1RXS J232953 Research Articles

Search for radio pulsations in four Anomalous X-ray Pulsars and discovery of two new pulsars

We report on observations of four southern Anomalous X-ray Pulsars (1RXS J170849.0−400910, 1E 1048.1−5937, 1E 1841−045 and AX J1845−0258) obtained at 1.4 GHz using the Parkes Radio Telescope. Radio pulsations from these sources have been searched (i) by directly folding the time-series at a number of trial periods centred on the value of the spin rate obtained from the X-ray observations; (ii) by performing a blind search and (iii) by using a code sensitive to single de-dispersed pulses, with the aim to detect signals similar to those of the recently discovered Rotating Radio Transients. No evidence for radio pulsations with an upper limit of ∼0.1 mJy for any of the four targets has been found. The blind search led to the serendipitous discovery of two new pulsars, rotating with a spin period of about 0.7 s and 92 m s, respectively, and to the further detection of 18 known pulsars, two of which were also detected in the single-pulse search.

Discovery of Luminous Pulsed Hard X‐Ray Emission from Anomalous X‐Ray Pulsars 1RXS J1708−4009, 4U 0142+61, and 1E 2259+586 byINTEGRALandRXTE

We investigated the time-averaged high-energy spectral characteristics of the persistent anomalous X-ray pulsars (AXPs) 1RXS J1708-4009, 4U 0142+61, 1E 2259+586, and 1E 1048.1-5937, using RXTE PCA (2-60 keV), RXTE HEXTE (15-250 keV), and INTEGRAL IBIS ISGRI (20-300 keV) data. We discovered hard spectral tails for energies above 10 keV in the total and pulsed spectra of AXPs 1RXS J1708-4009, 4U 0142+4009, and 1E 2259+586, but 1E 1048.1-5937 appeared to be too weak to be detected. Improved hard X-ray spectral information for AXP 1E 1841-045 is also presented. The pulsed and total spectra above 10 keV have power-law shapes, and there is so far no significant evidence for spectral breaks or bends up to ~150 keV. The pulsed spectra above 10 keV are exceptionally hard with indices for four AXPs in the range -1.0 to 1.0. Below 10 keV these indices were in the range 2.0-4.3, indicating the very drastic spectral changes in a narrow energy interval around 10 keV. The best-fit power-law models to the total spectra between ~10 and 150 keV are significantly softer, with indices measured for 1E 1841-045, 1RXS J1708-4009, and 4U 0142+61 in the range 1.0-1.4. For the latter AXPs the pulsed fractions are consistent with 100% around 100 keV but are different at 10 keV: ~10% for 4U 0142+61, ~25% for 1E 1841-045, and consistent with 100% for 1RXS J1708-4009. The luminosities of these total and pulsed spectral tails (10-150 keV) largely exceed the total available spin-down powers by factors ranging from ~100 to ~600. We also reanalyzed archival CGRO COMPTEL (0.75-30 MeV) data: no AXP detections can be claimed, and the obtained upper limits indicate for 1RXS J1708-4009, 4U 0142+61, and 1E 1841-045 that strong breaks or bends must occur somewhere between ~150 and 750 keV. We discuss predictions from first attempts to model our hard X-ray and soft gamma-ray spectra in the context of the magnetar model.

A Gemini observation of the anomalous X-ray pulsar 1RXS J170849-400910

The anomalous X-ray pulsars (AXPs) represent a growing class of neutron stars discovered at X-ray energies. While the nature of their multi-wavelength emission mechanism is still under debate, evidence has been recently accumulating in favor of their magnetar nature. Their study in the optical and infrared (IR) wavelengths has recently opened a new window to constrain the proposed models. We here present a brief overview of AXPs and our Gemini-South observation of 1RXS J170849-400910, which is a relatively bright AXP discovered with ROSAT and later found to be an 11 s X-ray pulsar by ASCA. The observation was taken with the near-IR imager Flamingos in J (1.25 μm), H (1.65 μm), and K s (2.15 μm). We confirm the recent detection by (ApJ, 589, L93–L96) of a source coincident with the CHANDRA source (candidate ‘A’). Our derived magnitudes of J = 20.6 (0.2), H = 18.6 (0.2), and K s = 17.1 (0.2) are consistent with those derived by (ApJ, 589, L93–L96), and indicate that if this source is indeed the IR counterpart to 1RXS J170849-400910, then there is no evidence of variability from this AXP. However, given the lack of IR variability and the relatively high IR to X-ray flux of this source when compared to the other AXPs, we conclude that this source is unlikely the counterpart of the AXP, and that the other source (candidate ‘B’) within the CHANDRA error circle should not be ruled out as the counterpart. Further monitoring of these sources and a deep observation of this complex field are needed to confirm the nature of these sources and their association with the AXP.

