Draft:Vela X-1

Vela X-1 is the name and designation for the first extrasolar X-ray source to be discovered in the constellation Vela.

But, if Vela XR-1 is actually the first X-ray source discovered in the constellation Vela, then how did Vela X-1 become so labelled and why?

The apparent first X-ray source discovered or detected in the constellation Vela on 20 September 1966 was designated Vela XR-1. No specific discrete X-ray source has been found since at that position B 1950.0 08 30 -40.

Notations
The astronomical X-number notation (X-#) for an astronomical X-ray source in a constellation has its start from the initial discoverers of an extrasolar X-ray source at the American Science and Engineering Company (ASE)-Massachusetts Institute of Technology (MIT). "[T]he constellation genitive (or an abbreviation) followed by X and a digit, such as SCO X-1 [is applied]."

Researchers at the US Naval Research Laboratory suggested "XR" (for X-Ray) rather than "X". This would be SCO XR-1 for the first X-ray source discovered in the constellation Scorpius.

"[I]t seems more logical to use this in the standard astronomical notation where the identification precedes the constellation, i.e., X-1 Scorpii." Among the first 17 catalogued astronomical X-ray sources are X1 Scorpii to X3 Scorpii, X1 and X2 Sagittarii, and X1 to X4 Cygni.

Later, the sources became simply the constellation (or an abbreviation) followed by X-#, e.g., Scorpius X-1 (Sco X-1).

Astronomy
"An image [on the page top right] of the surroundings of the comparatively bright OB star HD77581 and its (optically invisible) companion Vela X-1 was obtained with the 1.54-m Danish telescope at La Silla, through a narrow-band H-alpha filter. It clearly shows the presence of a typical bow shock, thus immediately confirming the runaway status of this system. In fact, this is one of the most 'perfect' bow shocks of parabolic form ever observed around an OB-runaway."

"Moreover, the orientation of the bow shock indicates that the system is moving towards the north; its origin must therefore lie somewhere south of its present position in the sky. It also turns out that the accordingly deduced path of HD77581 crosses a well-known OB-association with the designation Vel OB1 [Vela OB1 in yellow, middle left of the image on the right]."

"At the measured distance of Vel OB1 of about 6000 light-years, the observed proper motion and radial velocity of HD77581 indicate a space velocity of 90 km/sec. With this velocity, it would have taken HD77581 and its compact companion about 2.5 million years to travel the distance between Vel OB1 and its present position."

"This corresponds exactly to the expected time that has passed since the supernova explosion of the progenitor star of Vela~X-1, as deduced from the observed properties of the binary system."

X-radiation
Def. an action or process of throwing or sending out a traveling X-ray in a line, beam, or stream of small cross section is called X-radiation.

X-rays span 3 decades in wavelength, frequency and energy. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays.

Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.

Planetary sciences
The diagram on the right shows the 8.964416 d orbit of Vela X-1 around its companion. It is approximately to scale. The eccentricity of its orbit is 0.0885 ± 0.0025.

The companion HD 77581 is a spectral type B0.5Iab star with a radius of about 30 R⊙. The star rotates every 13.5 d.

Theoretical Vela X-1 astronomy
An approximate polygon of overlap between the Chodil polygon and that portion of the constellation Vela within is given at SIMBAD by "region(polygon,08 20 -43 00, 09 00 -37 00,10 00 -45 00,09 30 -50 00) & otype='X'". There are 461 known X-ray sources within this overlap region and inside Vela. This includes all three of the same HMXBs.

As the lower hour, lower declination border has been drawn to encompass more of Vela, it also includes a small corner of the constellation Puppis that contains the supernova remnant Puppis A, whereas the Gursky polygon of overlap does not contain any of Puppis.

The somewhat blurred term on the lower right of the image on the right is "Candidate_YSO", without the quotes, overwritten many times. In this overlap region there are 274 candidate young stellar objects (YSOs). There is a star-forming cluster RCW 38 marked by this blurred, multiply written term.

