September 1998. The wide band spectrum is well fit by both a cutoff power law, or a power law modified by a high energy cutoff, plus a fluorescence Iron line at 6.5 keV. The two models are statistically equivalent. The inclusion of a cyclotron resonance feature at ~36 keV -- corresponding to a magnetic field strength 3.2 (1+z) x 10^12 G, where z is the gravitational redshift -- improves significantly the X^2 for the cutoff model but only marginally for the power law plus high energy cutoff model. The statistical significance of our data is not adequate to discriminate between the two models, and even the "Normalized Crab Ratio" technique, successfully used to pinpoint cyclotron features in the spectra of other X-ray pulsars, is not conclusive in answering the question whether the feature is real or it is an artifact due to an improper modeling of the continuum used to fit the data.

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arXiv1999

A BeppoSAX study of the pulsating transient X0115+63: the first X-ray spectrum with four cyclotron harmonic features

The recurrent hard pulsating X-ray transient §was observed with \B on March 19, when the source was at a 2--10 keV flux level of $\sim$310~mCrab. We report on the high energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74 keV, 24.16 keV, 35.74 keV, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1:(1.9):(2.8):(3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field, when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for {\it four harmonically spaced lines} in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.

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arXiv1999

An anomalous low-state from the globular cluster X-ray source X1732-304 (Terzan 1)

During a BeppoSAX observation of the X-ray burst source X1732-304 located in the globular cluster Terzan 1 in 1999 April, the source was detected in an anomalously low-state with an X-ray luminosity of 1.4 x 10^33 erg/s$, about 300 times fainter than the lowest previously reported measure of the persistent flux. The spectral shape and intensity of this low-state emission is similar to that of a number of neutron star soft X-ray transients in quiescence, being well fit by a ~0.34 keV blackbody together with a power-law with a photon index of ~1.0. These similarities suggest that the same mechanism, such as the inhibition of accretion due to the propeller effect, is also operating in X1732-304. Alternatively, the X1732-304 low-state may be due to obscuration of the line of sight to the central neutron star, with only scattered and/or reflected X-rays being detected. The possibility that the source detected by BeppoSAX is not X1732-304, but a nearby dim source of the cluster field (what would make the observed dimming of X1732-304 even more remarkable) is also discussed.

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