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Her X-1 has been monitored nearly continuously in soft X-rays (2-12 keV) since 1996 February by the All-Sky Monitor on board the Rossi X-Ray Timing Explorer. We present an analysis of these observations, which include 23 contiguous 35 day cycles. We present the best-yet average 35 day cycle light curve for Her X-1. Thirty-five day light-curve featu...
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Context 1
... combination of the ASM and BATSE light curves was enough to classify the orbital phase of TOs as either 0.2 or 0.7, except for one case, cycle 16. Table 1 gives the 23 cycles, a calculated turn-on time and classiÐcation as either an 0.2 or 0.7 turn-on, the phase jump in orbital cycles (i.e., the di †erence between the time interval from the current turn-on to the next one and 20.5 orbital cycles), and the cycle length. The turn-on time is calculated as the ModiÐed Julian Date (MJD \ JD [ 2,440,000.5) of orbital phase 0.68 for the 0.7 TO main high states or as the time of orbital phase 0.23 for the 0.2 TO main high states. ...
Context 2
... analysis here gives one of the best determinations so far of the ordering of 0.2 TO and 0.7 TO 35 day cycles and the phase jumps. From Table 1, 20.5 orbits is the most common 35 day cycle length (11 cases), 21 orbits occurs 6 times, 20 orbits occurs 2 times, and 19.5 orbits occurs once (though this case is of low conÐdence). ...
Citations
... The 35-day cycle in X-ray and optical flux are caused by a precessing accretion disk [9,10] which also causes the 35-day cycle in pulse shape variations [1]. This 35-day cycle and accretion disk have been modelled by [11][12][13][14]. The neutron star is directly visible during MH State and, for a short time, during SH State but not during LS. ...
Observations of the X-ray binary system Her X-1 by the AstroSat Soft X-ray Telescope 1 (SXT) were carried out in 2020 through 2023 with the goals of measuring X-ray spectrum changes with the 35-day disk rotation phase and measuring eclipses at different 35-day phases. The four multi-day long observations were scheduled during: turn-on to Main High (MH) and MH rise; MH state; Short High (SH) state; and Low State (LS). 35-day phase was determined using monitoring observations with the Swift Burst Alert Telescope (BAT). Nine eclipses were observed at a range of 35-day phases, with at least one eclipse during each observation. Data with dips were separated from data without dips. The variation of X-ray spectral parameters vs. 35-day phase shows the following: eclipse parameters are nearly constant, showing that the scattering corona does not change with 35-day phase; dips show an increase in covering fraction but not column density compared to non-dip data; the 1 keV line normalization indicates an origin in a small region near the continuum emission region, likely the magnetospheric accretion flow from the inner disk onto the neutron star; and the change in blackbody normalization (area) shows that the emission region is much larger than the 1 keV emission region, and consistent with the inner edge of the accretion disk.
... The near-circular binary orbital path and the high orbital inclination (∼85°) lead to a 6 hr eclipse during the 1.7 day orbital period. The longer 35 day superorbital flux modulation is divided into four parts: an 11 day main-on state (with a steep increase, a maximum of variable length and a slower decay), an 8 day short-on state (substantially less bright), and two 8 day off states, separating the two on states (Scott & Leahy 1999;Leahy & Igna 2011). This flux modulation is thought to be due to precession of the accretion disk, providing variable obscuration of the X-ray emitting regions on the surface of the neutron star. ...
Recent observations of Her X-1 with NuSTAR (as well as with INTEGRAL, Swift, and Astrosat) have provided evidence that the nearly 20 yr-long decay of the cyclotron line energy since ∼1994 has ended and that a stable value has replaced the decay. Using the observations of the Hard X-ray Modulation Telescope (Insight-HXMT) performed between 2017 July and 2020 February, we analyze the spectra of Her X-1 in its main-on state, focusing on tracing the evolution of cyclotron line energy. While our analysis of eight main-on observations with Insight-HXMT (two in coordination with NuSTAR) shows significant differences with the results of NuSTAR, two earlier findings are confirmed: the positive correlation between the cyclotron line energy and the X-ray flux (source luminosity) and the constancy of the flux-normalized cyclotron line energy during 2017–2020—albeit with significant uncertainty about the absolute value.