Using a region(08 32 -40 10,120m) & otype='X' search on SIMBAD yields 25 X-ray sources within 2° of Vela XR-1. Most are apparently X-rays sources only, but three are more notable:
 * 1) 2MASS J08261004-3902050 is a pre-main sequence star of G9V spectral type and ROSAT X-ray source,
 * 2) HD 73882 is an eclipsing binary with one star an O9III at 2.17 mas and a ROSAT X-ray source, and
 * 3) 1RXS J083828.7-414402 is a rotationally variable star and a ROSAT X-ray source.

A region(08 32 -40 10,180m) & otype='X' search on SIMBAD yields 78 X-ray sources at or within 3° of Vela XR-1. Changing the otype to gamma-ray sources 'gam' or gamma-ray bursts 'gB' yields 8 and 2, respectively.

Entities
"Vela (the sails) is a southern constellation, one of the three parts into which Argo Navis was split."

The Vela Molecular Ridge (VMR) is a vast complex of giant molecular clouds visible in the direction of the constellation of Vela, where the most important clouds in the region are identified by the letters A, B, C and D, and in fact belong to two different complexes: the clouds A, C and D are located at an average distance of about 700-1000 parsecs (2300-3300 light years) and are related to the OB association Vela R2, while cloud B is located at a greater distance, up to 2000 parsecs (6500 light years) away, and is physically connected to the extended Vela OB1 association.

Radio 12CO observations of the region showed the ridge to be composed of several clouds, each with masses 100,000–1,000,000 M⊙. This cloud complex lies on the sky in the direction of the Gum Nebula (foreground) and the Carina–Sagittarius Spiral Arm (background).

Stellar formation activity is confirmed by the discovery of several associations of T Tauri stars, particularly in the VMR D cloud, as well as by the presence of several open clusters heavily obscured and deeply immersed in the gas observable at infrared wavelengths.

The brightest and warmest stars of the Vela R2 association illuminate some filaments of gas that shine with a bluish light, typical of reflection nebulae, such as NGC 2626 nebulae which belongs to the VMR D cloud and hosts some stars presenting H&alpha; emission and the famous Herbig-Haro object HH 132.

The stellar components, however, are partially visible to the naked eye and combine to form a rich star field, characteristic of the northwestern part of Vela; in particular, the area of ​​the sky visible between Lambda Velorum and Gamma Velorum is occupied by the Vela OB1 stellar association, physically linked to the Vela Molecular Ridge.

Named entities
The apparent first X-ray source discovered or detected in the constellation Vela on 20 September 1966 was designated Vela XR-1. No specific discrete X-ray source has been found since at that position B 1950.0 08 30 -40. From a subsequent ROSAT X-ray map of the region, it appears Vela XR-1 was a composite signal from nearby SNRs and X-ray binaries.

Vela XR-1
On or before 20 November 1967, Vela XR-1 was associated with Vela X or Vela Y (?).

As of 25 July 1969, Vela XR-1 had that name and position at B 1950.0 08 30 -40.

GX263+3 was at 08 57 -41 15.

On or before 21 February 1973 HD 77581 was considered an optical candidate for the X-ray source Vela XR-1.

As of 26 March 1974, the "variable X-ray source Vela XR-1 was discovered by Chodil et al. (1967) and studied by Giacconi et al. (1972), who catalogued it as 2U 0900-40."

On or before 31 March 1976, Vela XR-1 is used in discussions of other X-ray pulsars such as GX 1+4: "The "low state" of GX 1+4 is suggestive of the variable absorption seen to occur in several X-ray binary systems, e.g., Cen X-3, Vela XR-1, and Cyg X-3."

"We have analysed the shape of the profile as a function of the phase of the 283-s period pulsations of the associated X-ray source 4U 0900−40 (Vela XR-1)." is from on or before 15 August 1980.

As of or before 6 October 1967, "the X-ray binary system HD 77581 (=4U 0900-40)."

