As­tronomers have th­ese eyes in the sky

Mint ST - - TASTE -

How two cam­eras on board the In­ter­na­tional Space Sta­tion are help­ing in the study of black holes, neu­tron stars and X-ray sources

Su­per­man’s X-ray vi­sion fa­mously gave him en­hanced per­cep­tion, though his ca­pa­bil­ity had lit­tle to do with how X-rays ac­tu­ally work. But what would the uni­verse re­ally look like if one had “X-ray eyes”? We now have cam­eras that al­low us to an­swer this ques­tion, and th­ese have opened up a fas­ci­nat­ing win­dow into new physics.

A beam of X-rays car­ries thou­sand of times more en­ergy than a beam of vis­i­ble light. So, for X-rays to be gen­er­ated, there needs to be highly en­er­getic cos­mic phe­nom­ena, in­clud­ing mat­ter swirling around black holes and neu­tron stars at near light speeds, au­ro­rae near the mag­netic poles of plan­ets in the so­lar sys­tem, and mil­lion-de­gree plas­mas sur­round­ing ex­ploded stars.

The birth of X-ray as­tron­omy was, in fact, a happy ac­ci­dent. In the 1960s, sci­en­tists search­ing for X-rays from the moon, launched sound­ing rock­ets with X-ray de­tec­tors. In­stead, they dis­cov­ered X-rays from a pre­vi­ously un­seen neu­tron star in the Milky Way, to­gether with X-rays from grow­ing black holes. It was quickly rec­og­nized that the skies must be teem­ing with hot and dense X-ray-emit­ting sources, and the past 60 years have seen the new field of “X-ray as­tron­omy” grow and de­velop. The 2002 No­bel Prize for physics was awarded to one of the pi­o­neers of this field, Ric­cardo Gi­ac­coni, for rec­og­niz­ing its fun­da­men­tal im­por­tance.

X-rays are ab­sorbed and scat­tered by air, so it is nec­es­sary to get above the earth’s at­mos­phere to de­tect them from space. And the In­ter­na­tional Space Sta­tion (ISS) pro­vides an ex­cel­lent plat­form for mount­ing X-ray cam­eras. Cur­rently, Ja­pan has a cam­era on­board (called Maxi or the mon­i­tor of all-sky X-ray image), as does the US (the NICER or neu­tron star in­te­rior com­po­si­tion ex­plorer tele­scope).

Maxi, oper­a­tional since 2009, scans the en­tire sky with each ISS or­bit, and is re­spon­si­ble for dis­cov­er­ing sev­eral new black holes and neu­tron stars in the Milky Way ev­ery year. As their name sug­gests, black holes are “black” and do not emit light. But as mat­ter spi­rals at high speeds to­wards a black hole, it can be­come hot and bright, glow­ing at the sear­ing tem­per­a­tures of mil­lions of de­grees as­so­ci­ated with X-ray ra­di­a­tion. Th­ese are ex­tremely vi­o­lent en­vi­ron­ments, with con­di­tions chang­ing faster than the blink of an eye. There­fore, bright and vari­able X-ray sources are ex­cel­lent sign­posts of black holes grow­ing by ac­tively swal­low­ing sur­round­ing mat­ter. As­tronomers be­lieve that there ought to be many thou­sands of grow­ing black holes in the Milky Way. But only a few dozen have been se­curely iden­ti­fied so far. Maxi is help­ing us dis­cover more of th­ese.

NICER is a rel­a­tively new cam­era, oper­a­tional since mid-2017, per­form­ing de­tailed ob­ser­va­tions of in­di­vid­ual X-ray sources. Its strength is the abil­ity to col­lect a large num­ber of X-rays with high time ac­cu­racy. This en­ables us to fol­low the mo­tions of mat­ter swirling around neu­tron stars and black holes with un­prece­dented fi­nesse. A key ob­jec­tive of the mis­sion is to study the com­po­si­tion of neu­tron stars—dead stars whose den­sity and pres­sures are higher than in atomic nu­clei. A sin­gle tea­spoon full of neu­tron star mat­ter would have about as much mass as a moun­tain on earth! Un­der­stand­ing the be­hav­iour of mat­ter in this ex­treme regime could her­ald new physics.

X-ray as­tron­omy is still a rel­a­tively young field. The light-col­lect­ing area of X-ray tele­scopes re­mains small, equiv­a­lent to am­a­teur op­ti­cal tele­scopes. But tech­nol­ogy con­tin­ues to im­prove, and other na­tions are now par­tic­i­pat­ing in the ex­cit­ing quest to de­tect X-rays from the sky. In­dia’s first as­tron­omy-ded­i­cated satel­lite, Astrosat, also has a strong fo­cus on X-rays. In fact, In­dia has an es­tab­lished her­itage in black hole stud­ies. Its legacy is rooted in the the­o­ret­i­cal work pi­o­neered by Subrah­manyan Chan­drasekhar, and stretches into the fu­ture with the up­com­ing con­struc­tion of the in­ter­na­tional grav­i­ta­tional wave ob­ser­va­tory arm in Ma­ha­rash­tra. But the ISS, or sim­i­lar space plat­forms en­vi­sioned by other na­tions, will likely con­tinue to serve for some time as one of the best, low-cost van­tage points for ex­per­i­ments that can only be done from space, in­clud­ing X-ray as­tron­omy.

The author is an as­tronomer work­ing at the School of Physics & As­tron­omy at the Univer­sity of Southamp­ton.

(clock­wise, from left) An image of the full sky in X-rays, as seen by Ja­pan’s Maxi cam­era, show­ing the glow­ing hot vicini­ties of black holes, neu­tron stars, and su­per­nova rem­nants in the Milky Way; a pic­ture of NICER; Maxi is lo­cated on the Ja­panese Ex­per­i­ment Mod­ule of the ISS; and the X-ray con­cen­tra­tors in NICER.

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