... The 35-day cycle length can vary from 33 to 37 days ([16]). Turn-on of the 35-day cycle is defined by the sharp rise in flux at start of the Main High state (see next paragraph), and was proposed to start only at orbital phases 0.23 and 0.68 by [17]. This implies the 35-day cycle is either an integer or half-integer number of binary orbits long, with the most common value of 20.5 orbits. ...
... The 35-day cycle has 8 substates: Main High (MH) turn-on, MH, MH decline, Low State (LS) 1, Short High (SH) turn-on, SH, SH decline, and LS 2. These are obtained by dividing MH and SH states into their 3 different parts, and distinguishing the two LS. The MH state (including the 3 subdivisions) lasts 10 -12 days, the SH state lasts 5 -7 days, and MH and SH are separated by LS lasting 8 -10 days [17]. In addition, Her X-1 has Anomalous Low States (ALS) (e.g. ...
... According to [34] [36] dips can also be caused by partially ionized matter or by cool clumps of material immersed in hot ionized gas. [17] analyzed the pre-eclipse dips using RXTE/ASM data, confirming the marching phenomenon where the di ps progress to earlier orbital phases as the 35-day phase increases. [24] modeled the stream-accretion disk impact and concluded that the dips properties can be explained by this model when the impact site of the accretion stream on the accretion disk obscures the X-ray source. ...
... The X-ray emission of the system features three timescales-the 1.24 s pulsation of Her X-1, the 1.7 day binary period, and the super-orbital 35 day cycle. The counterprecession of a twisted-tilted accretion disk is believed to be the reason causing the 35 day cycle (Petterson 1975;Scott & Leahy 1999;Scott et al. 2000;Leahy 2002). ...
... The MH lightcurves in different energy bands are shown in Figure 1 (panels (c) and (d)). The previous analysis to obtain a time-averaged MH orbital lightcurve was done using ASM data (Scott & Leahy 1999, their Figure 4). Our ASM MH lightcurve is shown in Figure A1 (panel (b)). ...
... orbital lightcurve is shown in Figure 1 (panels (c) and (d)). It is consistent with previously measured MH orbital lightcurves (Scott & Leahy 1999;), but here is measured in several energy bands. We used the multiband lightcurves, subdivided into substates of MH, to derive the absorption column density, N H , and the transmission fraction, f, versus orbital phase (Figure 4, right column). ...
Hercules X-1/HZ Hercules (Her X-1/HZ Her) is an X-ray binary monitored by multiple X-ray missions since the last century. With the abundance of long-term observations, we present a complete set of orbital lightcurves of Her X-1/HZ Her during the six states of the 35 day cycle in multiple energy bands. These illustrate in detail the changing lightcurve caused by the rotating twisted-tilted accretion disk surrounding the neutron star. The orbital lightcurves during the main high state are analyzed in 0.05 35 day phase intervals. These show the regular occurrence of pre-eclipse dips that march to earlier orbital phases as the 35 day phases increase. From the multiband lightcurves, we derive the time-average orbital phase dependence of column density for photoelectric absorption and energy-independent transmission as a function of 35 day phase. The X-ray lightcurves during low states are similar in shape to the optical low-state lightcurve, but X-ray leads optical by ≃0.04–0.08 in orbital phase.
... The X-ray emission of the system features three timescales -the 1.24-second pulsation of Her X-1, the 1.7-day binary period, and the super-orbital 35-day cycle. The counter-precession of a twisted-tilted accretion disc is believed to be the reason causing the 35-day cycle (Petterson 1975;Scott & Leahy 1999;Scott et al. 2000;Leahy 2002). ...