On or before 3 December 1996, "4U0900-40 (Vela XR-1)=HD77581".

"The best measured values of the mass of 4U0900-40 (= Vela XR-1), 1.86 ± 0.16 and 1.93 ± 0.20 solar masses, make it the most massive neutron star known."

Stated on or before 5 January 2005: "The best measured values of the mass of 4U 0900-40 (= Vela XR-1), 1.86 ± 0.16 M⊙ (Barziv et al.) and 1.93 ± 0.20 M⊙ (Abubekerov et al.), make it a leading candidate for the most massive neutron star known."

Vela X-1
On or before 21 May 1971: "It is possible that the source Vela X-1 has a similar origin3."

On or before 9 June 1972: "Several [bright stars] are being actively pursued as candidates, [...] HD 77581 for Vela XR-1".

From the table in the article, "2U 0900-40 0.007 [Area Error Box] 220767 [SAO star] (6.7 mag Bo I) Vela X-1 [Comment]".

It appears to be the case that Vela X-1 was assigned to 2U 0900-40.

On or before 29 July 1976: "We suggest a model for the complex pulse shapes of the long period, pulsating X-ray sources, Vela X-1 and AO535+26."

Backgrounds
The graph on the right contains the proportional "counter data from [the Nike-Tomahawk] flight [on] September 20, 1966. Dashed and solid lines are the cosmic-ray-induced and diffuse X-ray background, [respectively]. Counting statistics are shown [by error bars] on typical points. Discrete sources [Vela XR-1 and Cygnus XR-2] appear with [a] triangular shape characteristic of the collimator."

Neutrals
"Circumstellar structures around [blue super giant stars] BSGs have been predominantely identified as bow shocks around runaway stars."

"Runaway stars have large stellar velocities (above 30 km s−1) resulting from dynamical interactions in (dense) clusters or from a supernova explosion in a binary system. These stars can thus travel at supersonic speeds through the local medium giving rise to “bow shocks” as their stellar winds interact with the surrounding medium, which has been previously ionised by stellar photons from the hot star (Weaver 1977). The occurrence of such bow shocks has been shown to depend primarily on the ISM conditions (Huthoff & Kaper 2002)."

"Once formed, the size, shape and morphology of a bow shock depends on both stellar (wind kinetic energy and stellar velocity) and interstellar parameters (density and temperature). In particular the ratio v*/vwind indicates whether or not instabilities are likely to develop (Dgani et al. 1996), and the stand-off distance between the star and the apex of the shock is determined from the pressure balance between the stellar wind and the ISM (see analytical results by Wilkin 1996 and simulations by e.g. Comeron & Kaper 1998, Blondin & Koerwer 1998)."

"For Vela X-1, the peak emission of the dust emission is co-spatial with the most prominent Hα arc seen in the supposed direction of space motion [figure on the right]: it is concluded that the outer shock is radiative, but the inner shock is adiabatic, though some Hα emission possibly related to (part of) the inner termination shock is also detected."

Gamma rays
"Vela X-1 is, however, only 6.4° from the Vela pulsar, the brightest object in the sky at 100 MeV energies, and the presence of a weak γ-ray source in the direction of Vela X-1 cannot therefore be ruled out."

It "would appear that the γ-ray emission of Vela X-1 must be due to a different mechanism than that which produces the X-ray emission, perhaps acceleration of protons or nuclei followed by nuclear interaction resulting in γ-rays from π0 decay."

X-rays
"An X-ray source was observed in the constellation Vela from an attitude-controlled Aerobee 150 rocket launched from the White Sands Missile Range on February 2, 1968. The object, which may be the previously reported Vel XR-1 (Chodil et al. 1967), lies close to the galactic plane; we designate it as GX263+3."

