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Register a new userIntermittent dipping in a low-mass X-ray Binary
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Authors
Duncan K. Galloway
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Periodic dips observed in approx. 20% of low-mass X-ray binaries are thought to arise from obscuration of the neutron star by the outer edge of the accretion disk. We report the detection with the Rossi X-ray Timing Explorer of two dipping episodes in Aql X-1, not previously a known dipper. The X-ray spectrum during the dips exhibited an elevated neutral column density, by a factor between 1 and almost two orders of magnitude. Dips were not observed in every cycle of the 18.95-hr orbit, so that the estimated frequency for these events is 0.10 (+0.07,-0.05)/cycle. This is the first confirmed example of intermittent dipping in such a system. Assuming that the dips in Aql X-1 occur because the system inclination is intermediate between the non-dipping and dipping sources, implies a range of 72-79 deg. for the source. This result lends support for the presence of a massive (> 2 M_sun) neutron star in Aql X-1, and further implies that approx. 30 additional LMXBs may have inclinations within this range, raising the possibility of intermittent dips in those systems also. Thus, we searched for dips from 24 other bursting systems, without success. For the system with the largest number of dip phases covered, 4U 1820-303, the nondetection implies a 95% upper limit to the dip frequency of 1.4E-3/cycle.
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In this paper, we report on the available X-ray data collected by INTEGRAL, Swift, and XMM-Newton during the first outburst of the INTEGRAL transient IGR J17451-3022, discovered in 2014 August. The monitoring observations provided by the JEM-X instruments on-board INTEGRAL and the Swift/XRT showed that the event lasted for about 9 months and that the emission of the source remained soft for the entire period. The source emission is dominated by a thermal component (kT~1.2 keV), most likely produced by an accretion disk. The XMM-Newton observation carried out during the outburst revealed the presence of multiple absorption features in the soft X-ray emission that could be associated to the presence of an ionized absorber lying above the accretion disk, as observed in many high-inclination low mass X-ray binaries. The XMM-Newton data also revealed the presence of partial and rectangular X-ray eclipses (lasting about 820 s), together with dips. The latter can be associated with increases in the overall absorption column density in the direction of the source. The detection of two consecutive X-ray eclipses in the XMM-Newton data allowed us to estimate the source orbital period at 22620.5(-1.8,+2.0) s (1{sigma} c.l.).
(abridged) We analyzed the archived XMM-Newton observation of the poorly studied low-mass X-ray binary XTE J1710-281 performed in 2004 that covered one orbital period of the system (3.8 hr). The source shows dips as well as eclipses, hence it is viewed close to edge-on. We modeled the spectral changes between persistent and dips in the framework of the partial covering model and the ionized absorber approach. The persistent spectrum can be fit by a power law with a photon index of 1.94(+-0.02) affected by absorption from cool material with a hydrogen column density of 0.401(+-0.007)*10^22 cm^-2. The spectral changes from persistent to deep-dipping intervals are consistent with the partial covering of the power-law emission, with the covering fraction increasing from 26% during shallow dipping to 78% during deep dipping. We do not detect any absorption lines from highly ionized species such as FeXXV. The upper-limits we derive on their equivalent width (EW) are not constraining. Despite not detecting any signatures of a warm absorber, we show that the spectral changes are consistent with an increase in column density (4.3(-0.5;+0.4)*10^22 cm^-2 during shallow dipping to 11.6(-0.6;+0.4)*10^22 cm^-2 during deep dipping) and a decrease in ionization state of a highly-ionized absorber (10^2.52 during shallow dipping to 10^2.29 erg.s^-1.cm during deep dipping), associated with a slight increase in the column density of a neutral absorber. The parameters of the ionized absorber are not constrained during persistent emission. The warm absorber model better accounts for the ~1 keV depression visible in the pn dipping spectra, and naturally explains it as a blend of lines and edges unresolved by pn. A deeper observation of XTE J1710-281 would enable this interpretation to be confirmed.
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.
We report on Chandra X-ray Observatory (CXO) High-Energy Transmission Grating (HETG) spectra of the dipping Low Mass X-ray Binary (LMXB) 1A 1744-361 during its July 2008 outburst. We find that its persistent emission is well modeled by a blackbody (kT ~ 1.0 keV) plus power-law ($Gamma$ ~ 1.7) with an absorption edge at 7.6 keV. In the residuals of the combined spectrum we find a significant absorption line at 6.961+/-0.002 keV, consistent with the Fe XXVI (hydrogen-like Fe) 2 - 1 transition. We place an upper limit on the velocity of a redshifted flow of v < 221 km/s. We find an equivalent width for the line of 27^+2_-3 eV, from which we determine a column density of 7+/-1x10^17 cm^-2 via a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on the ionization parameter of > 10^3.6 erg cm/s. The properties of this line are consistent with those observed in other dipping LMXBs. Using Rossi X-ray Timing Explorer (RXTE) data accumulated during this latest outburst we present an updated color-color diagram which clearly shows that 1A 1744-361 is an atoll source. Finally, using additional dips found in the RXTE and CXO data we provide an updated orbital period estimate of 52+/-5 minutes.
We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer (RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We find that outside the dips the tracks in the colour-colour and hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll sources in the middle and upper parts of the banana branch. The tracks show secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a function of the position in the CD. This is the first time power spectra of this source are presented. No quasi-periodic oscillations (QPOs) are found. The power spectra are dominated by very low frequency noise (VLFN), characteristic for atoll sources in the banana state, and band limited noise (BLN) which is not reliably detected but may, uncharacteristically, strengthen and increase in frequency with spectral hardness. The VLFN fits to a power law, which becomes steeper when the source moves to the harder part of the CD. We conclude that 4U 1624-49 is an atoll source which in our observations is in the upper banana branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.
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