... The MH light-curves in different energy bands are shown in Figure 1 (panels c and d). The previous analysis to obtain a time-averaged MH orbital light-curve was done using ASM data Scott & Leahy (1999) (their Fig. 4). Our ASM MH light-curve is shown in Figure A.1 (panel b). ...
... The 35-day phase averaged (phase 0.9 to 1.1) orbital light-curve is shown in Figure 1 (panels c and d). It is consistent with previously measured MH orbital light-curves (Scott & Leahy 1999;), but here is measured in several energy bands. We used the multi-band light-curves, subdivided into sub-states of MH, to derive the absorption column density, N H , and the transmission fraction, f , vs. orbital phase (Figure 4, right column). ...
Hercules X-1/HZ Hercules (Her X-1/HZ Her) is an X-ray binary monitored by multiple X-ray missions since last century. With the abundance of long-term observations, we present a complete set of orbital light-curves of Her X-1/HZ Her during the six states of the 35-day cycle in multiple energy bands. These illustrate in detail the changing light-curve caused by the rotating twisted-tilted accretion disc surrounding the neutron star. The orbital light-curves during Main-High (MH) state are analyzed in 0.05 35-day phase intervals. These show the regular occurrence of pre-eclipse dips which march to earlier orbital phase as 35-day phase increase. From the multi-band light-curves we derive time-average orbital phase dependence of column density for photoelectric absorption and energy-independent transmission as a function of 35-day phase. The X-ray light-curves during Low States are similar in shape to the optical Low State light-curve, but X-ray leads optical by $\simeq$0.04 to 0.08 in orbital phase.
... The 35-day period is caused by the precessing accretion disc around the neutron star, which causes periodic obscuration of the X-ray emitting accretion column on the neutron star [3]. The 35-day cycle normally shows a few day variations in its period [2] [4], but sometimes disappears for a few to several months: these periods are called anomalous low states (ALS) [5]. ...
... However, a consistent explanation of the physical mechanism causing the twisted disk was not obtained until the radiation-driven model of [4]. Almost at the same time, long term observations of the 35-day cycle gave the first measurements of the average shape of the 35-day light-curve [5]. The modelling study of [6] utilized that data to infer the shape of the twisted accretion disk. ...
... Long term monitoring observations of Her X-1 by Rossi X-ray Timing Explorer (RXTE) All-Sky Monitor (ASM) were analyzed by [22][23][24] and most recently by [25]. RXTE/ASM operated from 1996 until 2013. ...
... The CC method includes template fitting, and in our iterative application of the method, it has the advantage that the template is derived from the data. The CC method is used because it has better properties compared to previously used methods, such as defining a fiducial point on the light-curve, like Turn-On (TO) [23]. Defining a fiducial point is difficult when the light-curve has strong variability, like the 35-day cycle of Her X-1: different fiducial points give different results because of the changing shape of the light-curve. ...
... The first evidence that TO occured at specific orbital phases, 0.2 and 0.7, was presented by [1], with 9 observed TOs were concentrated near those orbital phases. The study of the 35-day cycle by [23] assumed TOs occured only at orbital phase 0.2 and 0.7 to create average 35-day light-curves for 0.2 and for 0.7 TOs. TOs measured using the CC method applied to RXTE/ASM data were presented by [22], which showed a uniform distribution of orbital phases. ...
Hercules X-1 (Her X-1) has been monitored by MAXI and by Swift/BAT for over a decade. Those observations are analyzed to measure the shape and energy dependence of the long-term average of the 35-day cycle of Her X-1. The cross-correlation (CC) method is used to determine peak times and cycle lengths. Swift/BAT data produces better 35-day times because of the gaps in the MAXI data. Using Swift/BAT-derived times, average 35-day cycle light-curves are created for multiple energy bands: MAXI’s 2–20 keV, 2–4 keV, 4–10 keV and 10–20 keV bands and Swift/BAT’s 15–50 keV band. The durations of the different states of the 35-day cycle are measured better than previously. We find clear changes in X-ray softness ratio with 35-day phase, and detect persistent features in the 35-day cycle. These include column density changes during turn-on of Main High and of Short High states, and persistent absorption dips during the bright part of Main High and of Short High states.