The image on the upper right "shows the counting rates plotted against time during the maneuver in which the new source was observed. The peaks labeled 1, 2, 3, anf 4 all refer to this source. Peaks 1 and 2 determine a location that is the same within experimental error as the location determined by peaks 3 and 4. The reduction in background level from 130 to 150 sec after launch occurs when a large portion of the field of view falls below the horizon."

The second image down on the right shows the most "probable celestial locations defined by the peaks in [the upper image on the right] and counting-rate ratios are shown as line segments. Separations between intersections are consistent with a single X-ray source and statistical errors in determination of times of peak counting rates. Shaded area around intersections, and enlargement, show the region of uncertainty of the source. Lines labeled 1st pass and 2d pass refer to the center of the field of view during scan. Area inclosed by dashed lines is the region of uncertainty [well within Vela and the Chodil polygon] of Vel XR-1 as reported by Chodil et al. (1967)."

"The location of the source as α(1950) = 8h57m, δ(1950) = -41°15', with a region of uncertainty of about 3 square degrees, as shown in the [second figure down on the right]. The associated galactic coordinates are lII = 263.3°, bII = 2.9°. [...] This source lies about 3° from the position of the source Vel XR-1 as reported by Chodil et al. (1967) and is within its region of uncertainty. The two objects are probably coincident, since we see no other sources in the vicinity."

Actually, according to NASA's Universal coordinate converter, the X-ray source at lII = 263.3°, bII = 2.9° is 5.26° from Vela XR-1 at lII = 259° 08' 33.8", bII = 00 19' 35.7" not about 3°.

Hard X-rays
"Vela X-1 is the prototype of the classical sgHMXB systems. Recent continuous and long monitoring campaigns revealed a large hard X-rays variability amplitude with strong flares and off-states. This activity has been interpreted invoking clumpy stellar winds and/or magnetic gating mechanisms. We are probing if the observed behaviour could be explained by unstable hydrodynamic flows close to the neutron star rather than the more exotic phenomena. We have used the hydrodynamic code VH-1 to simulate the flow of the stellar wind with high temporal resolution and to compare the predicted accretion rate with the observed light-curves. The simulation results are similar to the observed variability. Off-states are predicted with a duration of 5 to 120 minutes corresponding to transient low density bubbles forming around the neutron star. Oscillations of the accretion rate with a typical period of ∼ 6800 sec are generated in our simulations and observed. They correspond to the complex motion of a bow shock, moving either towards or away from the neutron star. Flares are also produced by the simulations up to a level of 1037 erg/s."

"INTEGRAL discovered huge hard X-ray variability and off-states in Vela X-1 and other high mass X-ray binaries. Hydrodynamic simulations allowed us to discover the likely source for such variations occuring on time scales of hours. These variations are related to oscillations of the accretion rate (with a typical period of ∼ 6800 sec) corresponding to the complex motion of a bow shock forming between the neutron star and the massive companion, moving either towards or away from the neutron star."

The image on the right shows a hydrodynamic simulation of stellar winds that qualitatively reproduce the hard X-ray variations observed in Vela X-1.

The hydrodynamic patterns are induced "by the gravitation of the neutron star on the stellar wind of the massive companion".

The image on the right is a "snapshot of the density (blue to red for (1-10) 10-14 cm-3 close to the neutron star (black arrow). The companion star is on the left. The image side is about 5 1012 cm. The bow shock in red creates low density bubbles (in blue) that are quenching accretion on the neutron star."

Soft X-rays
The image on the right is a "map of the Vela-Puppis region. The radio brightness contours are 2650 MHz isotherms from Milne (1968a) and from Milne and Hill (1969). The X-ray source positions of LRL and ASE are shown. The large solid circle is a shell model used to fit the Vel XR-2 0.3 - 2 keV data of Seward et al. (1971). The dashed ellipse indicates the 0.2 - 0.3 keV emission. The accuracy of the location of the X-ray source Pup A is 0.3°. Also shown is the center of a circumferential magnetic field pattern (Milne 1968a), the location of the pulsar PSR 0833-45 and the optical filaments (wispy lines)."