First evidence of a cyclotron feature in an anomalous X-ray pulsar

Abstract We report on the results of the longest uninterrupted observation of the Anomalous X-ray Pulsar 1RXS J170849–400910 obtained with the BeppoSAX satellite in August 2001. The best-fit phase-averaged spectrum was found to be an absorbed power law plus blackbody model, with photon index Γ∼2.4 and a black body temperature of kTbb∼ 0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ∼8.1 keV (significance level of 4ς), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9 × 1011G and 1.6 × 1015G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

The Formation of Cataclysmic Variables with Brown Dwarf Secondaries

The present-day formation of cataclysmic variables (CVs) with brown dwarf (BD) secondaries (0.013 M? ? Ms ? 0.075 M?) is investigated using a population synthesis technique. Results from the latest, detailed models for BDs have been incorporated into the population synthesis code. The present-day orbital period distribution of zero-age CVs (ZACVs) that form with BD secondaries is calculated. For our models, we find that ZACVs with BD secondaries have orbital periods in the range 46 minutes to 2.5 hr. We also find that ZACVs with BD secondaries comprise 18% of the total, present-day ZACV population. In addition, we find that 80% of ZACVs with BD secondaries have orbital periods shorter than 78 minutes. This implies that 15% of the present-day ZACV population should have orbital periods shorter than the observed orbital period minimum for CVs. We also investigate the dependence of the present-day formation rate of CVs with BD secondaries on the assumed value of the common envelope efficiency parameter, ?CE, for three different assumed mass ratio distributions in zero-age main-sequence (ZAMS) binaries. Surprisingly, we find that the common envelope process must be extremely inefficient (?CE < 0.1) in order for CVs with BD secondaries not to be formed. Finally, we find that the progenitor binaries of ZACVs with BD secondaries have ZAMS orbital separations of smaller than 3 AU and ZAMS primary masses between ~1 and 10 M?, with ~75% of the primary masses of smaller than ~1.6 M?. Interestingly, these ranges in orbital separation and primary mass place the majority of the progenitor binaries within the so-called brown dwarf desert. The implications of these results are discussed in the context of V485 Cen and 1RXS J232953.9+062814, the only known CVs with orbital periods shorter than the period minimum, CVs above the period minimum that are strongly suspected of containing BD secondaries (such as WZ Sge), common envelope evolution involving very low mass secondaries, and the BD mass function in ZAMS binaries.

Anomalous Ultraviolet Line Flux Ratios in the Cataclysmic Variables 1RXS J232953.9+062814, CE 315, BZ Ursae Majoris, and EY Cygni, Observed with theHubble Space TelescopeSpace Telescope Imaging Spectrograph

Brief Hubble Space Telescope Space Telescope Imaging Spectrograph spectroscopic snapshot exposures of the cataclysmic variables 1RXS J232953.9+062814, CE 315, BZ UMa, and EY Cyg reveal very large N V/C IV line flux ratios, similar to those observed in AE Aqr. Such anomalous line flux ratios have so far been observed in 10 systems and presumably reflect a different composition of the accreted material compared to the majority of cataclysmic variables. We discuss the properties of this small sample in the context of the recent proposal by Schenker et al. that a significant fraction of the present-day population of cataclysmic variables may have passed through a phase of thermal timescale mass transfer.

The Infrared Counterpart to the Anomalous X-Ray Pulsar 1RXS J170849-400910

We report the discovery of the likely IR counterpart to the anomalous X-ray pulsar (AXP) 1RXS J170849-400910 based on the Chandra High Resolution Camera (imaging detector) X-ray position and the deep optical/IR observations carried out from ESO telescopes and the Canada-France-Hawaii Telescope during 1999-2002. Within the narrow uncertainty region, we found two relatively faint (K' = 20.0 and K' = 17.53) IR objects. Based on their color and position in the J-K' versus J-H diagram, only the brighter object is consistent with the known IR properties of the counterparts to other AXPs. No variability was detected for this source, which is similar to what is observed in the case of 4U 0142+614. Like in other AXPs, we found that the IR flux of 1RXS J170849-400910 is higher than expected for a simple blackbody component extrapolated from the X-ray data. If confirmed, this object would be the fourth IR counterpart to a source of the AXP class and would make the IR excess a likely new characteristic of AXPs.