... A third, superorbital modulation, thought to be caused by the precession of the accretion disc, has a period of 35 d (Giacconi et al. 1973). The 35 d cycle shows two 'on' states, in which the pulsed emission is observed: a 'Main-On' of about 10 d and a 5-d 'Short-On', separated by two 'off' states during which the emission from the neutron star is occulted by the disc (Gerend & Boynton 1976;Boynton, Crosa & Deeter 1980;Scott & Leahy 1999;Scott, Leahy & Wilson 2000;Klochkov et al. 2008;Vasco et al. 2013). ...
We employ our new model for the polarized emission of accreting X-ray pulsars to describe the emission from the luminous X-ray pulsar Hercules X-1. In contrast with previous works, our model predicts the polarization parameters independently of spectral formation, and considers the structure and dynamics of the accretion column, as well as the additional effects on propagation due to general relativity and quantum electrodynamics. We find that our model can describe the observed pulse fraction and the pulse shape of the main peak, as well as the modulation of the cyclotron line with phase. We pick two geometries, assuming a single accretion column or two columns at the magnetic poles, that can describe current observations of pulse shape and cyclotron modulation with phase. Both models predict a high polarization fraction, between 60 and 80 per cent in the 1–10 keV range, that is phase and energy dependent, and that peaks at the same phase as the intensity. The phase and energy dependence of the polarization fraction and of the polarization angle can help discern between the different geometries.
... The rapid rise to MH occurs over a timescale of ∼3hr (e.g., Figure 3 of Scott et al. 2000) and is called turn-on. We analyzed 23 cycles from RXTE/ASM observations by were analyzed by Shakura et al. (1998) and by Scott & Leahy (1999). These studies measured accurately the mean 35 day lightcurve for the first time, measured dip behavior, and derived sets of turn-on times. ...
... The top panel of Figure 1 shows a sample of the Swift/BAT lightcurve of Her X-1. The 35 day cycle of Her X-1, as quantified by, e.g., Scott & Leahy (1999) and Leahy (2004c), is clearly seen in the data; Swift/ BAT has time in TT, while the standard time system for astronomical sources is barycentric dynamical time (TDB), which is TT transformed to the solar system barycenter. The maximum difference between the two time systems in the Swift/BAT observations is 0.0016582s, corresponding to a difference in orbital phase of´-1.1 10 8 . ...
... The orbital phase of the 35 day turn-on is a regular subject of interest in the literature. Early references often found turn-on to preferentially occur near orbital phase 0.2 or 0.7 (e.g., Scott & Leahy 1999 and references therein). In contrast, Leahy & Igna (2010) analyzed orbital phases of turn-on observed with RXTE/ASM and found a uniform distribution. ...
Swift/BAT and RXTE/ASM observations have monitored the X-ray binary system Her X-1 for approximately 14.5 years each, and both were monitoring Her X-1 for a period of ~5.5 years. Here we study the 35-day cycle using these observations. Using a cross-correlation method we find the times of peaks of the 35-day cycles for ~150 cycles observed by Swift/BAT and ~150 cycles observed by RXTE/ASM. These cycles include ~60 observed with both instruments. The noise level of the RXTE/ASM measurements is larger than that of Swift/BAT, resulting in larger uncertainty in peak times. The distribution of 35-day cycle lengths can be fit with a Gaussian with mean 34.79 d and $\sigma$ of 1.1 d. The distribution of orbital phases of 35-day cycle peaks is well fit by a uniform distribution, with 76 percent of the cycles, plus a Gaussian distribution peaked at orbital phase ~0.5, with 24 percent of the cycles. We construct the long-term average 35-day lightcurve in the 15-50 keV band from Swift/BAT, and in the 2-12 keV band from RXTE/ASM. The high energy band shows more variability in the Short High state and the low energy band shows more variability in the Main High state. This is consistent with a precessing accretion disk model as cause of the 35-day cycle.