Soft thermal X-rays can originate from supernova remnants. Vela XR-1 as originally detected exhibited higher energy X-rays, but it "is possible that the source Vela X-1 has a similar origin3." Reference3 is Gursky from 1968.

Infrareds
On the right is a combined Spitzer IRAC (3.6 µm blue, 4.5 µm green, 8.0 µm red) image of the bow shock around Vela X-1.

Circumstellar clouds
On the right is a "HIRAS image of the circumstellar environment of Vela X-1 at 60 µm. The arc-like structure of a wind bow-shock is clearly resolved. The arrow indicates the direction of the system's space velocity. East is to the left and North is up; the image size is 0.5° X 0.5°."

Supernova remnants
"A source of hard X-radiation has been detected in the general direction of the Vela X supernova remnant."

On the right is a "Rosat image of the Vela supernova remnant and its surroundings, for X-ray energies above 1.3 keV, showing the big Vela remnant, the small Puppis-A supernova remnant in the upper right, and in this image newly discovered remnant RX J0852.0-4622 in the lower left (Aschenbach, 1998)."

"The supernova remnant RX J1713.7-3946 [...] was the first remnant where the shell structure was detected in VHE gamma rays, demonstrating that the supernova shock wave accelerates particles. The detailed interpretation is complicated by the fact that there is significant uncertainty in a number of crucial parameters, such as the distance to the remnant and the characteristics of the ambient interstellar medium. It is therefore important to enlarge the ensemble of supernova remnants studied in VHE gamma rays, which allows one to average over such parameters and to extract the key features. "Vela Junior", or RX J0852.0-4622 is supernova remnant discovered in 1998 in ROSAT X-ray images (Aschenbach 1998, Aschenbach at al. 1999). The remnant is only a faint radio emitter (Duncan & Green, 2000) and was not identified in earlier radio surveys. Age and distance are estimated to 680 years and 200 pc (Aschenbach at al. 1999). It has been argued that three nearby supernova explosions might be responsible for spikes in the nitrate abundance found in South Pole ice cores (Rood et al, 1979); with an age around 700 years, the Vela Junior supernova might be responsible for a fourth, previously unidentified spike (Burgess and Zuber, 2000) (Fig. 1). The CANGAROO instrument reported the detection at the 6 sigma level of VHE gamma rays from the north-western part of Vela Junior, based on about 100 h of observations (Katagiri et al., 2005)."

"With the H.E.S.S. telescopes, a clear signature of Vela Junior in VHE gamma rays was detected in only 3.2 h of exposure, with a significance of 12 sigma [...]. In the H.E.S.S. image, a shell-like morphology is clearly visible, coincident with the X-ray morphology of the remnant. The radius of the shell is almost 2 degrees, with a peak of VHE emission in the north-western (upper-left) section of the remnant. The total flux from the remnant is at the same level as the flux from the Crab Nebula, which makes it one of the strongest galactic sources of VHE gamma rays. The close resemblance between the X-ray image and the gamma-ray image is demonstrated by a correlation coefficient of 0.7, obtained by dividing the image into bins of 0.4 by 0.4 degrees. X-ray images show a compact X-ray source at the center of the remnant (AX J0851.9-4617.4, Slane et al. 2001), which could be a neutron star created in the explosion; in the H.E.S.S. image, no significant gamma-ray excess from this object is detected."

In a table of probable "identifications of x-ray sources and supernova remnants", Vela XR 1 is matched with "Vela Y (or X?)".

Vela X is at the position B1950.0 RA 08 32.5 Dec -45 35 with a distance of 600 pc.

Vela Y is at the position B1950.0 RA 08 43.5 Dec -43 42 with a distance of 1200 pc.

Binary stars
According to SIMBAD there are two low-mass X-ray binaries (LXBs) at about 2.9° of the original position for Vela XR-1:
 * 1) 4U 0836-42 (Ginga 0836-429) and
 * 2) 4U 0836-429.