Evidence of a Cyclotron Feature in the Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849−400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in August 2001. The best-fit phase-averaged spectrum was an absorbed power law plus blackbody model, with photon index Gamma~2.4 and a blackbody temperature of kT_bb~0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ~8.1 keV (a significance of 4 sigma), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9x10^11 G and 1.6x10^15 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

A Time Evolution Study of the Superhumps of the Dwarf Nova 1RXS J232953.9+062814 by Wavelet Transform

The time evolution behaviour of the superhumps of the dwarf nova1RXS J232953.9+ 062814 is investigated with the wavelet analysis method. On the basis of two nights CCD photometry performed during its first superoutburst as well as other published brightness data, we reveal the superhump's time-dependence as a function of periods and time. Our light curves, which phased in the rapid decay ending portion of the superoutburst and in the dawn of a following normal outburst, are important to help trace the superhump evolution for the star. Both the superhump period and the orbital period of the binary system are detected in the present data. We obtain P sh=0.0458±0.0002d and P orb=0.0450±0.0002d. They agree with those existing values. The two periods exchanged their roles during the superhump evolution. The general profile of brightness fading over the outbursts roughly followed an exponential decay law or a form of a five-order polynomial.

Superhump Evolution in the Ultrashort Period Dwarf Nova 1RXS J232953.9+062814

Abstract We report on the evolution of superhumps and late superhumps in an ultrashort period dwarf nova, 1RXS J232953.9$+$062814, during the superoutburst in 2001 November. Ordinary superhumps were observed throughout a plateau phase, a rapid fading phase, and a rebrightening phase. During the plateau phase, the superhump period increased with time at a large rate of $P_\mathrm{dot} = 1.19 \pm 0.24 \times 10^{-4}$. In conjunction with the rebrightening phenomenon, these characteristics indicate that an accretion disk expanded further outward from the $3:1$ resonance radius, which caused a large amount of left over matter at the outer disk, even after the superoutburst. In the post-outburst phase, we detected late superhumps superimposed on dominant double-peak modulations. Late superhumps were observed at least for 10 d without a significant period change. We detected the first normal outburst of this object on 2001 December 26. The interval between the superoutburst and this normal one is 53 d. This short recurrence time supports a high mass-transfer rate in this system. Concerning the evolutionary status of 1RXS J232953.9$+$062814, we propose that it is a progenitor of AM CVn stars on the evolutionary course of the cataclysmic variable channel in which systems have a secondary star with a hydrogen-exhausted core.

Superhumps in Cataclysmic Binaries. XXII. 1RXS J232953.9+062814

ABSTRACTWe report photometry of a recently discovered dwarf nova with a remarkably short 64.2 minute orbital period. In quiescence, the star’s light curve is that of a double sinusoid, arising from the “ellipsoidal” distortion of the Roche‐lobe–filling secondary. During superoutburst, common superhumps develop with a period 3%–4% longer than Porb. This indicates a mass ratio M2/M1 = 0.19 ± 0.02, a surprisingly large value in so compact a binary. This implies that the secondary star has a density 2–3 times higher than that of other short‐period dwarf novae, suggesting a secondary enriched by H‐burning prior to the common‐envelope phase of evolution. We estimate i = 50° ± 5°, M1 = 0.63+0.12-0.09 M⊙, M2 = 0.12+0.03-0.02 M⊙, R2 = 0.121+0.010-0.007 R⊙, and a distance to the binary of 180 ± 40 pc.

Discovery of a Dwarf Nova Breaking the Standard Sequence of Compact Binary Evolution

Abstract We revealed that the dwarf nova 1RXS J232953.9+062814 is an SU UMa-type system with a superhump period of $ 66.774\pm 0.010 \,\mathrm{min}$. The short period strongly indicates that the orbital period of this object is below the period minimum of cataclysmic variables. The superhump period is $ 4.04\pm 0.02$% longer than the photometric period during quiescence ($ 64.184\pm 0.003 \,\mathrm{min}$), which is probably associated with the orbital period. Although the standard evolutionary scenario of cataclysmic variables predicts lower mass-transfer rates in systems with shorter orbital periods, we found firm evidence of a relatively high mass-transfer rate from its large proper motion and bright apparent magnitude. Its proximity indicates that we have overlooked a number of objects in this new class. With the analogous system of V485 Cen, these objects establish the first subpopulation in hydrogen-rich cataclysmic variables below the period minimum.