Both of these are also gamma-ray sources.

Another close X-ray source is Puppis A a supernova remnant (SNR) at about 3.2° from the original position of Vela XR-1.

Ginga 0834-430 is a high mass X-ray binary (HXB) located at J2000.0 RA 08h 35m 55.0s -43° 11' 06" (Einstein satellite), according to SIMBAD. It is also a gamma-ray source and within the Chodil polygon. The high mass companion is spectral class B0-2III-Ve. This source is about 3.1° from the position for the original position of Vela XR-1.

The four sources mentioned above are much closer to the original position for Vela XR-1. All emit higher energy X-rays such as were detected by the original rocket launch. And, three of the four are within Vela. An overlapping higher energy X-ray signal from all four is highly likely to be interpreted as a signal from inside Vela. No significant higher energy X-ray sources occur above -38° to -37°.

LM Velae is a high mass X-ray binary located at J2000.0 RA 08h 40m 47.79180s -45° 03' 30.2384" (optical), according to SIMBAD. And, it is within the Chodil polygon. The high mass companion is a spectral type O8.5Ib with a parallax of 0.03 mas. This star or binary is also a gamma-ray source and an ultraviolet source.

Vela X-1 is the prototypical detached high-mass X-ray binary (HMXB).

GP Velae (HD 77581) is an HXB located at J2000.0 RA 09h 02m 06.86048s -40° 33' 16.9033" (optical), according to SIMBAD. The high-mass companion is spectral type B0.5Iae with a parallax of 0.94 mas. It is also a gamma-ray source and an ultraviolet source. Vela X-1 is at about 5.7° from the original position of Vela XR-1.

Of the three HXBs within the Chodil polygon GP Velae is by far the closest to Earth and thereby the most likely to be detected by the Nike-Tomahawk rocket.

Supernovas
It is a type of stellar remnant (a compact star) that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons.

Neutron stars are theorized as the radiation source for anomalous X-ray pulsars (AXPs), binary pulsars, high-mass X-ray binaries, intermediate-mass X-ray binaries, low-mass X-ray binaries (LMXB), pulsars, and soft gamma-ray repeaters (SGRs).

"The [image on the right] shows two observations of [the] neutron star [RX J0822-4300] obtained with the Chandra X-ray Observatory over the span of five years, between December 1999 [on the left] and April 2005 [on the right]. By combining how far it has moved across the sky with its distance from Earth [at about 7,000 light years], astronomers determined the cosmic cannonball is moving at over 3 million miles per hour, one of the fastest moving stars ever observed. At this rate, RX J0822-4300 [at (J2000) RA 08h 23m 08.16s Dec -42° 41' 41.40" in Puppis] is destined to escape from the Milky Way after millions of years, even though it has only traveled about 20 light years so far."

Pulsars
According to SIMBAD, Vela X is a gamma-ray pulsar located at B1950.0 08 31 27.9 -45 01 06.

The Vela pulsar is in the "region of the Gum Nebula [...] the time-averaged flux determination [...] is (7.2 ± 1.4) X 10-4 cm-2 s-1 keV-1, in the energy interval from 23 to 80 keV.".

This "hard X-ray source [is] coincident with the radio source Vela X centered at α = 8h 32', δ = -45°. Since the 20° FWHM aperture of the Harnden X-ray telescope probably included Vel XR-1 which is less than 6° away from Vela X, the flux reported [...] could have come from Vel XR-1 (Peterson 1972)."

The second image down on the right "is a figure that overlays the H-alpha and X-ray wavebands. (This has been done only in an approximate manner, for illustrative purposes only!) The X-ray is shown in blue/green with the optical remaining in red. [...] the bright X-ray region north of the pulsar is bounded by the optical filaments. The bright X-ray blob to the NW (upper right) is actually a separate supernova remnant, Puppis A, which is much farther away (about 2000 parsecs, or 6500 light years) in the next spiral arm out in this direction)."

Satellites
Three X-ray sources detected by the Uhuru satellite occur within the Chodil polygon: 2U 0821 -42, 2U 0832 -45, and 2U 0900 -40. Each of these has a point of maximum probability density, and four corners of a region for 90 percent confidence. Here they are listed in B 1950.0 coordinates.
 * 1) 2U 0821 -42 08 21 26 -42 39 36 08 21 43 -42 31 12 08 20 48 -42 39 00 08 20 48 -42 51 00 08 21 43 -42 43 12
 * 2) 2U 0832 -45 08 32 29 -45 07 12 08 33 19 -45 04 48 08 32 24 -45 00 00 08 31 36 -45 10 48 08 32 00 -45 16 12
 * 3) 2U 0900 -40 09 00 19 -40 22 48 09 00 41 -40 23 24 09 00 19 -40 19 12 08 59 55 -40 21 00 09 00 19 -40 25 12

Each of these was tentatively cross identified with possible previous X-ray source detections:
 * 1) 2U 0821 -42 Vela XR-2 Puppis A
 * 2) 2U 0832 -45 Vela XR-1 Vela XR-2 Vela X
 * 3) 2U 0900 -40 GX 263 +3 Vela XR-1 Vela I

"The OSO-7 UCSD hard X-ray telescope scanned Vela XR-1 [indicated in the diagram on the right], located at α = 9h0.3m, δ = -40.4°, from 1971 December 17 to 1972 January 22. The flux in the 7-26 keV range shows evidence for periodic intensity variations of at least a factor of 20, with a period of 8.7 ± 0.2 days."

Balloons
"In the course of a high-energy X-ray sky survey carried out for 8 hours in a balloon flight from Mildura, Australia, on October 15, 1967, we [obtained] results from 20 to 74 keV in the form of a sky map [the figurre above] in galactic coordinates in which the data have been lumped in 5° X 5° boxes. The number of vertical lines in each box indicates the number of standard deviations by which the count rate is above the background rate. No vertical line means zero-to-one standard deviation above the background rate, one vertical line means between one and two standard deviations above the background rate, etc. One should note that this number is not necessarily proportional to the count rate above background, since the exposure time differs from box to box. The dots indicate positions of previously observed X-ray sources in the area we scanned. The circles indicate the positions of conspicuous supernova remnants and strong radio sources."

In the figure above, right of center (0,0) is the 270° notation. Two squares over further to the right, above the galactic equator is a 5° X 5° square containing a black dot at l = 259 08 33.8 b = -00 19 35.7, or just above the plane, that is the originally reported location for Vela XR-1.

"A balloon was launched from Paraná, Argentina, on 1970 November 25 at 04:23 UT."

Sounding rockets
"A rocket observation on May 13, 1970 confirmed that Vela-X and Puppis A are strong sources of soft X-rays. X-rays from Vela-X come from a broad region of sky with a diameter of ~ 6.5°. This region is neither centered on the Vel X radio source nor on the Vela pulsar. [...] Assuming the X-rays are thermal in origin, approximate temperatures of 2.7 x 106 °K and 4.3 x 106 °K can be derives for Vel X and Pup A respectively."

"A Terrier-Sandhawk rocket was launched at 0803 UT, 1970 May 13, from Kauai, latitude 22.5° N. Apogee was at 306 km, and an altitude control system (ACS) provided scans in two different directions through the Vela region at a rate of 0.8° per second."

"On 14 May 1968 a rocket-borne proportional counter detected x-rays of energies between 200 and 800 eV from a source in the constellation Vela. It is believed that the source is the Wolf-Rayet binary γ2 Velorum. The count rate increases with decreasing energy, and below 500 eV the count rate is higher than that of Sco XR-1. The observation described locates the source to within approximately 2° of γ2 Velorum. Until a positive correlation is made, the designation Vela XR-2 is suggested."

Hypotheses

 * 1) Vela X-1 may not be the first X-ray source discovered in the constellation Vela.