1RXS J232953.9+062814: A Dwarf Nova with a 64 Minute Orbital Period and a Conspicuous Secondary Star

We present spectroscopy and time-series photometry of the newly discovered dwarf nova 1RXS J232953.9+062814. Photometry in superoutburst reveals a superhump with a period of 66.06(6) minutes. The low state spectrum shows Balmer and HeI emission on a blue continuum, and in addition shows a rich absorption spectrum of type K4 +- 2. The absorption velocity is modulated sinusoidally at P_orb = 64.176(5) min, with semi-amplitude K = 348(4) km/s. The low-state light curve is double-humped at this period, and phased as expected for ellipsoidal variations. The absorption strength does not vary appreciably around the orbit. The orbital period is shorter than any other cataclysmic variable save for a handful of helium-star systems and V485 Centauri (59 minutes). The secondary is much hotter than main sequence stars of similar mass, but is well-matched by helium-enriched models, indicating that the secondary evolved from a more massive progenitor. A preliminary calculation in which a 1.2 solar-mass star begins mass transfer near the end of H burning matches this system's characteristics remarkably well.

1RXS J232953.9+062814: a New SU UMa Dwarf Nova below the Period Minimum

1RXS J232953.9+062814 was identified as a cataclysmic variable by Wei et al. (1999). Four low-resolution spectra of 1RXS J232953.9+062814 were obtained by using the 2.16-m telescope of the National Astronomical Observatories, in which two of them were at outburst, and the other two were at quiescence. The system is about 16.8 B and 16.5 V at quiescence, and 12.6 B and 12.6 V at outburst. The quiescent spectra were dominated by double-peaked Balmer emissions, which indicates a hydrogen-rich system with a high-inclination accretion disc. MgH and TiO absorption bands appeared in the quiescent spectrum imply a companion with a spectral type of early M dwarf. If we take it as a M0 dwarf, the system is located at a distance of 350 pc with a proper motion velocity 150 km s$^{-1}$. The superhump period of 0.046311 days (Uemura et al. 2001) was confirmed by our V photometry. The short period and the hydrogen-rich nature reveal that this system is another SU Ursae Majoris-type dwarf nova below the period minimum after V485 Centauri. 1RXS J232953.9+062814 is one of the most important systems for studying the evolutionary scenario of cataclysmic variables since it is much brighter than V485 Cen.

The Pulse-Phase–dependent Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849−400910

We report on the results of a 50ks BeppoSAX observation of 1RXS J170849-400910, one of the five (plus a candidate) known anomalous X-ray pulsars. The BeppoSAX data are consistent with a power-law plus blackbody spectral decomposition, making 1RXS J170849-400910 the fourth source of this class for which such a spectral decomposition was found. The inferred power-law slope and blackbody temperature are Gamma~2.6 and kT_BB~0.46keV, respectively. We found significant energy-dependence of the pulse profile, a remarkable feature for an AXP. By using the power-law plus blackbody decomposition we detected a significant variation in at least one spectral parameter, the power-law photon index, as a function of the pulse phase. This is the first significant detection of spectral parameter variation in an AXP. The implications of these results are briefly discussed.

Discovery of an 11-s X-Ray Pulsar in the Galactic-Plane Section of the Scorpius Constellation

During a galactic-plane survey with ASCA in 1996 September, we detected a relatively bright, soft source at R.A. = 17h8m46s.6, DEC. = −40°9′27″ (J2000), and discovered an 11-s X-ray pulsation from the source. This source has been identified with the ROSAT source 1RXS J170849.0−400910. From a timing analysis of the source, we obtained a barycentric pulse period of P = 10.99759±0.00005 s with a broad sinusoidal shape of a pulse fraction of ∼ 30%. The energy spectrum in the 0.8–10 keV region is very soft, and can be fitted by a power-law model with a photon index of 3.5 and an absorption column density of 1.8 × 1022 cm−2. The observed pulse-phase-averaged flux in the range 0.8–10 keV is 4.3 × 10−11 erg cm−2 s−1, which corresponds to 1.7 × 10−10 erg cm−2 s−1 after correcting for soft X-ray absorption. During an observation interval of about 14 hr, neither a significant change in the pulsation period, nor a significant variation in the phase-averaged flux was detected. From these X-ray properties, we suggest that this newly discovered X-ray pulsar might be a member of a small subgroup of “anomalous” X-ray pulsars with a period close to 6–9 s.